Camp. Biochem. Physioi. Vol. 103A, No.
0300-9629/92 $5.00 + 0.00
1, Pp. 169-171,1992
Printed in Great Britain
0 1992Pergamon Press Ltd
COMPARATIVE NUTRITION PAPERS MINI REVIEW AGE INFLUENCES
ON AMINO ACID INTESTINAL TRANSPORT M. P. VINARDELL
Departament de Ciencies Fisioiogiques Humanes i de la Nutricio, Facultat de Farmacia, Joan XXIII s/n, 08028 Barcelona, Spain. Tel.: 93-490-78-49 Fax: 93-490-82-74 (Recehed
26 February 1992)
Abstract-l.
intestinal absorption of amino acids show a decrease with aging in different animal species such as avians, rodents and ruminants. 2. The different intestinal segments show different absorptive capacities, the jejunum being the most absorbent. 3. Chickens show the biggest capacity for amino acid absorption close to hatching. 4. In rodents the third week seems to be the period of increased transport capacity.
GENERAL CHARACTERKSTICS OF INTESTINAL AMINO ACID ABSORPTION
The absorption of amino acids by the intestine can occur passively through a paracellular or a cellular route and actively through cellular pathways. Analy sis of amino acid transport is complicated by the presence of multipie transporters with overlapping specificity. At least 12 different classes of transporters have been described for non epithelial cells in vertebrates (Christen~n 1984, 1985): (a) five Na+-dependent systems for neutral amino acids; A (alanine), ASC (alanine, serine, cysteine), Gly, N (~Iutamine), b-system; (b) three Na+-independent system also for neutral amino acids; L (leucine), T (tryptophan), asc (alanine, serine, cysteine); (c) one system for basic amino acids (Y + ); (d) three systems for acidic amino acids: X& (glutamic acid, aspartic acid), Xi (aspartic acid), X, (glutamic acid). Studies have been made to design unusual amino acids that are carried by a single transporter and therefore can be used to test for the presence of the characteristic transporter in other tissues or species. acid membrane Amino
trunsport
across
the
brush
horder
Na+ -Dependent amirro acid transport. Nat-Dependent transport was first shown for alanine in vesicles isolated from rat intestine. Similar to glucose transport, Na+-dependent neutral amino acid transport is electrogenic in many species, including humans (Stevens et al., 1982, 1984) who have similar amino acid fluxes and properties for several neutral amino acid transporters. The brush border membrane contains at least four different Na+-dependent transport systems for neutral amino acids. 1. Most neutral amino acids are substrates for the neutral brush border (NBB) system. 2. PhenylaIanine and methionine are also transported by another Na’-dependent
system, termed PHE. 3. The transport capacity for imino acids is very high because of the IMINO system, which also accepts a-methyl-aminoisobutyric acid, although it excludes alanine and other shortchain amino acids. 4. Munck (1985) has characterized a Na+-dependent amino acid transport system in the intact rabbit distal ileum. Na+-Independent amino acid transport. Na+-lndependent facilitated diffusion of amino acids can be demonstrated in the brush border membrane in the absence of Na+. It is a minor pathway for amino acid absorption. Amino acid transport acrass the basoiateral plasma membrane
The models for transepithelial amino acid absorption require exit pathways in the basolateral membrane. The L-system appears to provide the major route. It has a broad specificity for neutral amino acids including cysteine and glutamine (Stevens et al., 1984; Hopfer et al., 1976). AGE DIFFERENCES
The amino acid transport capacity of the intestine seems to show differences with the age of animals similar to other substrates such as sulfate (Batt, I969), or monosaccharides (Vinardell, l990). There are three different periods in the life of the animal, according to their transport capacity. The first period is before and after birth showing differences in developing amino acid transport. The second corresponds to the adult animal with a decrease in absorption compared to youngest animals. The third are the old and very old animals when the transport capacity seems to fall dramatically. The different intestinal segments also show differences with age and can be studied by taking into account the intestinal region. 169
170
M.P. VINARDELL
Yolk sac region The yolk and yolk sac correspond to the protoplasm that serves as nourishment for the growing embryo and assumes the function of the intestine during the fetal period. The developmental pattern of tissue accumulation of amino acids by the yolk sac is apparently different in different species of animals. A neutral amino acid pump, while absent in the yolk sac of the rat, is active in that of the rabbit, guinea pig and chicken. Toward the end of gestation of the rabbit when the neutral pump is at its highest level the active transport of glycine, proline and lysine cannot be detected. The amino acids accumulated by this system in the rabbit were: valine, isoleucine, phenylalanine, alanine and methionine (all of the t-configuration). In the chicken, glycine accumulation was noted at the earliest period studied (9 days of incubation). The