Prostaglandins Leukotrienes and Essential IQ Longman Group UK Ltd 1991
Fatty Acids
(1991) 42. 197-200
Effect of Dietary Calcium on Renal Prostaglandins S. Katayama+, Y. Maruno, A. Itabashi, M. Inaba, S. Akabane, K. Tanaka, K. Tanaka, M. Shibuya, S. Kawazu, J. Ishii, R. Kusuhara*, T. Wakabayashi* and Y. Kagawa* The Fourth Department of Medicine, Saitama Medical School, 38 Morohongo, Moroyamacho, Irumagun, Saituma 350-04, Japan and *Kagawa Nutrition College, Tokyo, Japan (Reprint requests to X) ABSTRACT.
The present study was designed to clarify the possible role of renal prostaghtndms (PGs) on blood pressure (BP) regulation during calcium (Ca) restriction or supplementation. Twelve normotensive women with a mean age of 21.2 years participated in the study. After 1 week of normal Ca intake (mean +_ SE, 536 rt 2 mg/day), a low-Ca diet (163 + 1 mg/day) was given for a further 1 week. Additional asparagine Ca (3 g as Ca/day) was also given to half of the subjects. BP, heart rate, and serum total and ionized Ca concentrations were measured at the end of each period. Levels of Ca, sodium, PGE2, 6-ketoPGFt, and thromboxane (TX) B2 excreted into urine were also determined. The plasma level of ionized Ca was significantly increased without any change in total Ca in both groups. Low and high Ca intake decreased and increased urinary Ca excretion by 28% and 56%, respectively. BP was not altered after Ca deprivation or loading. However, urinary PGE2 excretion was significantly augmented from 668.9 +_ 68.1 to 959.7 f 183.1 ngday by Ca loading, whereas Ca deprivation decreased PGEz excretion (695.4 + 1Ofi.l to 513.2 + 55.2 ngday). No changes were observed in 6-keto-PGFt, or TXBz urinary excretion. These results suggest that renal PGEz. synthesis is stimulated or decreased by l-week Ca loading or deprivation, indicating a possible antihypertensive role of renal PGE2 during high-Ca intake in hypertensives.
pressure regulation. In addition, no data are available about the effect of dietary calcium .on renal prostaglandins (PGs), some of which have a profound vasodilative and natriuretic action. Therefore, the present study was undertaken to clarify the effects of dietary calcium on blood pressure as well as urinary PG excretion in normotensive women.
INTRODUCTION Low dietary calcium has been considered the best nutrient predictor of high blood pressure in the United States (l), although reanalyses of the same data using more appropriate weighting factors for age failed to support this hypothesis (2, 3). Oral calcium supplementation has been reported to decrease blood pressure in patients with essential hypertension (4, 5), and a large’increase in calcium intake has been found to reduce blood pressure in hypertensive rats (6, 7). However, in other studies where calcium supplements have been given, no changes in blood pressure have been observed in either normotensives or hypertensives (8-12). Recent studies have demonstrated that intracellular free calcium in platelets, which may affect vascular smooth muscle cell contraction and hence arteriolar resistance, is elevated in hypertensives in comparison with normotensive controls (13). Thus, there are conflicting, and at times irreconcilable, data concerning the role of calcium in blood
SUBJECTS AND METHODS Twelve healthy female volunteers aged 21.2 It 0.4 (mean + SE) years with a mean body weight of 49.0 + 1.7 kg participated in the study, having given their informed consent. For the entire length of the study, they were allowed to live in the metabolic house at Kagawa Women’s Nutritional College with continuing their usual daily activities. During the baseline run-in period, the normal calcium diet was given. After 1 week normal calcium diet, they were randomized into two groups, i.e. low calcium group and high calcium group. A low calcium diet containing approximately 200 mg of calcium/day was given all of them during another 1 week. High calcium intake was achieved by prescription of asparagine
Date received 9 September 1990 Date accepted 20 November 1990 197
198
Proataghmdina Leukotrienes
and Essential Fatty Acids
Table 1 Mean daily energy and nutrient intake during the control diet and experimental
Energy (kcal) Protein (g) Lipid (g) PUFA (n) ._.
