Effect of Increased Postmilking Prolactin Concentrations on Lactation, Plasma Metabolites, and Pancreatic Hormones in Lactating Goats's2 N. JACQUEM- and E. C. PRlGGE3 Division of Animal and Veterinary Sciences West Virginia University Morgantown 26606-6108 INTRODUCTION
ABSTRACT
Prolactin (PRL) is a lactation-promoting hormone enabling the mammary cells to differentiate fully at parturition and initiating the onset of copious milk secretion (10). Although depression of PRL concentrations during an established lactation in nomuminant animals results in an immediate decline in milk production (10). the role of PRL postpartum remains uncextain in ruminant animals. Numerous studies (7, 17,27) have failed to note a change in lactational performance of milk-producing ruminant animals with alterations in PRL concentrations. Although the involvement of basal PlU during an established lactation is questionable, indirect evidence suggests that milking- or suckling-induced PRL release may participate in maintenance of milk secretion. In all species, milking or suckling induces release of PRL (7), and this release declines as lactation advances (7). Milk yields are positively correlated with s e m P l U concentrations immediately postmilking in cows (21) and women (3). When suckling-induced PRL release was Stimulated, milk secretion could be maintained up to 2 mo in mice (23). Similarly, heifers milked and suckled in early lactation showed an increased milk yield as compared to heifers milked only Abbreviation key: CTRL = control, PRL = (25). Increasing milking frequency from two to prolactin, PRL2 = prolactin at 2 mg/h, PRL4 = three times daily also enhances milk yield (28). These results suggest that the milking- or suckprolactin at 4 m a . ling-induced PRL release may be of physiological importance. In 1983, Bauman and Elliot (5) proposed that PRL may play a part in homeorhetic Received February 20, 1990. control of metabolism during lactation. AdminAccepted July 18, 1990. istration of PRL elicits increases in FFA conhblished with the approval of the director of Ihe West centrations in humans (8), dogs (32), and cows Virgiaia Agricultural a d Forestry Experiroent Slation as (31). Furthermore, a switch in lipogenesis from Scientific Papa Number 2210. "me authors Wish to acknowledge the National Insti- the mammary gland to the liver and adipose tute of Arthritis, Diabetes, and Digestive and Kidney Dis- tissue is observed when suckling-induced or eases (NIADDK) for gift of the purified prolactin and basal PRL releases are inhibited in lactating thanlr E. Townsend for statistical help. rats (1). Prolactin has also been shown to in%'o whom reprint requests should be sent.
Six lactating dairy goats were utilized in a replicated 3 x 3 Latin square design to examine the effect of increased postmilking prolactin concentrations on lactational performance. Thirty minutes after each milking, either placebo or pituitaryderived ovine prolactin at 2 mg/h or 4 mg/h was infused via jugular cannula for 2 h. Goats were housed in a controlled environment at 19'C and a cycle of 16 h light and 8 h dark. Mean plasma prolactin concentrations after millring were increased 100 and 180% for 2 and 4 mg/h prolactin infusion, respectively. Milk yield was slightly but significantly increased by 2 mg/h prolactin infusion compared with placebo and 4 mgh prolactin infusion. However, milk composition and yields of milk components were not influenced by treatments. No differences in dry matter intake, digestibility of the diet, or N retention were observed. Circulating concentrations of glucose, nonesterified fatty acids, insulin, and glucagon were not altered. (Key words: prolactin, lactation, goats)
1991 J Dairy Sci 74:10%114
109
110
JACQUEMET AND PRIGGE
crease N retention in humans (6) and sheep (9) and to affect glucose metabolism in nonrcuninant animals (12). The purpose of this experiment was to determine if elevation of postmilking PRL concentrations affects milk production and composition, N retention, and the plasma concentrations of glucose, FFA and pancreatic hormones in lactating dairy goats. MATERIALS AND METHODS
Six lactating goats of either Toggenburg or Saanen b d weighing an average of 49.5 kg were used in a replicated 3 x 3 Latin square design. The animals were milked three times daily at 0700 h, 1300 h. and 1900 h. Thirty minutes after each milking, either a placebo [conaol (CraL)] or prolactin at 2 mg/h (PRL.2) or 4 mg/h (PRL4) was infused via an indwelling left jugular catheter (Intracath 16 ga 12 in needle 14 ga, Deseret Inc., Sandy, UT) at a rate of .16 Wmin for 2 h using a constant infusion pump (Gilson Medical Electronics, Middleton, Wl). The purpose of the PRL infusion was to prolong and intensify the natural phenomenon of milking-induced PRL release. Infusion solutions were prepared as described by Jacquemet and Prigge (20). The amount of PRL infused was calculated to increase PRL concentrations to a maximum of 400 and 800 n g / d Each experimental or infusion period lasted 6 d A 1 5 d preliminary period was utilized, during which feed intake and milk yield were monitored. The mom temperature was maintained at lYC, and the lighting schedule was 16 h light and 8 h dark. A 50% alfalfa hay and 50% concentrate diet was fed ad libitum. The composition and analysis of the ration were as described by Jacquemet and Prigge (20). Sampling Technlques and Chemical Analysts