yolk sac of the chicken possesses transport systems for amino acids. Significant amounts of free amino acids appear in the yolk sac during the first few days of incubation and their concentration increases during the first 7 days (Tausing, 1965; Williams, 1954). During the second week their concentration in the yolk falls. Absorption of amino acids from the yolk must exceed hydrolysis during this period. Tausing (1965) showed that (2-“C]glycine injected into the yolk of eggs from the 5th to 11th days of incubation is incorporated into the proteins of the developing embryo. Holdworth and Wilson (1967) indicated that an amino acid transport system is present throughout most of embryonic life, decreasing in activity at the time of hatching. The decrease in glycine accumulation at the time of hatching is associated with a change in aflinity. Electron micrographs show that microvilii are present in a regular pattern as early as 10 days before hatching. The guinea pig yolk sac possessed the capacity for active accumulation of glycine, proline, lysine and valine, the transport of valine being particularly weak. As in the chicken, the transport activity declined to low levels just before the end of the term (Butt and Wilson, 1968). Small intestine Absorption of t-Tyr, L-Phe and L-Trp was significantiy reduced in jejunal rings of 24-month-old rats. Moreover a trend toward reduced tissue uptake was also present in the 12-month-old rats compared with 6-month-old rats, suggesting that diminished uptake may be a continuous process with aging (Navad and Winter, 1988). Penzes and Boross (1974~) using an in uiro technique in female rats, reported that K, values of t_-Trp and t-Phe were lowest in younger animals (6 months) compared with adult (12 months) and old (27 months) rats. In other in uivo studies by the same authors a lower affinity for the transporter was reported for L-Gly, L-Ala and L-Lys in older rats compared with adult animals. No consistent trend was found in these in vivo observations. Winter rr al. (1971) measured blood radioactivity for 60min after intragastric administration of radiolabeled cysteine and reported enhanced absorption at 5 min
in 18-month-old rats compared with 2-month-old animals. There is a similarity in tryptophan absorption by young and old rats. But the phenylalanine and proline intestinal affinity measured by the Michaelis-Menten kinetics, should be considered as an age-dependent phenomenon that decreases as the organism ages (Penzes and Boross, 1974~). In previous papers Penzes (1970, 1974a, 1974b) reported that there are no real differences in the intestinal absorption of dibasic amino acids (lysine and arginine) as the rat becomes older. Young and adult rats showed the same characteristics as old rats. fi-Alanine transport in the day-old chick was much more pronounced in the ileum than in the jejunum. These results may suggest that either a greater abundance of carriers exists in the ileum or that a qualitatively different mechanism is present (Lerner et al., 1976). A decrease in leucine transport by duodenum, jejunum and ileum has been found in chicken from 1 week to 3 weeks (Gonzalez and Vinardell, 1992). In contrast to the results of Lerner et al. (1976), the jejunum has the biggest absorptive capacity in chickens from 1 day to 15 weeks. Absorption of free amino acids from Be-Lys is highest at birth in the pig and in the rabbit (Smith, 1983). Large intestine Week-old chicks transport methionine in the large bowel by a process characterized by low apparent affinity (K,) and relatively high capacity (V,,,,,); 2-year-old chickens, on the other hand, appear to transport methionine in the colon by a separate process which has a high affinity and a lower capacity (Lerner et al., 197.5). Accumulation of L-proline followed the same pattern in the small intestine at 3-4 days as at 31 days, although accumulation by the colon was observed only in younger rats and mice (Batt and Schachter, 1969). Evidence for accumulation of t_-valine by the adult rat colon has been reported (Evered, 1968). In newborn chicks the colon and cecum can accumulate glycine in Ditro which disappears within a few days (Holdsworth and Wilson, 1967). UISCUSSION
The third week seems to be a critical period in the development of intestinal function in rodents. At this time sulfate (Batt, 1969) and hexose {Calingaert and Zorzoli, 1965) transport in mice increases, the surface area of mucosal cells is enhanced by development of microvilli (Overton, 19651, and levels of alkaline phosphatase in the duodenum reach maximal values (Moog, 1950, 1962). Among the possible mechanisms which might trigger the changes at the third week is the action of adrenal steroids. The adrenal cortex of the mouse grows most rapidly from 14 to 17 days, with possible increased secretion in the second week (Moog et ul., 1954). The reason for the decrease in transport observed with age could be attributed to a decrease in microvilli number or the number of enterocytes (Batt, 1969; Holdsworth, 1967). Alternatively a decrease in the number of carriers in the brush border may also be responsibIe (Scharrer er al., 1979).