;;FA k)
Carbohydrate (g) Fiber (g) Calcium (mg) Phosphorus (mg) Sodium (mg) Potassium (mg) Magnesium (mg)
Baseline
Low Ca
159.5 + 2 71.4 + 0.4 55.3 + 0.4 12.1 + 0 8.5 + 0.1 1.4 + 0 201.1 _+0.1 3.0 + 0 538 + 2 942 I!I 1 2229 + 3 2495 +- 10 211 + 1
1459 80.5 31.9 13.2 6.7 2.0 209.4 3.5 163 754 2791 2460 192
+ f + + + f + f + + + + +
Baseline 26** 0.3** 0.3** 0** 0** o** 6.3* 0.1** I** 4** 0** 39*” 4**
1577 71.1 55.3 12.1 8.6 1.4 197.8 3.0 534 942 2215 2470 209
It + * + + + + + + + + + +
diet.
High Ca 28 0.4 0.4 0 0.1 0 4.5 0 3 15 17 31 3
1478 80.8 32.0 13.2 6.7 2.0 213.8 3.5 3163 756 2791 2465 193
+ + + + + f + f + + + + f
0.4** 0.3** O.l** 0** O** o** 0.2** o** l** l** l** 17 1**
Values are means + SD (n = 6). PUFA: polyunsaturated fatty acids, SAFA: saturated fatty acids, P/S: ratio of PUFA to SAFA. **: p < 0.01 vs. controls, *: p < 0.05 vs. controls.
Table 2 Effects of calcium restriction or loading on serum levels of total and ionized calcium, urine volume, and urinary excretion of calcium, phosphorus, sodium, potassium and magnesium. Calcium intake
Normal
Low
Normal
Serum calcium total (mg/dl) ionized (mmol/l)
9.4 I!Z0.14 1.12 4 0.02
9.3 f 0.02 1.21 + o.oz**
9.3 ?I 0.15 1.10 + 0.02
9.4 + 0.08 1.15 f 0.01**.++
1113 + 73
887 + 36**
963 + 74
970 rt 73
56.3 206 39.1 24.3 39.0
40.7 188 44.7 19.7 28.2
66.1 229 45.2 29.0 40.4
Urine volume (ml/day) Urinary excretion (mg/day) Calcium Phosphorus Sodium Potassium Magnesium
+ + + + +
7.8 25 6.1 2.5 5.3
+ + + f f
3.6* 26 6.2 2.3 5.3
+ + f + +
High
4.7 21 4.5 3.2 4.5
103.0 146 48.3 24.7 38.6
f k f + +
6.7**‘++ 14** 7.6 1.7 4.9
Each value is the mean + SE (n = 6) at the end of each period. **. p < 0.01 vs. controls, *: p < 0.05 vs. controls. +‘: p < 0.01 between groups.
calcium (3 g as calcium/day). They were asked to drink only distilled water instead of tap water. Each meal was prepared by a well trained dietitian and kept frozen until analysis of energy and content of protein, carbohydrate and fat as well as calcium, sodium, potassium and magnesium. Blood pressure and pulse rate were measured 3 times at 07:OO at the recumbent position followed by 5 min sitting at the final day during each period using the same random zero sphygmomanometer. Diastolic blood pressure was recorded at the disappearance of Korotoff sound V. Two consecutive 24 h urinary samples were collected with 10 ml of 1 N HCl at the end of normal calcium diet and, during and at the end of low- or high-calcium intake (3rd and 4th, 6th and 7th day) to determine 24 h sodium, potassium, calcium, phosphate, magnesium and creatinine excretion. A venous blood sample was obtained to determine serum total and ionized calcium concentration at the end of each period. Calcium was measured by atomic absorption spectrophotometry. Serum ionized calcium was determined by using a calcium-selective electrode.
Sodium and potassium were measured by flame photometry. Urinary PGE2, 6-keto;PGF,, and thromboxane (TX) B2 were determined by specific radioimmunoassay following purification with a SEP-PAK Cts cartridge, the details of which have been reported previously (14, 15). The average of all measurements was used for subsequent analysis and all data were expressed as the mean + SE, except where otherwise specified. For statistical analysis, Student’s t-test was used. When p was less than 0.05, the data were considered to be significantly different from each other.