During the experimental period, milk samples were collected three times daily, fecal samples twice daily, and feed and refusal samples once daily, and prepared and analyzed as described by Jacquemet and Prigge (20). Urine was collected twice daily, and an aliquot was preserved with HzSO4 and stored at 4'C until N was analyzed by the Kjeldahl procedure (4). On d 5 of infusion, right jugular cannulas J
d of Dairy Scirnce Vol. 74, No. 1, 1991
were inserted for blood sampling. On the last day of each experimental period. 15-ml blood samples were taken every 20 min, stamng 10 min before the am. milking, for 16 h and every hour thereafter for 8 h. Three milliliters of blood were immediately transferred into chilled test tubes to be assayed for glucagon as described by Jacquemet and Prigge (20). An additional 3 ml of blood were discharged into chilled test tubes containing .1 ml of sodium citrate for FFA analysis. Plasma FFA wnmtrations were determined by enzymatic analysis (Walk0 Chemicals, Dallas, Tx). The remainder of the blood was transferred to heparinized test tubes for glucose, insulin, and PRL assays. Plasma glucose and hormone concentrations were determined as described by Jacquemet and Prigge (20). Interassay and intraassay coefficients of variation were 12 and 7%, 11 and 6%, and 12 and 8% for insulin, glucagon, and PRL., respectively. Statlstlcal Analyses
The experiment was analyzed as a replicated 3 x 3 Latin square design (30). The mean of each experimental unit was determined for each variable describing milk output, intake, digestibility, and N metabolism, and an ANOVA was conducted. Glucose, FFA, and hormone profiles were analyzed as a split-plot over time within a replicated Latin square. A conservative F test was used for the effects of time and treatment x time interaction (15). Polynomial regressions over time weze also fitted for each experimental unit (2), and an ANOVA was performed for each set of regression coefficients obtained. RESULTS AND DISCUSSION Dietary Intake, Nitrogen Metabolism, and Milk Production
The influence of treatments on means of intake and digestibility of DM, CP, and ADF, and N metabolism are presented in Table 1. Increased postmilking PRL concentrations did not affect intake or digestibility (-.lo), which is in agreement with other studies in which basal PRL concentrations were altered (14, 20, 24). Based on N retention data, the average amount of N retained per day should be equivalent to 38 g of tissue protein However, N
111
MILIUNG-LNDUCED PROLACI"
TABLE 1. Effect of infusion of placebo (CTRL),prolactin at 2 @ (pRL2), and prolactin at 4 mg/h (PRL4) on mean intake and apparent digestibility of DM, CP, and ADP,and on N metabolism~~
Intakes DM, g/d Mi,
MWd
CP, g/d ADP, gld
CTRL
Treatments F%u2
2308 4.9 328 533
2399 5.1 338 536
67.7 65.2 58.7
PRLA
SED'
2284
4.9 324 514
73 .1 10 24
682 65.0 572
67.5 64.2 58.9
1.5 1.8 2.4
52.4 34.4 18.1 18.3 10.5 5.6
543 35.3 18.9 18.4 10.6 6.3
51.8 33.1 18.1 17.0 6.5
1.7 1.8 .7 3.7 .3 3.5
10.0
11.3
9.2
7.3
Digestibilities
DM, % CP, % ADF, % N Metabolism, g/d N Intake
Digestible N Fecal N Urinruy N MiIkN Retained N
10.2
Retained N, Percentage N intake.
'SED =
stan-
error of ttre difference.
balance studies often overestimate N retention (13). Limitation of validity of N balance studies aside, N retention was not affected by PRL infusion. Similarly, when basal PRL concentrations were increased in sheep via PRL injection (22), no change in N retention was observed. However, Brinklow and Forbes (9) observed an increased N retention in sheep infused with PFZL. Mean milk yield was slightly but significantly i n ~ ~ (k.05) e d When PRL W ~ Sinfused at 2 mg/h after milking (PRL2) as compared to control or PRL infused at 4 mg/h aftei
milking ( P U ) (Table 2). Others (18, 21) noted a positive correlation between average milk production and average serum PRL concentration after milking. However, Hart (18) noticed that in the goat the release of large quantities of PRL at milking does not necessarily parallel high milk production. Peters et al. (26) reported an increased milking-induced PRL release in cattle exposed to a long photoperiod, with a concomitant improved mills yield. In this experiment, milk production apparently increased with increased postmilking PRL concentrations up to a certain yield, above
TABLE 2. E f f e c t of infusion of placeh (CTaL),prolaain at 2 mg/n (Prua), and prolactin at 4 mg/h (PRL4) on milk production and composition. Treatments
MiIk yield, g/d Fat, 96 Fat yield, @d
sm,% Protei4 % Protein yield, g/d
CTRL
PRL2
PRLA
SED1
241ga 2.60 65.3 7.93 2.78 66.9
2483b 2.37 58.1 7.97 2.78 67.7
2385a 2.46 61.6 7.88 2.77 64.9
37.4 .19 6.9 .ll .04 1.5
".%ems in the same mw with difterent superscripts differ (R.01). 'SED = Standard error of the dmerence. Journal of Dairy Science Vol. 74. No. 1, 1991
112
JACQUEMET AND PRIGGE
TABLE 3. Effect of infusion of placebo (m), prolacth at 2 m a (PRL2), and prolactin at 4 mg/h (PRLA) on plasma metabolites and hormonc concentrations.
