Several abnormalities of calcium metabolism

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1 AJH 1991; 4: Intestinal Absorption of Calcium and Calcium Metabolism in Patients With Essential Hypertension and Normal Renal Function Merit Gadallah, Shaul G. Massry, Roberto Bigazzi Ronald L Horst, Peter Eggena, and Vito M. Campese Several abnormalities of calcium metabolism have been described in patients with essential hypertension, and they have been linked to the pathogenesis of hypertension. Intestinal calcium absorption has been shown to be decreased in rats with spontaneous hypertension, but it has not been studied in patients with essential hypertension. In these studies we have for the first time measured intestinal absorption of calcium (using oral and intravenous administration of 47 Ca), along with other parameters of calcium metabolism, in 14 patients with essential hypertension and normal renal function and in 16 normal subjects. There was no difference in serum total or ionized calcium, serum phosphorus, parathyroid hormone (PTH), 25-hydroxyvitamin D (25(OH)D), 1,25-dihydroxyvitamin D (l,25(oh) 2 D), and 24,25-dihydroxyvitamin D(24,25(OH) 2 D) among hypertensives and normotensives. The urinary excretion of calcium, on the other hand, was greater in hypertensive than in normotensive subjects (195 ± 33 ν 107 ± 13 mg/24 h, Ρ <.05). There was also no difference in intestinal absorption of calcium after 2 and 24 h among hypertensives and normotensives. When hypertensive patients were stratified according to plasma renin activity (PRA) we found that patients with low PRA had higher intestinal absorption of calcium at 2 h (23 ± 2.9 ν 18 ± 0.6%, Ρ <.05) but not at 24 h. Serum total and ionozed calcium, PTH, and l,25(oh) 2 D were not different between patients with low and those with normalhigh PRA. The major derangement of calcium metabolism in patients with essential hypertension is hypercalciuria. This abnormality is more pronounced in patients with low PRA, and it may lead to increased vitamin D-dependent intestinal absorption of calcium. Am J Hypertens 1991;4: KEY WORDS: Calcium, hypertension, hypercalciuria, vitamin D. Several abnormalities of calcium metabolism have been described in patients with essential hypertension, as well as in rats with spontaneous hypertension (SHR). These abnormalities From the Division of Nephrology, Department of Medicine, University of Southern California School of Medicine, Los Angeles, California (MG, SGM, RB, PE, VMC); and the National Animal Disease Center, USD A, Agricultural Research Service, Ames, Iowa (RLH). This study was partially supported by National Institute of Health (NIH) grants DK 2955, HL35629, by NIH National Center for Research Resources of the General Clinical Research Center's grant MOl RR-43, and by a grant from the Wright Foundation. Address correspondence and reprint requests to Vito M. Campese, MD, Professor of Medicine, USC School of Medicine, Division of Nephrology, 2025 Zonal Avenue, GNH 4250, Los Angeles, CA include hypercalciuria, reduced serum ionized calcium, increased cytosolic calcium, increased serum levels of parathyroid hormone (PTH), abnormal levels of vitamin D 3, abnormalities in cell membrane transport of calcium, and decreased dietary calcium intake. 1,2 McCarron 1 has suggested that patients with essential hypertension and SHR display a defect in intestinal absorption of calcium, and that this derangement, combined with decreased dietary calcium intake and hypercalciuria, leads to a negative calcium balance and to hypocalcemia, which would ultimately result in hypertension. Intestinal calcium absorption has not been studied in patients with essential hypertension, and the data on 1991 by the American Journal of Hypertension, Inc /91/$3.50