RESULTS The mean intake of energy and nutrients is summarized in Table 1. During the low-calcium diet, the women received 163 f 1 (SD) mg calcium (range 140-205 mg) daily. To reduce the intake of calcium, the intake of energy, lipid and phosphorus was less than that in the control diet, whereas carbohydrate and protein intake was increased during the low-
Dietary Calcium on Renal Prostaglandins
199
Table 3 Effects of calcium restriction or loading on body weight, blood pressure and heart rate.
High
Calcium intake
Normal
LOW
Normal
Body weight (kg) Mean blood pressure (mmHg) recumbent sitting Heart rate (beats/min) recumbent sitting
48.8 + 2.5
49.0 + 2.4
49.2 + 2.5
49.0 + 2.6
76.4 + 4.5 69.6 + 4.3
74.8 + 2.9 70.0 + 3.1
72.3 + 3.8 64.2 + 2.5
70.7 + 4.1 73.1 + 3.4
53.7 + 4.0 64.7 + 3.5
59.0 + 4.2 69.0 z!z3.6
61.8 + 3.4 68.7 f 3.8
62.0 f 4.7 69.7 rt 3.0
Each value is the mean + SE (n = 6) at the end of each period.
-HIGH
Ca INTAKE
-LOW
Ca INTAKE
M-Y
wlldw
6.KETO.PGF,a I
I
TmKmeOXAME 81
600 t
interest was the finding that urinary PGE2 excretion decreased significantly 3 or 4 days after initiation of the low-calcium diet and tended to remain low even after 6 or 7 days (Fig.). In contrast, urinary PGE2 excretion was augmented by calcium loading, although the difference from individual basal values was not significant. However, PGE2 excretion in the high-calcium group was significantly higher than that in the low-calcium group. Urinary excretion of 6-keto-PGF,, and TXB2 showed no changes after dietary calcium restriction or loading.
DISCUSSION
Fig. Urinary excretion of PGE,, 6-keto-PGF,, and thromboxane Bz before, during (i.e. Day 3 and 4) and at the end of calcium restriction (clear circles) or loading (solid circles). Each value is the mean f SE (n = 6). **. p < 0.05 vs. individual controls, +: p < 0.05 between high-calcium group and low-calcium group.
calcium diet. The P/S ratio in the experimental diet was higher than that in the control diet. In addition,
the intake of minerals such as sodium and magnesium was higher than during the control period. The high-calcium group received almost the same dietary contents, except that their calcium intake amounted to 3163 + 1 (3140-3205) mg/day. As shown in Table 2, low calcium intake significantly decreased the urine volume. Urinary excretion of calcium decreased, but not significantly, without any changes in phosphorus excretion. On the other hand, calcium loading significantly increased the urinary excretion of calcium and decreased phosphorus excretion. Urinary excretion of sodium, potassium or magnesium was not altered by changes in dietary calcium intake. Manipulation of dietary calcium content did not affect the serum calcium level. However, the serum level of ionized calcium significantly increased after either calcium loading or restriction. As shown in Table 3, blood pressure and heart rate in either a recumbent or sitting position was not changed by calcium loading or restriction. Of
The present study demonstrated that high calcium intake augmented urinary excretion of PGE*, whereas low calcium intake decreased urinary PGEz excretion. However, the urinary excretion of other prostanoids such as 6-keto-PGF,, and TXB2 was not affected during calcium deprivation or loading. Urinary excretion of PGEz or PGF,, has been reported to reflect renal PG synthesis to a great extent (16). In addition, we have demonstrated that oral administration of aspirin at 300 mg/day, sufficient to suppress platelet TXA2 synthesis, decreased urinary TXB, excretion by 60%) suggesting that 40% of urinary TXBz may originate from the kidneys (14). Thus. an increase or decrease in urinary PGE;! excretion after dietary calcium supplementation or restriction may reflect renal PGEz synthesis. To our knowledge, this is the first observation in humans that dietary calcium intake affects renal PGE2 synthesis. The calcium intake achieved in the present study was extraordinarily high or low. In fact, the concentration of serum ionized ,calcium was increased not only during calcium loading but also during calcium restriction, indicating that calcium may be mobilized from bones during low calcium intake. Intestinal calcium absorption is reported to be almost zero when oral calcium intake is less than 5 mg/kg/day (17). In contrast, calcium absorption is non-saturable at intakes of more than above 10 mg/kg/day, although the slope of intestinal absorption against oral intake is not steep. Therefore, intestinal calcium absorption at intakes of more