Glucose, mg/dl Free fkHy acids, CCeqlL
Insolin, PUrn Glucagon, P W Molar insulin:glucagon
48.4 270.6 24.7 142.1 4.7 317.2'
52.6 280.7 24.1 142.4 45
575.1b htaain, ng/ml q b * ' in~ tbe same row with merent superscripts differ (~c.01). 'SED = Standard error of the difference.
43.9 259.7 23.6 1402 4.6 807.P
5.1 22.0 4.9 9.9 1.5 782
which milk yield stabilized or slightly ment (20) indicate that neither enhanced postdecreased. In over 12 different situations, the milking nor increased basal PRL afiFect the biological action of PRL has been shown to regulation of glucose homeostasis in lactating follow a "bell-shaped" dose response curve; the goats, in agreement with other studies of nunieffect is detectable at low or physiological con- nant animals (14, 22, 27). Although PRL increases plasma nonestercentrations but either disappears or is reversed at higher concentrations (19). Regulation of ified fatty concentrations in nonruminant PRL receptors is complex, and an optimal in- animals (8, 32), our results confirm other crease in PRL concentrations at milking may studies (22, 27) in which PRL was reported induce an optimal cellular response. Although ineffective in altering FFA concentrations in milk fat percent and yield were numerically ruminant animals. Williams et al. (31) reported lower for PRL2 compared with CI'RL or an increased FFA concentration after PRL inPRZA, no significant difference was observed jection in cows; however, the response was (F5.10) (Table 2). Similarly, SNF percentage, dependent on the pH of the PRL solution inprotein percentage, and yield were not influ- jected. Increased postmilking PRL concentrations enced by PRL infusion (h.10). had no effect on plasma pancreatic hormones (h.10) (Table 3). Similarly, other studies in Plasma Metabolite and which basal PRL concentrations have been alHormone Concentrations tered in nuninant animals reported no change in Mean plasma metabolite and hormone con- plasma insulin and glucagon concentrations centrations are presented in Table 3. Prolactin (14, 22, 27). concentrations were increased by PRL infusion The observed increased milk yield when 2 (R.01) (Table 3). After milking, plasma PRL mg/h of PRL was infused after milking cannot concentrations averaged 600 ng/ml for CTRL be explained by a change in partitioning of and increased by 100%for PRL2 and 180% for nutrients. Although our study did not measure PRL4 (Figure 1). Such high values after milk- blood flow, PRL may have increased blood ing for PRIA were considered unphysiological. flow to the mammary gland Decreased cardiac Values from 8 to lo00 n g / d after milking have output by removal of suckling stimulus obbeen reported for normal goats (17, 18); this served in lactating rats was prevented by PRL huge vahtion can be attributed to the effects of injection (26), and mammary gland flow of season or stage of lactation. cows has been shown to increase during milkPlasma glucose concentrations were numeri- ing (11). The slightly higher glucose concentracally higher for PRL;! compared with PRL4 tions coupled with an elevated blood flow may and CTRL, but no significant difference was provide the mammary gland with an increased observed (f5.10) (Table 3). Although glucose glucose supply, thus enhancing milk producmetabolism is altered by PRL in laboratory tion. As has been reprted by others, response animals (29), this study and a previous experi- to PRL was not linear. However, although Journal of Dairy Science Vol. 74, No. 1, 1991
MILKING-INDUCED PROLACl"
2,100
I
I
F-l
r-7
I
r-l
MF 7
MF
I
I
7:00
13:OO
19:00
7
E .
113
MF Q
7
0 ';. .
1,800
ui I-
1,500
a
E
1,200
8
8900
0 '
1
6:30
TIME, h Figure 1. Effect of infusion of placebo (*), prolactin at 2 mgh (q, or prolactin at 4 @ (A) on plasma prolactin concentrations over time. I = Infusion; M = * , F = feeding.
milking-induced PRL release may participate in mammary metabolism during lactation, the physiological role of PRL, although still unclear, seems to be marginal, at least for the infusions used in this study. Postmilking r e lease of PRL does not appear to represent a major regulatory process of milk production or nutrient partitioning during an established lactation in ruminant animals selected for high milk yield. REFERENCES 1 Agius, L., A. M.Robinson, I. R G h d , and D. H. Williamson 1979. Alterations in the rate of lipogenesis in vivo in maternal liver and adipose h e on premature weaning of lactatiug rats. A possible regulatory role of prolactin. Biochean. J. 180689. 2Allen. 0. B., J. H. Burton, and I. D. Holt 1983. Analysis of repeated measurements from animal experiments using polynomial regression. J. Anim. Sci. 57765. 3 Aono, R., T. Shioji, T. Shda, and K. Kurachi. 1977. Thc initiation of human lactation and prolactin response to suckling. J. Cliu Endocrinol. Metab. 44: 1101.