2 AJH-MAY 1991-VOL 4, NO. 5, PART 1 INTESTINAL ABSORPTION OF CALCIUM IN HYPERTENSION 405 intestinal calcium absorption in SHR are controversial. Schedl et al 3 and Lucas et al 4 have demonstrated a decrease in maximal transport rate and net total transport of calcium in 12-week-old SHR. It has been suggested that the alteration in vitamin D metabolism may be responsible for this abnormality. 3 Other investigators, however, have observed increased intestinal absorption of calcium, positive calcium balance, and increased serum levels of l,25(oh) 2 D levels in prehypertensive 3-week-old SHR. 5 Finally, no abnormality in in vitro calcium uptake by everted gut sacs from these animals was reported. 6 The reasons for these differences are not readily apparent, but they may be related to differences in experimental protocols. It is also noteworthy that in many studies of calcium metabolism in hypertension, renal function has not been properly evaluated, despite the well-known fact that even moderate reduction in glomerular filtration rate is associated with changes in calcium metabolism. 78 The purpose of this study was to determine whether patients with primary hypertension and normal renal function display abnormalities of intestinal absorption of calcium and of other parameters of calcium metabolism, compared with normal control subjects. METHODS Sixteen normal subjects aged 30 to 64 years (40 ± 2.6 [SEM]) and 14 patients with essential hypertension aged 39 to 65 years (55 ± 1.8) were studied. The normal subjects included seven men and nine women; the hypertensive group was made up of seven men and seven women. Six of the normal subjects were black, three white, six Hispanic, and one Oriental. Nine of the hypertensive patients were black, one white, two Hispanic, and two Oriental. Patients were considered to be hypertensive if their blood pressure was found to be equal or greater than 140/90 mm Hg during three consecutive visits to the outpatient clinic. Antihypertensive medications were withheld at least 2 weeks before the study. A diagnosis of secondary hypertension was adequately excluded by the finding of normal SMA-19, urinalysis, and creatinine clearance, by the absence of abdominal bruits, and by radiologic studies, when clinically indicated. Patients with previous history of liver diseases, diabetes, adrenal dysfunction, metabolic bone diseases, thyroid disease, or gastrointestinal diseases were excluded from the study. Patients taking birth control pills, glucocorticoids, phosphate binding antacids, or sex hormones, or those on weight-reducing diets were also excluded from the study. After the nature and purpose of the study were explained, informed consent was obtained and the patients were instructed to continue their usual diet and to record their whole dietary intake for the 3 days prior to the study. The day before the study, all subjects collected 24-h urine samples for analysis of sodium, potassium, calcium, magnesium, phosphorus, and creatinine. The day of the study, after fasting overnight, all subjects were admitted to the Clinical Research Center of the Los Angeles County University of Southern California Medical Center, where they remained supine for 1 h from 8:00 to 9:00 AM. At the end of the hour, blood pressure and heart rate were measured in triplicate, and blood was drawn from an antecubital vein for measurement of serum sodium, potassium, total and ionized calcium, phosphorus, magnesium, PTH, 25(OH)D, 24,25(OH) 2 D, and l,25(oh) 2 D, plasma catecholamines and plasma renin activity (PRA). Plasma renin activity was also measured after 10 and 60 min of upright posture. Thereafter, intestinal calcium absorption was measured by the method of Curtis et al, 9 as modified by Coburn et al, 10 using oral and intravenous administration of 47 Ca. The oral isotope was delivered in 200 mg of calcium gluconate as a carrier. Forearm radioactivity was measured 2 and 24 h after both intravenous and oral doses by a large volume scintillation counter (Armac, Packard Instruments Co, Downers Grove, 111.). The fraction of intestinal absorption of ingested 47 Ca was calculated as previously described. 