200
Prostaghndins Leukotrienes and Essential Fatty Acids
than 10 mg/kg/day rarely exceeds 5 mg/kg/day. In fact, urinary calcium excretion in the present study was about 100 mg/day, which was almost the same as that in the previous study in which 1400-1600 mg of calcium was given (11. 12). Calcium ion is an important factor in the excitationcontraction coupling of vascular smooth muscle cells. Calcium chloride infusion into the renal, brachial or coronary artery has been reported to result in constriction of these vessels (B-20). Okahara et al (21) demonstrated that intrarenal infusion of calcium chloride at 0.68 mEq/min in dogs produced a 13-fold increase in the renal PGE2 secretion rate associated with a marked increase in renal blood flow followed by a gradual decrease to below the preperfusion level, suggesting that renal PGE? may modify the renal vasoconstriction induced by calcium infusion. It is unlikely that altered renal PGEZ synthesis during a high- or low-calcium diet would be influenced by changes in the content of lipid, especially polyunsaturated fatty acid, since the P/S ratio was increased with the low-calcium diet even though the lipid content was lower. Although blood pressure or sodium excretion was not altered by changes in dietary calcium in the present study, augmented or diminished renal PGE? synthesis during calcium supplementation or restriction may play some physiological role in blood pressure regulation and/or sodium homeostasis. Further study to elucidate the role of renal PGs during calcium supplementation, which has been reported to decrease blood pressure in some investigations, will be needed. References 1. McCarron D A, Morris C D, Henry H J. Stanton 2. 3.
4.
5.
J L. Blood pressure and nutrient in the United States. Science 1984; 224: 1392-1397. Feinleib N. Lenfant C, Miller S A. Hypertension and calcium. Science 1984; 226: 384-386. Semoos C. Coooer R. Kovar M G. Johnson C. Drizh T, Yetely E. Dietary calcium and blood’ pressure in National Health and Nutrition Examination Surveys 1 and II. Hypertension 1986; 8: 1067-1074. MacCarron D A. Is calcium more important than sodium in the pathogenesis of essential hvoertension? Hvoertension 1985: 7: 607-627. S;;azzullo P, Sia; A, Guglielmi S, Di Carlo A, Galletti F, Cirillo M, Manicini M. Controlled trial of long-term calcium supplementation in essential hypertension. Hypertension 1986; 8: 1084-1088.
6. Ayachi S. Increased dietary calcium lowers blood pressure in the spontaneously hypertensive rats. Metabolism 1979; 28: 1234-1238. 7. Stern N. Lee D B N, Silis V, Beck F W J, Deftos L, Manolga S C. Sowers J R. Effects of high calcium intake on blood pressure and calcium metabolism in young SHR. Hvoertension 1984: 6: 639-646. .. 8. Belizan J H, Villar J, Pineda 0, Gonzalez A E, Saintz E. Garrera G. Sibrian R. Reduction of blood pressure with calcium supplementation in young adults. JAMA 1983; 249: 1161-1165. 9. Johnson N E, Smith E L, Freudenheim J L. Effects on blood pressure of calcium in persons with mild to moderate hypertension. Ann Intern Med 1985; 103: 825-831. -. 10. Cappuccio F P, Markandu N D, Beymon G W, Shore A C. MacGregor G A. Effect of increasinev calcium intake on urynary sodium excretion in normotensive subjects. Clin Sci 1986; 71: 453-456. 11. Cappuccio F P, Mankandu N D, Singer D R J, Smith S J, Shore A C, MacGregor G A. Does oral calcium supplementation lower high blood pressure? A double blind study. J Hypert 1987; 5: 67-71. 