4Association of Official Analytical Chemists. 1984. Official methods of analysis. 19th ed. As=. Offic. Anal. them., washington$Dc. 5Baaman. D. E., and J. M. Elliot 1983. Control of nutrient partitioning in lactating ruminant Page 437 in Biochemistry of lactation. T. B. Mepham, ed. Elsevier Biomedical Press, Elsevier, North Holland. 6 Beck, J. C., A. Gonda, M. A. Hamid, R. 0.firgen, D. Rubinstein, and E. E.McGarry. 1964. Some metabolic changes induced by primate growth homnone and purified ovine prolactin Metabolism 13:1108. 7B& N.F.G., H. A. Tucker, and W. D. Oxendef. 1979. M;rmmary arterial and venous concentrabionsof prolactin in lactatiug cows after milking or administration of thyrotr~~phin-~leasing hormone or ergocrypthe. Endocrinology 104:111. 8 Berle, P., E.Finsterwalder, and M. Apostolakis. 1974. Comparative studies on the effect of hmnan growth hormonej human prolactin and human placental lactogen on lipid metabolism. Horm. Metab. Res. 6347. gBrinklow, B. R., and J. M. Forbes. 1983. Prolactin infusion causes increasednitrogenretention in lambs in continuous darkness. Roc. NUB. SOC. 4238A. lOCowie, A. T., I. A. Po& and I. C. Hart. 1980. Lactation. Page 146 in Hormonal control of lactation. Monographs on docrinology. Vol. 15. Natl. Inst Res. Dairy-@. Univ. Reading, Engl.
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11 Davis, S. R., and R. J. Collier. 1985. k L ? y blood flow and regulation of substrate supply for milk synW s . J. Dairy Sci. 6&1041. ~ 12 De Bodo, R C., and N.A~WCUICI.1958. h d i hypcrd t i v i I y and physiological insalin antagonists. physiol. Rev. 38389. D. C. 1966. The balance trial and its limita13tions. Page 51 in Recent advances in animal nutrition. J. T. Abnuns, ed. Little, Brown and Company, Boston, MA. 14 Faichoey, G. J., and T.N. Barry. 1986. Weds of mild hcat exposare and suppression of prolactin secretion on gastmintestinal tract function and temperature regulation in sheep. Ausst J. Biol. Sci. 3985. 15 Gill, J. L.,and H.D. Hafs. 1971. Analysis of repeated measllranents of animals. J. Anim. Sci. 33:331. 16HanWeu. A., and J. L. Linzell. 1972. Elevation of the cardiac output in the rat by prolactin and growth hormone. J. Endocriwl. 53:lvii. 17 Hart. I. C. 1973. Effect O f ~ - B I O ~ O C IonJ J milk yield and the level of prolactin and p w t h hormone in the blood of the goat at milking. J. Endodnol. 57179. s prolactin in serial 18 Hart, I. C. 1975. C o ~ f m t i o n of blood samples from goats before, during and after milking thmughout lactation J. Jkdocrinol. 64:305. 19 H m b i n , D. F. 1979. Cellular basis of prolactin actiom involvement of cyclic WleOtida, p~lyamines,pro~tagladins, steroids, thymid hormones, NaK ATPase and calcium: relevance to breast m c e r and the mensbnal cycle. Med. Hypotheses 5599. POJacquemet, N., and E. C. Prigge. 1990. Effect of prolactin infusion on lactation, glucose kinetics. and pancreatic hormortes in lactating goats. J. Dairy Sci. 735433. 21 Koprowski, J. A., and H. A. Tucker. 1973. Senrm prolactin during various physiological states and its relationship to milk production in the bovine. Endocrinology 92:1480. 22 Manns, J. G., and J. M. Boda. 1%5. Effect of ovine growth hormone and prolactin on blood glucose, serum
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O
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insulin, plasma nonesteaifid fatty acids and amino nitrogen in sheep. EQdocnno ' logy 761109. 23 Nagasawa, H., and R. Yanai. 1976. Mammary nucleic acids and pituitary prolaain secretion during prolonged lactation in mice. J. Endocrinol. 70389. 24 Ngongoni, N. T.,J. J. Robinson, RHB. Kay, R.GA. Stephenson, T. Atkinson, I. Grant, and G. Henderson. 1987. The &ect of altering the hormone status of ewes on the outflow rate of protem supplements from the rumen and so on protein degradability. Auim. Rod. 44: 395. 25 Peel, C. J., I. B. Robinson, and A. A. McGowan 1979. Effects of multiple suby dairy heifem for short periods before or after calving on subsequent milk yields. Aud J. Exp. Agric. 19535. 26Peters, R R, L. T. Chapin, R. S. JZmery, and H. A. Tucker. 1981. Millr yield, feed intake, prolactin, growth hormone and glucocorticoid response of cows light. J. Dairy Sci. W1671. ~ ~to ~supplemented W 27Plaut, K., D. E. Baaman, N. Agergaard, and R. M. A k a . 1987. Effect of exogenous prolactin on lactational pexformance of dairy cows. Domest Anim. EndoQinol. 4279. 28Poole, D. A. 1982. The effects of milking cows three times daily. Auim. Rod.34197. 29Rathgeb, I., B. Winkler, R. Steele, and N. Altszuler. 1971. IBect of ovine prolactin administration on glucose metabolism and plasma insolin levels in the dog. Endocrioology 88:718. 3OSAS@ User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst, W., Ciuy, NC. 31 Williams. W. P., A. G. Weisshaar, and G. E. Lauterback 1966. Lactogenic hormone effects on plasma nonesterified fatly acids and blood glucose concenlrations. J. Dairy Sci. 49106. 32WinLle1, B.,I. Rathgeb, R. Steele, and N. Altszuler. 1971. Effect of ovine prolactin administration on free fatty acid metabolism in the normal dog. Endocrinology 88:1349.