Age inguences on amino acid intestinal transport
The transport of amino acids against a concentration gradient could be supported by energy derived from glycolysis just before birth in the rabbit (Wilson and Lin, 1960). Whereas in the guinea pig, Rosenberg (1966) has demonstrated the development of the transport system of lysine which parallels the development of Na’,K+-ATPase of the intestine. The loss of the capacity of the caecum for active transport of glycine is almost certainly due to the loss of the columnar absorptive cells which occurs in the organ at this time, the vi& becoming more extended and the proportion of goblet cehs increasing. The observations reported in some studies may be interpreted in different ways. One explanation is that the aging process results in a decrease in affinity for the transport process by the amino acids. Estimates of apparent K, are affected by the thickness of the unstirred layer and the diffusion coefficient of the substrate (Thomson, 1980). Thus, a reduction in height of the unstirred water layer in older animals could result in an underestimation of the apparent K,. Information about the effect of aging on the unstirred water layer thickness is conflicting. Although aging was found to increase the thickness of the unstirred layer in the rabbit jejunum (Thomson, 1979) no agerelated changes in unstirred water layer have been found in the rat intestine (Hoilander and Dadufalza, 1983). REFERENCES Batt E. R. (1969) Sulfate accumulation by mouse intestine: the influence of age and other factors. Am. J. Physiof 217, 1101-I104. Batt E. R. and Schachter S. (1969) Developmental pattern of some intestinal transport mechanism in newborn rats and mice. Am. J. Ph~~~o/. 216, 1~~1068. Butt H. and Wilson T. H. (1968) Development of sugar and amino acid transport by intestine and yolk sac of the guinea pig, Am. J. Physiol. 215, 14681477. Calineaert A. and Zorzoli A. (1965) . , The influence of age on 6-deoxy-D-glucose accumulation by mouse intestine. J. Geronr. 20, 21 l-214. Christensen H. N. (1984) Organic ion transport during seven decades: the amino acids. Biochim. biophys. Acta 779, 255-269. Christensen H. N. (1985) On the strategy of kinetic discrimination of amino acid transport systems. J. ~~rn~~~~~ Biof. 84, 97-103. Evered D. F. and Nunn P. B. (1968) Uptake of amino acids by mucosa of rat colon in vitro. Eur. J. Biochem. 4, 301-304. Gonzalez G. and Vinardell M. P. (1992) Age-dependent changes on 3-oxy-methyl-n-glucose and leucine intestinal absorption in chickens. Poult. Sci. (in press). Holdsworth C. D. and Wilson T. H. (1967) Development of active sugar and amino acid transport in the yolk sac and intestine of chicken. Am. J. Physiol. 212, 233-240. Hollander D. and Dadufalza V. D. (1983) Aging: its influence on the intestinal unstirred water layer thickness, surface area and resistance in the unanesthetized rat. Can. 1. Phy~jof. Pharmac. 61, 1501-1508. Hopfer U., Sigrist-Nelson I(., Ammann E. and Murer H. (1976) Differences on neutral amino acid and glucose transport between brush-border and basolateral plasma membrane of intestinal epithelial cells. J. Cell Physiof. 89, 805-S IO. Lerner J., Sattekneyer P. and Rush R. (1975) Kinetics of methionine influx into various regions of chicken intestine. Comp. Biochem. Physiol. SOA, 113-120.