12. Siani A, Strazzullo P, Guglielmi S, Pacioni D, Giacco A, Lacone R, Manicini M. Controlled trial of low calcium versus high calcium intake in mild hypertension. J Hypert 1988; 6: 253-256. 13. Erne P, Bolli P, Burgisser E, Buhler F R. Correlation of platelet calcium with blood pressure. Effect of antihypertensive therapy. N Engl J Med 1984; 310: 1084-1088. 14. Katayama S. lnaba M, Maruno Y, Omoto A, Kawazu S, Ishii J. Increased thromboxane B, excretion in diabetes mellitus. J Lab Clin Med 1987: 109: 711-717. 15. Omoto A. Katayama S, Inaba M, Maruno Y, Watanabe T, Kawazu S, Ishii J. Effects of thromboxane AZ synthesis inhibition on renal renin release and protein excretion in diabetic rats. Jpn J Nephrol 1988; 30: 67-73. 16. Frolich J C, Wilson T W, Sweetmas B J, Simgel M, Nies A S. Carr K, Watson T, Oates J A. Urinary prostaglandins. Identification and origin. J Clin Invest 1975; 55: 763-770. 17 Wilkinson R. Absorption of calcium, phosphorus and magnesium. In Calcium, Phosphorus and Magnesium Metabolism (Nordis BEC. Ed.), New York, Churchill Livingstone, 1975. 18 Watkins B E, Davis J 0, Lohmeier T E, Freeman R H. Intrarenal site of action of calcium on renin secretion in dogs. Cir Res 1976: 39: 847-853. 19. Frolich E D. Scott J B, Haddy F J. Effect of cations on resistance and responsiveness of renal and forelimb vascular beds. Am J Phvsiol 1962: 203: 583-587. 20. Scott J B, Frolich E D, Hardin R A, Haddy F J. Na+, K’, Ca ‘+ and Mg++ action on coronary vascular resistance in the dog heart. Am J Physiol 1961; 1095-1100. 21. Okahara T, Abe Y, Imanishi M, Yukimura T, Yamamoto K. Effect of calcium on prostaglandin Ez release in dogs. Am J Physiol 1981; 241: F77-F84.
Fatty Acids
(1991) 42. 197-200
Effect of Dietary Calcium on Renal Prostaglandins S. Katayama+, Y. Maruno, A. Itabashi, M. Inaba, S. Akabane, K. Tanaka, K. Tanaka, M. Shibuya, S. Kawazu, J. Ishii, R. Kusuhara*, T. Wakabayashi* and Y. Kagawa* The Fourth Department of Medicine, Saitama Medical School, 38 Morohongo, Moroyamacho, Irumagun, Saituma 350-04, Japan and *Kagawa Nutrition College, Tokyo, Japan (Reprint requests to X) ABSTRACT.
The present study was designed to clarify the possible role of renal prostaghtndms (PGs) on blood pressure (BP) regulation during calcium (Ca) restriction or supplementation. Twelve normotensive women with a mean age of 21.2 years participated in the study. After 1 week of normal Ca intake (mean +_ SE, 536 rt 2 mg/day), a low-Ca diet (163 + 1 mg/day) was given for a further 1 week. Additional asparagine Ca (3 g as Ca/day) was also given to half of the subjects. BP, heart rate, and serum total and ionized Ca concentrations were measured at the end of each period. Levels of Ca, sodium, PGE2, 6-ketoPGFt, and thromboxane (TX) B2 excreted into urine were also determined. The plasma level of ionized Ca was significantly increased without any change in total Ca in both groups. Low and high Ca intake decreased and increased urinary Ca excretion by 28% and 56%, respectively. BP was not altered after Ca deprivation or loading. However, urinary PGE2 excretion was significantly augmented from 668.9 +_ 68.1 to 959.7 f 183.1 ngday by Ca loading, whereas Ca deprivation decreased PGEz excretion (695.4 + 1Ofi.l to 513.2 + 55.2 ngday). No changes were observed in 6-keto-PGFt, or TXBz urinary excretion. These results suggest that renal PGEz. synthesis is stimulated or decreased by l-week Ca loading or deprivation, indicating a possible antihypertensive role of renal PGE2 during high-Ca intake in hypertensives.
pressure regulation. In addition, no data are available about the effect of dietary calcium .on renal prostaglandins (PGs), some of which have a profound vasodilative and natriuretic action. Therefore, the present study was undertaken to clarify the effects of dietary calcium on blood pressure as well as urinary PG excretion in normotensive women.