10 Dietary records were also obtained from each patient to determine the dietary intake of sodium, potassium, calcium, phosphorus, and magnesium over the 3 days preceding the study. Total serum and urine calcium and magnesium were determined by atomic absorption spectrophotometer (Model 503, Perkin-Elmer Co., Instrument Division, Norwalk, Conn.). Serum and urine phosphorus and creatinine were determined by Technicon autoanalyzer (Technicon Instrumentation Co., Tarry town, N.Y.). Serum and urine sodium and potassium were measured by Flame Photometry (Instrumentation Laboratory, Lexington, Mass.). Serum PTH was measured by radioimmunoassay, 11 and 25(OH)D, 24,25(OH) 2 D, and l,25(oh) 2 D were measured by competitive protein binding assays. 12 Plasma catecholamines were measured by high performance liquid chromatography with electrochemical detection (Shimadzu, Kyoto, Japan). Plasma renin activity was measured by radioimmunoassay. 13 The data were statistically analyzed by Student's t test. RESULTS The clinical and biochemical data in the hypertensive patients and the normal subjects are given in Table 1. Both systolic and diastolic blood pressure were significantly higher (P <.01) in the hypertensive patients than in normal subjects. No differences were observed in serum levels of total and ionized calcium, phosphorus, magnesium, PTH, vitamin D metabolites, PRA, norepinephrine, and epinephrine. The urinary excretion of sodium, potassium, phosphorus, and magnesium were similar among hypertensives and normotensives. The urinary excretion of calcium, on the other hand, was

3 406 GAD ALLAH ET AL AJH-MAY 1991-VOL 4, NO. 5, PART 1 TABLE 1. CLINICAL AND HORMONAL CHARACTERISTICS OF NORMAL SUBIECTS AND PATIENTS WITH ESSENTIAL HYPERTENSION Hypertensives Normotensives Number of patients Age (years) 55 ± ± 2.6* SBP (mm Hg) 157 ± ± 1.7* DBP (mm Hg) 92 ± ±1.5* Heart rate (beats/min) 72 ± ±2.6 Creatinine clearance 104 ± ±5.4 (ml/min) Serum calcium (mg/ 9.5 ± ± 0.09 dl) Serum Ca 2+ (mmol/l) 1.2 ± ±0.03 Serum magnesium 2.0 ± ± 0.05 (mg/dl) Serum phosphorus 3.2 ± ±0.10 (mg/dl) Serum PTH (ßLEq/ 3.4 ± ±0.88 ml) 25(OH) D (ng/ml) 27 ± ±4.0 l,25(oh) 2 D (pg/ml) 32 ± ± ,25(OH) 2 D (ng/ml) 1.4 ± ± 0.30 Intestinal absorption of calcium (%) 2h 20 ± ± h 31 ± ± 1.3 Plasma norepinephrine 170 ± ± 14.2 (pg/ml) Plasma epinephrine 54 ± ±9.8 (pg/ml) PRA (ng/ml/h) Supine 2.7 ± ±0.63 Upright 60' 3.0 ± ±0.77 Urine sodium (meq/ 245 ± ± h) Urine calcium (mg/ 195 ± ± 12* 24 h) Urine magnesium 93 ± ±11 (mg/24 h) Urine phosphorus 740 ± ± 55 (mg/24 h) All values, except that for number of patients, are expressed mean ± SEM. SBP, systolic blood pressure; DBP, diastolic blood pressure; PTH, parathyroid hormone; PRA, plasma renin activity. * Ρ <.05, compared with the hypertensive group. our patients in those with low and those with normalto-high PRA based on our own nomogram relating PRA to urinary sodium excretion, we found that five had low and nine had normal-to-high PRA. We did not detect any difference in total (9.7 ± 0.2 ν 9.4 ± 0.2 mg/dl) or ionized serum calcium (1.22 ± 0.04 ν 1.18 ±0.04 mmol/l) or in serum magnesium (2.7 ± 0.07 ν 2.7 ± 0.07 mg/dl), PTH, and vitamin D 3 metabolites among patients with low and those with normal-to-high PRA. Patients with low PRA had a slightly higher urinary excretion of calcium than patients with normal-to-high PRA (247 ± 67 ν 167 ± 34 mg/24 h), but the difference was not statistically significant (P <.2). Furthermore, patients with low PRA had significantly greater (P <.05) urinary excretion of sodium (296 ± 48 ν 163 ± 48 meq/24 h) than patients with normal-to-high PRA. Patients with low PRA also manifested slightly higher levels of l,25(oh) 2 D than normal subjects (37 ± 6.0 ν 26 ± 2.3 pg/ml) and patients with normal-to-high PRA (30 ± 3.0 pg/ml), but the differences were not statistically significant. Two hours after the administration of 47 Ca, low-pra patients displayed increased intestinal absorption of calcium compared with patients with normal-to-high PRA (23 ± 2.9 ν 18 ± 0.66, Ρ <.05) (Figure 1). The estimated 24-h intake of sodium was 2,476 ± 313 mg, of potassium 2>132 ± 164 mg, of calcium 518±89mg, of phosphorus 1,016 ± 65 mg, and of magnesium 211 ± 26 mg in hypertensive patients. In normal subjects, the estimated 24-h intake of sodium was 2,581 ± 265 mg, of potassium 2,420 ± 207 mg, of calcium 526 ± 71 mg, of phosphorus 1,047 ± 87 mg, and of magnesium 236 ± 29 mg. The differences in the significantly greater (P <.05) in hypertensive