171
Lerner .I., Burrill P. H., Sattelmeyer P. A. and Anicki C. F. (1976) Developmental patterns of intestinal transport mechanisms in the chick. Comp. Biochem. Physiof. !%A, 109%III. Moog F. (1950) The functional differentiation of the small intestine I. The accumulation of alkaline phosphomonoesterase in the duodenum of the chick. J. exp. 2001. 115, 109-126. Moog F., Bennett C. J. and Dean C. M. (1954) Growth and cytochemistry of the adrenal gland of the mouse from birth to maturity. Anat. Rec. 120, 873-891. Moog F. (1962) Developmental adaptation of alkaline phosphatases in the small intestine. Fed. Proc. Fedn Am. Sots exp. Biof. 21, 51-56. Munck B. G. (1985) Transport of neutral and cationic amino acid across the brush-border membrane of rabbit ileum. J. Membrane Biol. 83, 15-24. Navad F. and Winter C. G. (1988) Effect of aging on intestinal absorption of aromatic amino acids in vitro in the rat. Am. J. Physfol. 254, G63M636. Overton J. (1965) Fine structure of the free cell surface in developing mouse intestinal mucosa. J. exp. Zool. 159, 195-201. Penzes L. (1970) intestinal transfer of L-arginine in relation to age. Exp. Geront. 5, 193-201. Penzes L. (1974a) Further data on the age-dependent intestinal absorption of dibasic amino acid. Exp. Gerunf. 9, 259-262. Penzes L. (1974b) Intestinal absorption of glycine, L-alanine, L-leucine in the old rat. Exp. Geront. 9, 245-252. Penzes L. and Boross M. (1974~) Intestinal absorption of some heterocyclic and aromatic amino acids from the aging gut. Exp. Geront. 9, 253-258. Rosemberg I. H. (1966) Development of fetal guinea pig small intestine: amino acid transport and sodiumpotassium-dependent ATPase (Na+-K+-ATPase). Fed. Proc. 25, 456. Scharrer E,, Liebich H. G., Rabb W. and Prombarger N. (1979) Influence of age and rumen development on intestinal absorption of galactose and glucose in lambs. A functional and mo~hological study. Zentbf. Vet. Med. A. 26, 955105. Smith M. W. (1983) Amino acid and peptide transport across the mammalian small intestine. In: Wrh Inr. Symp, Protein Metabolism and Nutrition, Ed. INRA Publications, I. pp. 21 l--232. Stevens B. R., Rosas H. J. and Wright E. M. (1982) Multiple transport pathways for neutral amino acids in rabbit jejunal brush border vesicles. J. Membrane Bfol. 66, 2 13-225. Stevens B. R., Kaunitz J. D. and Wright E. M. (1984) Intestinal transport of amino acid and sugars. Advances using membrane vesicles. A. Rev. Physfof. 46, 417433. Tausing M. P. (1965) incorporation of amino acids into chick oroteins during embrvonic growth. Can. J. Biochem. Physiol. 43, 1099~lilo.. Thomson A. B. R. (1979) Unstirred layer and age-dependent changes in rabbit jejunal D-glucose transport. Am. J. Physiol. 236, E685-E69 I. Thomson A. B. R. and Dietschy J. (1980) Intestinal kinetic parameters: effects of unstirred layers and transport preparation. Am. J. Fhysioi. 239, G372-G377. Vinardell M. P. (1990) Age differences on ileai glucose absorption in rat. Me&. Ag. Dew. 51, 243-247. Williams M. A., Da Costa W. A., Newman L. H. and Marshall L. M. (1954) Free amino acids in the yolk during the development of the chick. Nature 173, 490. Wilson T. H. and Lin E. C. C. (1960) Active transport by intestines of fetal and newborn rabbits. Am. J. Physioi. 199, 1030-1032. Winter D., Dobre V. and Oenu S. (1971) Cystein-S3’ absorption in old rats. Exp. Geront. 6, 367-371.