INTRODUCTION Low dietary calcium has been considered the best nutrient predictor of high blood pressure in the United States (l), although reanalyses of the same data using more appropriate weighting factors for age failed to support this hypothesis (2, 3). Oral calcium supplementation has been reported to decrease blood pressure in patients with essential hypertension (4, 5), and a large’increase in calcium intake has been found to reduce blood pressure in hypertensive rats (6, 7). However, in other studies where calcium supplements have been given, no changes in blood pressure have been observed in either normotensives or hypertensives (8-12). Recent studies have demonstrated that intracellular free calcium in platelets, which may affect vascular smooth muscle cell contraction and hence arteriolar resistance, is elevated in hypertensives in comparison with normotensive controls (13). Thus, there are conflicting, and at times irreconcilable, data concerning the role of calcium in blood
SUBJECTS AND METHODS Twelve healthy female volunteers aged 21.2 It 0.4 (mean + SE) years with a mean body weight of 49.0 + 1.7 kg participated in the study, having given their informed consent. For the entire length of the study, they were allowed to live in the metabolic house at Kagawa Women’s Nutritional College with continuing their usual daily activities. During the baseline run-in period, the normal calcium diet was given. After 1 week normal calcium diet, they were randomized into two groups, i.e. low calcium group and high calcium group. A low calcium diet containing approximately 200 mg of calcium/day was given all of them during another 1 week. High calcium intake was achieved by prescription of asparagine
Date received 9 September 1990 Date accepted 20 November 1990 197
198
Proataghmdina Leukotrienes
and Essential Fatty Acids
Table 1 Mean daily energy and nutrient intake during the control diet and experimental
Energy (kcal) Protein (g) Lipid (g) PUFA (n) ._.
;;FA k)
Carbohydrate (g) Fiber (g) Calcium (mg) Phosphorus (mg) Sodium (mg) Potassium (mg) Magnesium (mg)
Baseline
Low Ca
159.5 + 2 71.4 + 0.4 55.3 + 0.4 12.1 + 0 8.5 + 0.1 1.4 + 0 201.1 _+0.1 3.0 + 0 538 + 2 942 I!I 1 2229 + 3 2495 +- 10 211 + 1
1459 80.5 31.9 13.2 6.7 2.0 209.4 3.5 163 754 2791 2460 192
+ f + + + f + f + + + + +
Baseline 26** 0.3** 0.3** 0** 0** o** 6.3* 0.1** I** 4** 0** 39*” 4**
1577 71.1 55.3 12.1 8.6 1.4 197.8 3.0 534 942 2215 2470 209
It + * + + + + + + + + + +
diet.
High Ca 28 0.4 0.4 0 0.1 0 4.5 0 3 15 17 31 3
1478 80.8 32.0 13.2 6.7 2.0 213.8 3.5 3163 756 2791 2465 193
+ + + + + f + f + + + + f
0.4** 0.3** O.l** 0** O** o** 0.2** o** l** l** l** 17 1**
Values are means + SD (n = 6). PUFA: polyunsaturated fatty acids, SAFA: saturated fatty acids, P/S: ratio of PUFA to SAFA. **: p < 0.01 vs. controls, *: p < 0.05 vs. controls.
Table 2 Effects of calcium restriction or loading on serum levels of total and ionized calcium, urine volume, and urinary excretion of calcium, phosphorus, sodium, potassium and magnesium. Calcium intake
Normal
Low
Normal
Serum calcium total (mg/dl) ionized (mmol/l)
9.4 I!Z0.14 1.12 4 0.02
9.3 f 0.02 1.21 + o.oz**
9.3 ?I 0.15 1.10 + 0.02
9.4 + 0.08 1.15 f 0.01**.++
1113 + 73
887 + 36**
963 + 74
970 rt 73
56.3 206 39.1 24.3 39.0
40.7 188 44.7 19.7 28.2
66.1 229 45.2 29.0 40.4
Urine volume (ml/day) Urinary excretion (mg/day) Calcium Phosphorus Sodium Potassium Magnesium
+ + + + +
7.8 25 6.1 2.5 5.3
+ + + f f
3.6* 26 6.2 2.3 5.3
+ + f + +
High
4.7 21 4.5 3.2 4.5
103.0 146 48.3 24.7 38.6
f k f + +
6.7**‘++ 14** 7.6 1.7 4.9
Each value is the mean + SE (n = 6) at the end of each period. **. p < 0.01 vs. controls, *: p < 0.05 vs. controls. +‘: p < 0.01 between groups.