ABSTRACT
Prolactin (PRL) is a lactation-promoting hormone enabling the mammary cells to differentiate fully at parturition and initiating the onset of copious milk secretion (10). Although depression of PRL concentrations during an established lactation in nomuminant animals results in an immediate decline in milk production (10). the role of PRL postpartum remains uncextain in ruminant animals. Numerous studies (7, 17,27) have failed to note a change in lactational performance of milk-producing ruminant animals with alterations in PRL concentrations. Although the involvement of basal PlU during an established lactation is questionable, indirect evidence suggests that milking- or suckling-induced PRL release may participate in maintenance of milk secretion. In all species, milking or suckling induces release of PRL (7), and this release declines as lactation advances (7). Milk yields are positively correlated with s e m P l U concentrations immediately postmilking in cows (21) and women (3). When suckling-induced PRL release was Stimulated, milk secretion could be maintained up to 2 mo in mice (23). Similarly, heifers milked and suckled in early lactation showed an increased milk yield as compared to heifers milked only Abbreviation key: CTRL = control, PRL = (25). Increasing milking frequency from two to prolactin, PRL2 = prolactin at 2 mg/h, PRL4 = three times daily also enhances milk yield (28). These results suggest that the milking- or suckprolactin at 4 m a . ling-induced PRL release may be of physiological importance. In 1983, Bauman and Elliot (5) proposed that PRL may play a part in homeorhetic Received February 20, 1990. control of metabolism during lactation. AdminAccepted July 18, 1990. istration of PRL elicits increases in FFA conhblished with the approval of the director of Ihe West centrations in humans (8), dogs (32), and cows Virgiaia Agricultural a d Forestry Experiroent Slation as (31). Furthermore, a switch in lipogenesis from Scientific Papa Number 2210. "me authors Wish to acknowledge the National Insti- the mammary gland to the liver and adipose tute of Arthritis, Diabetes, and Digestive and Kidney Dis- tissue is observed when suckling-induced or eases (NIADDK) for gift of the purified prolactin and basal PRL releases are inhibited in lactating thanlr E. Townsend for statistical help. rats (1). Prolactin has also been shown to in%'o whom reprint requests should be sent.
Six lactating dairy goats were utilized in a replicated 3 x 3 Latin square design to examine the effect of increased postmilking prolactin concentrations on lactational performance. Thirty minutes after each milking, either placebo or pituitaryderived ovine prolactin at 2 mg/h or 4 mg/h was infused via jugular cannula for 2 h. Goats were housed in a controlled environment at 19'C and a cycle of 16 h light and 8 h dark. Mean plasma prolactin concentrations after millring were increased 100 and 180% for 2 and 4 mg/h prolactin infusion, respectively. Milk yield was slightly but significantly increased by 2 mg/h prolactin infusion compared with placebo and 4 mgh prolactin infusion. However, milk composition and yields of milk components were not influenced by treatments. No differences in dry matter intake, digestibility of the diet, or N retention were observed. Circulating concentrations of glucose, nonesterified fatty acids, insulin, and glucagon were not altered. (Key words: prolactin, lactation, goats)
1991 J Dairy Sci 74:10%114
109
110
JACQUEMET AND PRIGGE
crease N retention in humans (6) and sheep (9) and to affect glucose metabolism in nonrcuninant animals (12). The purpose of this experiment was to determine if elevation of postmilking PRL concentrations affects milk production and composition, N retention, and the plasma concentrations of glucose, FFA and pancreatic hormones in lactating dairy goats. MATERIALS AND METHODS
Six lactating goats of either Toggenburg or Saanen b d weighing an average of 49.5 kg were used in a replicated 3 x 3 Latin square design. The animals were milked three times daily at 0700 h, 1300 h. and 1900 h. Thirty minutes after each milking, either a placebo [conaol (CraL)] or prolactin at 2 mg/h (PRL.2) or 4 mg/h (PRL4) was infused via an indwelling left jugular catheter (Intracath 16 ga 12 in needle 14 ga, Deseret Inc., Sandy, UT) at a rate of .16 Wmin for 2 h using a constant infusion pump (Gilson Medical Electronics, Middleton, Wl). The purpose of the PRL infusion was to prolong and intensify the natural phenomenon of milking-induced PRL release. Infusion solutions were prepared as described by Jacquemet and Prigge (20). The amount of PRL infused was calculated to increase PRL concentrations to a maximum of 400 and 800 n g / d Each experimental or infusion period lasted 6 d A 1 5 d preliminary period was utilized, during which feed intake and milk yield were monitored. The mom temperature was maintained at lYC, and the lighting schedule was 16 h light and 8 h dark. A 50% alfalfa hay and 50% concentrate diet was fed ad libitum. The composition and analysis of the ration were as described by Jacquemet and Prigge (20). Sampling Technlques and Chemical Analysts