0300-9629/92 $5.00 + 0.00
1, Pp. 169-171,1992
Printed in Great Britain
0 1992Pergamon Press Ltd
COMPARATIVE NUTRITION PAPERS MINI REVIEW AGE INFLUENCES
ON AMINO ACID INTESTINAL TRANSPORT M. P. VINARDELL
Departament de Ciencies Fisioiogiques Humanes i de la Nutricio, Facultat de Farmacia, Joan XXIII s/n, 08028 Barcelona, Spain. Tel.: 93-490-78-49 Fax: 93-490-82-74 (Recehed
26 February 1992)
Abstract-l.
intestinal absorption of amino acids show a decrease with aging in different animal species such as avians, rodents and ruminants. 2. The different intestinal segments show different absorptive capacities, the jejunum being the most absorbent. 3. Chickens show the biggest capacity for amino acid absorption close to hatching. 4. In rodents the third week seems to be the period of increased transport capacity.
GENERAL CHARACTERKSTICS OF INTESTINAL AMINO ACID ABSORPTION
The absorption of amino acids by the intestine can occur passively through a paracellular or a cellular route and actively through cellular pathways. Analy sis of amino acid transport is complicated by the presence of multipie transporters with overlapping specificity. At least 12 different classes of transporters have been described for non epithelial cells in vertebrates (Christen~n 1984, 1985): (a) five Na+-dependent systems for neutral amino acids; A (alanine), ASC (alanine, serine, cysteine), Gly, N (~Iutamine), b-system; (b) three Na+-independent system also for neutral amino acids; L (leucine), T (tryptophan), asc (alanine, serine, cysteine); (c) one system for basic amino acids (Y + ); (d) three systems for acidic amino acids: X& (glutamic acid, aspartic acid), Xi (aspartic acid), X, (glutamic acid). Studies have been made to design unusual amino acids that are carried by a single transporter and therefore can be used to test for the presence of the characteristic transporter in other tissues or species. acid membrane Amino
trunsport
across
the
brush
horder
Na+ -Dependent amirro acid transport. Nat-Dependent transport was first shown for alanine in vesicles isolated from rat intestine. Similar to glucose transport, Na+-dependent neutral amino acid transport is electrogenic in many species, including humans (Stevens et al., 1982, 1984) who have similar amino acid fluxes and properties for several neutral amino acid transporters. The brush border membrane contains at least four different Na+-dependent transport systems for neutral amino acids. 1. Most neutral amino acids are substrates for the neutral brush border (NBB) system. 2. PhenylaIanine and methionine are also transported by another Na’-dependent
system, termed PHE. 3. The transport capacity for imino acids is very high because of the IMINO system, which also accepts a-methyl-aminoisobutyric acid, although it excludes alanine and other shortchain amino acids. 4. Munck (1985) has characterized a Na+-dependent amino acid transport system in the intact rabbit distal ileum. Na+-Independent amino acid transport. Na+-lndependent facilitated diffusion of amino acids can be demonstrated in the brush border membrane in the absence of Na+. It is a minor pathway for amino acid absorption. Amino acid transport acrass the basoiateral plasma membrane
The models for transepithelial amino acid absorption require exit pathways in the basolateral membrane. The L-system appears to provide the major route. It has a broad specificity for neutral amino acids including cysteine and glutamine (Stevens et al., 1984; Hopfer et al., 1976). AGE DIFFERENCES
The amino acid transport capacity of the intestine seems to show differences with the age of animals similar to other substrates such as sulfate (Batt, I969), or monosaccharides (Vinardell, l990). There are three different periods in the life of the animal, according to their transport capacity. The first period is before and after birth showing differences in developing amino acid transport. The second corresponds to the adult animal with a decrease in absorption compared to youngest animals. The third are the old and very old animals when the transport capacity seems to fall dramatically. The different intestinal segments also show differences with age and can be studied by taking into account the intestinal region. 169
170
M.P. VINARDELL
Yolk sac region The yolk and yolk sac correspond to the protoplasm that serves as nourishment for the growing embryo and assumes the function of the intestine during the fetal period. The developmental pattern of tissue accumulation of amino acids by the yolk sac is apparently different in different species of animals. A neutral amino acid pump, while absent in the yolk sac of the rat, is active in that of the rabbit, guinea pig and chicken. Toward the end of gestation of the rabbit when the neutral pump is at its highest level the active transport of glycine, proline and lysine cannot be detected. The amino acids accumulated by this system in the rabbit were: valine, isoleucine, phenylalanine, alanine and methionine (all of the t-configuration). In the chicken, glycine accumulation was noted at the earliest period studied (9 days of incubation). The yolk sac of the chicken