calcium (3 g as calcium/day). They were asked to drink only distilled water instead of tap water. Each meal was prepared by a well trained dietitian and kept frozen until analysis of energy and content of protein, carbohydrate and fat as well as calcium, sodium, potassium and magnesium. Blood pressure and pulse rate were measured 3 times at 07:OO at the recumbent position followed by 5 min sitting at the final day during each period using the same random zero sphygmomanometer. Diastolic blood pressure was recorded at the disappearance of Korotoff sound V. Two consecutive 24 h urinary samples were collected with 10 ml of 1 N HCl at the end of normal calcium diet and, during and at the end of low- or high-calcium intake (3rd and 4th, 6th and 7th day) to determine 24 h sodium, potassium, calcium, phosphate, magnesium and creatinine excretion. A venous blood sample was obtained to determine serum total and ionized calcium concentration at the end of each period. Calcium was measured by atomic absorption spectrophotometry. Serum ionized calcium was determined by using a calcium-selective electrode.
Sodium and potassium were measured by flame photometry. Urinary PGE2, 6-keto;PGF,, and thromboxane (TX) B2 were determined by specific radioimmunoassay following purification with a SEP-PAK Cts cartridge, the details of which have been reported previously (14, 15). The average of all measurements was used for subsequent analysis and all data were expressed as the mean + SE, except where otherwise specified. For statistical analysis, Student’s t-test was used. When p was less than 0.05, the data were considered to be significantly different from each other.
RESULTS The mean intake of energy and nutrients is summarized in Table 1. During the low-calcium diet, the women received 163 f 1 (SD) mg calcium (range 140-205 mg) daily. To reduce the intake of calcium, the intake of energy, lipid and phosphorus was less than that in the control diet, whereas carbohydrate and protein intake was increased during the low-
Dietary Calcium on Renal Prostaglandins
199
Table 3 Effects of calcium restriction or loading on body weight, blood pressure and heart rate.
High
Calcium intake
Normal
LOW
Normal
Body weight (kg) Mean blood pressure (mmHg) recumbent sitting Heart rate (beats/min) recumbent sitting
48.8 + 2.5
49.0 + 2.4
49.2 + 2.5
49.0 + 2.6
76.4 + 4.5 69.6 + 4.3
74.8 + 2.9 70.0 + 3.1
72.3 + 3.8 64.2 + 2.5
70.7 + 4.1 73.1 + 3.4
53.7 + 4.0 64.7 + 3.5
59.0 + 4.2 69.0 z!z3.6
61.8 + 3.4 68.7 f 3.8
62.0 f 4.7 69.7 rt 3.0
Each value is the mean + SE (n = 6) at the end of each period.
-HIGH
Ca INTAKE
-LOW
Ca INTAKE
M-Y
wlldw
6.KETO.PGF,a I
I
TmKmeOXAME 81
600 t
interest was the finding that urinary PGE2 excretion decreased significantly 3 or 4 days after initiation of the low-calcium diet and tended to remain low even after 6 or 7 days (Fig.). In contrast, urinary PGE2 excretion was augmented by calcium loading, although the difference from individual basal values was not significant. However, PGE2 excretion in the high-calcium group was significantly higher than that in the low-calcium group. Urinary excretion of 6-keto-PGF,, and TXB2 showed no changes after dietary calcium restriction or loading.
DISCUSSION
Fig. Urinary excretion of PGE,, 6-keto-PGF,, and thromboxane Bz before, during (i.e. Day 3 and 4) and at the end of calcium restriction (clear circles) or loading (solid circles). Each value is the mean f SE (n = 6). **. p < 0.05 vs. individual controls, +: p < 0.05 between high-calcium group and low-calcium group.