During the experimental period, milk samples were collected three times daily, fecal samples twice daily, and feed and refusal samples once daily, and prepared and analyzed as described by Jacquemet and Prigge (20). Urine was collected twice daily, and an aliquot was preserved with HzSO4 and stored at 4'C until N was analyzed by the Kjeldahl procedure (4). On d 5 of infusion, right jugular cannulas J
d of Dairy Scirnce Vol. 74, No. 1, 1991
were inserted for blood sampling. On the last day of each experimental period. 15-ml blood samples were taken every 20 min, stamng 10 min before the am. milking, for 16 h and every hour thereafter for 8 h. Three milliliters of blood were immediately transferred into chilled test tubes to be assayed for glucagon as described by Jacquemet and Prigge (20). An additional 3 ml of blood were discharged into chilled test tubes containing .1 ml of sodium citrate for FFA analysis. Plasma FFA wnmtrations were determined by enzymatic analysis (Walk0 Chemicals, Dallas, Tx). The remainder of the blood was transferred to heparinized test tubes for glucose, insulin, and PRL assays. Plasma glucose and hormone concentrations were determined as described by Jacquemet and Prigge (20). Interassay and intraassay coefficients of variation were 12 and 7%, 11 and 6%, and 12 and 8% for insulin, glucagon, and PRL., respectively. Statlstlcal Analyses
The experiment was analyzed as a replicated 3 x 3 Latin square design (30). The mean of each experimental unit was determined for each variable describing milk output, intake, digestibility, and N metabolism, and an ANOVA was conducted. Glucose, FFA, and hormone profiles were analyzed as a split-plot over time within a replicated Latin square. A conservative F test was used for the effects of time and treatment x time interaction (15). Polynomial regressions over time weze also fitted for each experimental unit (2), and an ANOVA was performed for each set of regression coefficients obtained. RESULTS AND DISCUSSION Dietary Intake, Nitrogen Metabolism, and Milk Production
The influence of treatments on means of intake and digestibility of DM, CP, and ADF, and N metabolism are presented in Table 1. Increased postmilking PRL concentrations did not affect intake or digestibility (-.lo), which is in agreement with other studies in which basal PRL concentrations were altered (14, 20, 24). Based on N retention data, the average amount of N retained per day should be equivalent to 38 g of tissue protein However, N
111
MILIUNG-LNDUCED PROLACI"
TABLE 1. Effect of infusion of placebo (CTRL),prolactin at 2 @ (pRL2), and prolactin at 4 mg/h (PRL4) on mean intake and apparent digestibility of DM, CP, and ADP,and on N metabolism~~
Intakes DM, g/d Mi,
MWd
CP, g/d ADP, gld
CTRL
Treatments F%u2
2308 4.9 328 533
2399 5.1 338 536
67.7 65.2 58.7
PRLA
SED'
2284
4.9 324 514
73 .1 10 24
682 65.0 572
67.5 64.2 58.9
1.5 1.8 2.4
52.4 34.4 18.1 18.3 10.5 5.6
543 35.3 18.9 18.4 10.6 6.3
51.8 33.1 18.1 17.0 6.5
1.7 1.8 .7 3.7 .3 3.5
10.0
11.3
9.2
7.3
Digestibilities
DM, % CP, % ADF, % N Metabolism, g/d N Intake
Digestible N Fecal N Urinruy N MiIkN Retained N
10.2
Retained N, Percentage N intake.
'SED =
stan-
error of ttre difference.
balance studies often overestimate N retention (13). Limitation of validity of N balance studies aside, N retention was not affected by PRL infusion. Similarly, when basal PRL concentrations were increased in sheep via PRL injection (22), no change in N retention was observed. However, Brinklow and Forbes (9) observed an increased N retention in sheep infused with PFZL. Mean milk yield was slightly but significantly i n ~ ~ (k.05) e d When PRL W ~ Sinfused at 2 mg/h after milking (PRL2) as compared to control or PRL infused at 4 mg/h aftei
milking ( P U ) (Table 2). Others (18, 21) noted a positive correlation between average milk production and average serum PRL concentration after milking. However, Hart (18) noticed that in the goat the release of large quantities of PRL at milking does not necessarily parallel high milk production. Peters et al. (26) reported an increased milking-induced PRL release in cattle exposed to a long photoperiod, with a concomitant improved mills yield. In this experiment, milk production apparently increased with increased postmilking PRL concentrations up to a certain yield, above
TABLE 2. E f f e c t of infusion of placeh (CTaL),prolaain at 2 mg/n (Prua), and prolactin at 4 mg/h (PRL4) on milk production and composition. Treatments
MiIk yield, g/d Fat, 96 Fat yield, @d
sm,% Protei4 % Protein yield, g/d
CTRL
PRL2
PRLA
SED1
241ga 2.60 65.3 7.93 2.78 66.9
2483b 2.37 58.1 7.97 2.78 67.7
2385a 2.46 61.6 7.88 2.77 64.9
37.4 .19 6.9 .ll .04 1.5
".%ems in the same mw with difterent superscripts differ (R.01). 'SED = Standard error of the dmerence. Journal of Dairy Science Vol. 74. No. 1, 1991
112
JACQUEMET AND PRIGGE
TABLE 3. Effect of infusion of placebo (m), prolacth at 2 m a (PRL2), and prolactin at 4 mg/h (PRLA) on plasma metabolites and hormonc concentrations.