possesses transport systems for amino acids. Significant amounts of free amino acids appear in the yolk sac during the first few days of incubation and their concentration increases during the first 7 days (Tausing, 1965; Williams, 1954). During the second week their concentration in the yolk falls. Absorption of amino acids from the yolk must exceed hydrolysis during this period. Tausing (1965) showed that (2-“C]glycine injected into the yolk of eggs from the 5th to 11th days of incubation is incorporated into the proteins of the developing embryo. Holdworth and Wilson (1967) indicated that an amino acid transport system is present throughout most of embryonic life, decreasing in activity at the time of hatching. The decrease in glycine accumulation at the time of hatching is associated with a change in aflinity. Electron micrographs show that microvilii are present in a regular pattern as early as 10 days before hatching. The guinea pig yolk sac possessed the capacity for active accumulation of glycine, proline, lysine and valine, the transport of valine being particularly weak. As in the chicken, the transport activity declined to low levels just before the end of the term (Butt and Wilson, 1968). Small intestine Absorption of t-Tyr, L-Phe and L-Trp was significantiy reduced in jejunal rings of 24-month-old rats. Moreover a trend toward reduced tissue uptake was also present in the 12-month-old rats compared with 6-month-old rats, suggesting that diminished uptake may be a continuous process with aging (Navad and Winter, 1988). Penzes and Boross (1974~) using an in uiro technique in female rats, reported that K, values of t_-Trp and t-Phe were lowest in younger animals (6 months) compared with adult (12 months) and old (27 months) rats. In other in uivo studies by the same authors a lower affinity for the transporter was reported for L-Gly, L-Ala and L-Lys in older rats compared with adult animals. No consistent trend was found in these in vivo observations. Winter rr al. (1971) measured blood radioactivity for 60min after intragastric administration of radiolabeled cysteine and reported enhanced absorption at 5 min
in 18-month-old rats compared with 2-month-old animals. There is a similarity in tryptophan absorption by young and old rats. But the phenylalanine and proline intestinal affinity measured by the Michaelis-Menten kinetics, should be considered as an age-dependent phenomenon that decreases as the organism ages (Penzes and Boross, 1974~). In previous papers Penzes (1970, 1974a, 1974b) reported that there are no real differences in the intestinal absorption of dibasic amino acids (lysine and arginine) as the rat becomes older. Young and adult rats showed the same characteristics as old rats. fi-Alanine transport in the day-old chick was much more pronounced in the ileum than in the jejunum. These results may suggest that either a greater abundance of carriers exists in the ileum or that a qualitatively different mechanism is present (Lerner et al., 1976). A decrease in leucine transport by duodenum, jejunum and ileum has been found in chicken from 1 week to 3 weeks (Gonzalez and Vinardell, 1992). In contrast to the results of Lerner et al. (1976), the jejunum has the biggest absorptive capacity in chickens from 1 day to 15 weeks. Absorption of free amino acids from Be-Lys is highest at birth in the pig and in the rabbit (Smith, 1983). Large intestine Week-old chicks transport methionine in the large bowel by a process characterized by low apparent affinity (K,) and relatively high capacity (V,,,,,); 2-year-old chickens, on the other hand, appear to transport methionine in the colon by a separate process which has a high affinity and a lower capacity (Lerner et al., 197.5). Accumulation of L-proline followed the same pattern in the small intestine at 3-4 days as at 31 days, although accumulation by the colon was observed only in younger rats and mice (Batt and Schachter, 1969). Evidence for accumulation of t_-valine by the adult rat colon has been reported (Evered, 1968). In newborn chicks the colon and cecum can accumulate glycine in Ditro which disappears within a few days (Holdsworth and Wilson, 1967). UISCUSSION
The third week seems to be a critical period in the development of intestinal function in rodents. At this time sulfate (Batt, 1969) and hexose {Calingaert and Zorzoli, 1965) transport in mice increases, the surface area of mucosal cells is enhanced by development of microvilli (Overton, 19651, and levels of alkaline phosphatase in the duodenum reach maximal values (Moog, 1950, 1962). Among the possible mechanisms which might trigger the changes at the third week is the action of adrenal steroids. The adrenal cortex of the mouse grows most rapidly from 14 to 17 days, with possible increased secretion in the second week (Moog et ul., 1954). The reason for the decrease in transport observed with age could be attributed to a decrease in microvilli number or the number of enterocytes (Batt, 1969; Holdsworth, 1967). Alternatively a decrease in the number of carriers in the brush border may also be responsibIe (Scharrer er al., 1979).