calcium diet. The P/S ratio in the experimental diet was higher than that in the control diet. In addition,
the intake of minerals such as sodium and magnesium was higher than during the control period. The high-calcium group received almost the same dietary contents, except that their calcium intake amounted to 3163 + 1 (3140-3205) mg/day. As shown in Table 2, low calcium intake significantly decreased the urine volume. Urinary excretion of calcium decreased, but not significantly, without any changes in phosphorus excretion. On the other hand, calcium loading significantly increased the urinary excretion of calcium and decreased phosphorus excretion. Urinary excretion of sodium, potassium or magnesium was not altered by changes in dietary calcium intake. Manipulation of dietary calcium content did not affect the serum calcium level. However, the serum level of ionized calcium significantly increased after either calcium loading or restriction. As shown in Table 3, blood pressure and heart rate in either a recumbent or sitting position was not changed by calcium loading or restriction. Of
The present study demonstrated that high calcium intake augmented urinary excretion of PGE*, whereas low calcium intake decreased urinary PGEz excretion. However, the urinary excretion of other prostanoids such as 6-keto-PGF,, and TXB2 was not affected during calcium deprivation or loading. Urinary excretion of PGEz or PGF,, has been reported to reflect renal PG synthesis to a great extent (16). In addition, we have demonstrated that oral administration of aspirin at 300 mg/day, sufficient to suppress platelet TXA2 synthesis, decreased urinary TXB, excretion by 60%) suggesting that 40% of urinary TXBz may originate from the kidneys (14). Thus. an increase or decrease in urinary PGE;! excretion after dietary calcium supplementation or restriction may reflect renal PGEz synthesis. To our knowledge, this is the first observation in humans that dietary calcium intake affects renal PGE2 synthesis. The calcium intake achieved in the present study was extraordinarily high or low. In fact, the concentration of serum ionized ,calcium was increased not only during calcium loading but also during calcium restriction, indicating that calcium may be mobilized from bones during low calcium intake. Intestinal calcium absorption is reported to be almost zero when oral calcium intake is less than 5 mg/kg/day (17). In contrast, calcium absorption is non-saturable at intakes of more than above 10 mg/kg/day, although the slope of intestinal absorption against oral intake is not steep. Therefore, intestinal calcium absorption at intakes of more
200
Prostaghndins Leukotrienes and Essential Fatty Acids
than 10 mg/kg/day rarely exceeds 5 mg/kg/day. In fact, urinary calcium excretion in the present study was about 100 mg/day, which was almost the same as that in the previous study in which 1400-1600 mg of calcium was given (11. 12). Calcium ion is an important factor in the excitationcontraction coupling of vascular smooth muscle cells. Calcium chloride infusion into the renal, brachial or coronary artery has been reported to result in constriction of these vessels (B-20). Okahara et al (21) demonstrated that intrarenal infusion of calcium chloride at 0.68 mEq/min in dogs produced a 13-fold increase in the renal PGE2 secretion rate associated with a marked increase in renal blood flow followed by a gradual decrease to below the preperfusion level, suggesting that renal PGE? may modify the renal vasoconstriction induced by calcium infusion. It is unlikely that altered renal PGEZ synthesis during a high- or low-calcium diet would be influenced by changes in the content of lipid, especially polyunsaturated fatty acid, since the P/S ratio was increased with the low-calcium diet even though the lipid content was lower. Although blood pressure or sodium excretion was not altered by changes in dietary calcium in the present study, augmented or diminished renal PGE? synthesis during calcium supplementation or restriction may play some physiological role in blood pressure regulation and/or sodium homeostasis. Further study to elucidate the role of renal PGs during calcium supplementation, which has been reported to decrease blood pressure in some investigations, will be needed. References 1. McCarron D A, Morris C D, Henry H J. Stanton 2. 3.
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J L. Blood pressure and nutrient in the United States. Science 1984; 224: 1392-1397. Feinleib N. Lenfant C, Miller S A. Hypertension and calcium. Science 1984; 226: 384-386. Semoos C. Coooer R. Kovar M G. Johnson C. Drizh T, Yetely E. Dietary calcium and blood’ pressure in National Health and Nutrition Examination Surveys 1 and II. Hypertension 1986; 8: 1067-1074. MacCarron D A. Is calcium more important than sodium in the pathogenesis of essential hvoertension? Hvoertension 1985: 7: 607-627. S;;azzullo P, Sia; A, Guglielmi S, Di Carlo A, Galletti F, Cirillo M, Manicini M. Controlled trial of long-term calcium supplementation in essential hypertension. Hypertension 1986; 8: 1084-1088.
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