Glucose, mg/dl Free fkHy acids, CCeqlL
Insolin, PUrn Glucagon, P W Molar insulin:glucagon
48.4 270.6 24.7 142.1 4.7 317.2'
52.6 280.7 24.1 142.4 45
575.1b htaain, ng/ml q b * ' in~ tbe same row with merent superscripts differ (~c.01). 'SED = Standard error of the difference.
43.9 259.7 23.6 1402 4.6 807.P
5.1 22.0 4.9 9.9 1.5 782
which milk yield stabilized or slightly ment (20) indicate that neither enhanced postdecreased. In over 12 different situations, the milking nor increased basal PRL afiFect the biological action of PRL has been shown to regulation of glucose homeostasis in lactating follow a "bell-shaped" dose response curve; the goats, in agreement with other studies of nunieffect is detectable at low or physiological con- nant animals (14, 22, 27). Although PRL increases plasma nonestercentrations but either disappears or is reversed at higher concentrations (19). Regulation of ified fatty concentrations in nonruminant PRL receptors is complex, and an optimal in- animals (8, 32), our results confirm other crease in PRL concentrations at milking may studies (22, 27) in which PRL was reported induce an optimal cellular response. Although ineffective in altering FFA concentrations in milk fat percent and yield were numerically ruminant animals. Williams et al. (31) reported lower for PRL2 compared with CI'RL or an increased FFA concentration after PRL inPRZA, no significant difference was observed jection in cows; however, the response was (F5.10) (Table 2). Similarly, SNF percentage, dependent on the pH of the PRL solution inprotein percentage, and yield were not influ- jected. Increased postmilking PRL concentrations enced by PRL infusion (h.10). had no effect on plasma pancreatic hormones (h.10) (Table 3). Similarly, other studies in Plasma Metabolite and which basal PRL concentrations have been alHormone Concentrations tered in nuninant animals reported no change in Mean plasma metabolite and hormone con- plasma insulin and glucagon concentrations centrations are presented in Table 3. Prolactin (14, 22, 27). concentrations were increased by PRL infusion The observed increased milk yield when 2 (R.01) (Table 3). After milking, plasma PRL mg/h of PRL was infused after milking cannot concentrations averaged 600 ng/ml for CTRL be explained by a change in partitioning of and increased by 100%for PRL2 and 180% for nutrients. Although our study did not measure PRL4 (Figure 1). Such high values after milk- blood flow, PRL may have increased blood ing for PRIA were considered unphysiological. flow to the mammary gland Decreased cardiac Values from 8 to lo00 n g / d after milking have output by removal of suckling stimulus obbeen reported for normal goats (17, 18); this served in lactating rats was prevented by PRL huge vahtion can be attributed to the effects of injection (26), and mammary gland flow of season or stage of lactation. cows has been shown to increase during milkPlasma glucose concentrations were numeri- ing (11). The slightly higher glucose concentracally higher for PRL;! compared with PRL4 tions coupled with an elevated blood flow may and CTRL, but no significant difference was provide the mammary gland with an increased observed (f5.10) (Table 3). Although glucose glucose supply, thus enhancing milk producmetabolism is altered by PRL in laboratory tion. As has been reprted by others, response animals (29), this study and a previous experi- to PRL was not linear. However, although Journal of Dairy Science Vol. 74, No. 1, 1991
MILKING-INDUCED PROLACl"
2,100
I
I
F-l
r-7
I
r-l
MF 7
MF
I
I
7:00
13:OO
19:00
7
E .
113
MF Q
7
0 ';. .
1,800
ui I-
1,500
a
E
1,200
8
8900
0 '
1
6:30
TIME, h Figure 1. Effect of infusion of placebo (*), prolactin at 2 mgh (q, or prolactin at 4 @ (A) on plasma prolactin concentrations over time. I = Infusion; M = * , F = feeding.
milking-induced PRL release may participate in mammary metabolism during lactation, the physiological role of PRL, although still unclear, seems to be marginal, at least for the infusions used in this study. Postmilking r e lease of PRL does not appear to represent a major regulatory process of milk production or nutrient partitioning during an established lactation in ruminant animals selected for high milk yield. REFERENCES 1 Agius, L., A. M.Robinson, I. R G h d , and D. H. Williamson 1979. Alterations in the rate of lipogenesis in vivo in maternal liver and adipose h e on premature weaning of lactatiug rats. A possible regulatory role of prolactin. Biochean. J. 180689. 2Allen. 0. B., J. H. Burton, and I. D. Holt 1983. Analysis of repeated measurements from animal experiments using polynomial regression. J. Anim. Sci. 57765. 3 Aono, R., T. Shioji, T. Shda, and K. Kurachi. 1977. Thc initiation of human lactation and prolactin response to suckling. J. Cliu Endocrinol. Metab. 44: 1101.