Age inguences on amino acid intestinal transport
The transport of amino acids against a concentration gradient could be supported by energy derived from glycolysis just before birth in the rabbit (Wilson and Lin, 1960). Whereas in the guinea pig, Rosenberg (1966) has demonstrated the development of the transport system of lysine which parallels the development of Na’,K+-ATPase of the intestine. The loss of the capacity of the caecum for active transport of glycine is almost certainly due to the loss of the columnar absorptive cells which occurs in the organ at this time, the vi& becoming more extended and the proportion of goblet cehs increasing. The observations reported in some studies may be interpreted in different ways. One explanation is that the aging process results in a decrease in affinity for the transport process by the amino acids. Estimates of apparent K, are affected by the thickness of the unstirred layer and the diffusion coefficient of the substrate (Thomson, 1980). Thus, a reduction in height of the unstirred water layer in older animals could result in an underestimation of the apparent K,. Information about the effect of aging on the unstirred water layer thickness is conflicting. Although aging was found to increase the thickness of the unstirred layer in the rabbit jejunum (Thomson, 1979) no agerelated changes in unstirred water layer have been found in the rat intestine (Hoilander and Dadufalza, 1983). REFERENCES Batt E. R. (1969) Sulfate accumulation by mouse intestine: the influence of age and other factors. Am. J. Physiof 217, 1101-I104. Batt E. R. and Schachter S. (1969) Developmental pattern of some intestinal transport mechanism in newborn rats and mice. Am. J. Ph~~~o/. 216, 1~~1068. Butt H. and Wilson T. H. (1968) Development of sugar and amino acid transport by intestine and yolk sac of the guinea pig, Am. J. Physiol. 215, 14681477. Calineaert A. and Zorzoli A. (1965) . , The influence of age on 6-deoxy-D-glucose accumulation by mouse intestine. J. Geronr. 20, 21 l-214. Christensen H. N. (1984) Organic ion transport during seven decades: the amino acids. Biochim. biophys. Acta 779, 255-269. Christensen H. N. (1985) On the strategy of kinetic discrimination of amino acid transport systems. J. ~~rn~~~~~ Biof. 84, 97-103. Evered D. F. and Nunn P. B. (1968) Uptake of amino acids by mucosa of rat colon in vitro. Eur. J. Biochem. 4, 301-304. Gonzalez G. and Vinardell M. P. (1992) Age-dependent changes on 3-oxy-methyl-n-glucose and leucine intestinal absorption in chickens. Poult. Sci. (in press). Holdsworth C. D. and Wilson T. H. (1967) Development of active sugar and amino acid transport in the yolk sac and intestine of chicken. Am. J. Physiol. 212, 233-240. Hollander D. and Dadufalza V. D. (1983) Aging: its influence on the intestinal unstirred water layer thickness, surface area and resistance in the unanesthetized rat. Can. 1. Phy~jof. Pharmac. 61, 1501-1508. Hopfer U., Sigrist-Nelson I(., Ammann E. and Murer H. (1976) Differences on neutral amino acid and glucose transport between brush-border and basolateral plasma membrane of intestinal epithelial cells. J. Cell Physiof. 89, 805-S IO. Lerner J., Sattekneyer P. and Rush R. (1975) Kinetics of methionine influx into various regions of chicken intestine. Comp. Biochem. Physiol. SOA, 113-120.
171
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