4Association of Official Analytical Chemists. 1984. Official methods of analysis. 19th ed. As=. Offic. Anal. them., washington$Dc. 5Baaman. D. E., and J. M. Elliot 1983. Control of nutrient partitioning in lactating ruminant Page 437 in Biochemistry of lactation. T. B. Mepham, ed. Elsevier Biomedical Press, Elsevier, North Holland. 6 Beck, J. C., A. Gonda, M. A. Hamid, R. 0.firgen, D. Rubinstein, and E. E.McGarry. 1964. Some metabolic changes induced by primate growth homnone and purified ovine prolactin Metabolism 13:1108. 7B& N.F.G., H. A. Tucker, and W. D. Oxendef. 1979. M;rmmary arterial and venous concentrabionsof prolactin in lactatiug cows after milking or administration of thyrotr~~phin-~leasing hormone or ergocrypthe. Endocrinology 104:111. 8 Berle, P., E.Finsterwalder, and M. Apostolakis. 1974. Comparative studies on the effect of hmnan growth hormonej human prolactin and human placental lactogen on lipid metabolism. Horm. Metab. Res. 6347. gBrinklow, B. R., and J. M. Forbes. 1983. Prolactin infusion causes increasednitrogenretention in lambs in continuous darkness. Roc. NUB. SOC. 4238A. lOCowie, A. T., I. A. Po& and I. C. Hart. 1980. Lactation. Page 146 in Hormonal control of lactation. Monographs on docrinology. Vol. 15. Natl. Inst Res. Dairy-@. Univ. Reading, Engl.
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11 Davis, S. R., and R. J. Collier. 1985. k L ? y blood flow and regulation of substrate supply for milk synW s . J. Dairy Sci. 6&1041. ~ 12 De Bodo, R C., and N.A~WCUICI.1958. h d i hypcrd t i v i I y and physiological insalin antagonists. physiol. Rev. 38389. D. C. 1966. The balance trial and its limita13tions. Page 51 in Recent advances in animal nutrition. J. T. Abnuns, ed. Little, Brown and Company, Boston, MA. 14 Faichoey, G. J., and T.N. Barry. 1986. Weds of mild hcat exposare and suppression of prolactin secretion on gastmintestinal tract function and temperature regulation in sheep. Ausst J. Biol. Sci. 3985. 15 Gill, J. L.,and H.D. Hafs. 1971. Analysis of repeated measllranents of animals. J. Anim. Sci. 33:331. 16HanWeu. A., and J. L. Linzell. 1972. Elevation of the cardiac output in the rat by prolactin and growth hormone. J. Endocriwl. 53:lvii. 17 Hart. I. C. 1973. Effect O f ~ - B I O ~ O C IonJ J milk yield and the level of prolactin and p w t h hormone in the blood of the goat at milking. J. Endodnol. 57179. s prolactin in serial 18 Hart, I. C. 1975. C o ~ f m t i o n of blood samples from goats before, during and after milking thmughout lactation J. Jkdocrinol. 64:305. 19 H m b i n , D. F. 1979. Cellular basis of prolactin actiom involvement of cyclic WleOtida, p~lyamines,pro~tagladins, steroids, thymid hormones, NaK ATPase and calcium: relevance to breast m c e r and the mensbnal cycle. Med. Hypotheses 5599. POJacquemet, N., and E. C. Prigge. 1990. Effect of prolactin infusion on lactation, glucose kinetics. and pancreatic hormortes in lactating goats. J. Dairy Sci. 735433. 21 Koprowski, J. A., and H. A. Tucker. 1973. Senrm prolactin during various physiological states and its relationship to milk production in the bovine. Endocrinology 92:1480. 22 Manns, J. G., and J. M. Boda. 1%5. Effect of ovine growth hormone and prolactin on blood glucose, serum
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insulin, plasma nonesteaifid fatty acids and amino nitrogen in sheep. EQdocnno ' logy 761109. 23 Nagasawa, H., and R. Yanai. 1976. Mammary nucleic acids and pituitary prolaain secretion during prolonged lactation in mice. J. Endocrinol. 70389. 24 Ngongoni, N. T.,J. J. Robinson, RHB. Kay, R.GA. Stephenson, T. Atkinson, I. Grant, and G. Henderson. 1987. The &ect of altering the hormone status of ewes on the outflow rate of protem supplements from the rumen and so on protein degradability. Auim. Rod. 44: 395. 25 Peel, C. J., I. B. Robinson, and A. A. McGowan 1979. Effects of multiple suby dairy heifem for short periods before or after calving on subsequent milk yields. Aud J. Exp. Agric. 19535. 26Peters, R R, L. T. Chapin, R. S. JZmery, and H. A. Tucker. 1981. Millr yield, feed intake, prolactin, growth hormone and glucocorticoid response of cows light. J. Dairy Sci. W1671. ~ ~to ~supplemented W 27Plaut, K., D. E. Baaman, N. Agergaard, and R. M. A k a . 1987. Effect of exogenous prolactin on lactational pexformance of dairy cows. Domest Anim. EndoQinol. 4279. 28Poole, D. A. 1982. The effects of milking cows three times daily. Auim. Rod.34197. 29Rathgeb, I., B. Winkler, R. Steele, and N. Altszuler. 1971. IBect of ovine prolactin administration on glucose metabolism and plasma insolin levels in the dog. Endocrioology 88:718. 3OSAS@ User's Guide: Statistics, Version 5 Edition. 1985. SAS Inst, W., Ciuy, NC. 31 Williams. W. P., A. G. Weisshaar, and G. E. Lauterback 1966. Lactogenic hormone effects on plasma nonesterified fatly acids and blood glucose concenlrations. J. Dairy Sci. 49106. 32WinLle1, B.,I. Rathgeb, R. Steele, and N. Altszuler. 1971. Effect of ovine prolactin administration on free fatty acid metabolism in the normal dog. Endocrinology 88:1349.