than in normotensive subjects (195 ±33 ν 107 ± 13 mg/24 h). Intestinal absorption of calcium after 2 and 24 h in patients with essential hypertension was 20.1 ± 1.24% and 30.6 ± 2.07%, respectively. These values were not different from those in normal subjects who had intestinal absorption of calcium of 18.7 ± 1.05% and 31 ± 1.26% after 2 and 24 h, respectively. When we divided Low-PRA * Ρ < 0.05 compared with Normal-PRA Normal-PRA FIGURE 1. Intestinal absorption of calcium after 2 and 24 h in patients with low and normal PRA. The intestinal absorption of calcium at 2 h in patients with low PRA was higher (P <.05) than in patients with normal PRA.

4 AJH-NlAY 1991-VOL 4, NO. 5, PART 1 INTESTINAL ABSORPTION OF CALCIUM IN HYPERTENSION 407 intake of these minerals between normal and hypertensive subjects were not significant. DISCUSSION Several derangements of calcium metabolism have been described in rats with spontaneous hypertension (SHR) and in Dahl's salt-sensitive hypertensive rats. 1 Among those abnormalities is reduced intestinal calcium absorption, which has been ascribed to alterations in vitamin D metabolism. 3,4 McCarron has suggested that these abnormalities may play an important role in the pathogenesis of essential hypertension. The data concerning the abnormalities of calcium metabolism in human essential hypertension are limited and controversial. Moreover, in most of these studies the level of glomerular filtration rate was not taken into account despite the fact that moderate reduction in glomerular filtration could affect many parameters of calcium metabolism. 7,8 McCarron 14 was the first to report decreased concentrations of serum ionized but not of total calcium in patients with essential hypertension. Others, however, have found normal serum ionized calcium. 15,16 Resnick et al 17 observed a decrease in serum ionized calcium only in patients with low-renin and in salt-sensitive patients, but not in those with normal or high renin or in salt-resistant patients. The reasons for these differences are not evident, but they could be related to the fact that other variables can affect serum calcium, 18 " 21 including the degree of renal function. 7,8 In our studies of two groups of normotensive and hypertensive subjects with normal and comparable glomerular filtration rate, we were unable to find any difference in serum levels of total or ionized calcium. Moreover, we did not observe any difference in total or ionized calcium between patients with low and those with normal-to-high PRA. However, the number of patients studied in each subgroup was small, which could explain why we did not observe any difference in serum ionized calcium as previously reported by Resnick et al. 17 In patients with essential hypertension, no studies have been previously published on intestinal calcium absorption. Recently, Young et al 22 have shown that intestinal calcium absorption, estimated by the increment in urinary calcium excretion after an oral calcium load, was not significantly different between normal subjects and patients with essential hypertension. In our study, in which intestinal absorption of calcium was measured with oral and intravenous administration of 47 Ca, we did not find significant difference either in the early (2-h) vitamin D 3 -dependent intestinal calcium absorption, nor in the 24-h intestinal calcium absorption. Hypertensive patients with low PRA, however, manifested increased intestinal absorption of 47 Ca at 2 h. At 24 h, intestinal absorption of calcium was similar between patients with low and those with normal-tohigh PRA. These data must be viewed with some caution because the number of hypertensive patients studied was small. Moreover, the fact that the hypertensive patients were older, and that vitamin D-dependent calcium absorption declines with age, may explain why we could not find any difference in calcium absorption between hypertensive and normotensive patients despite higher calcium excretion in the former group. With this caveat, our data suggest that human subjects with essential hypertension have normal or increased intestinal absorption of calcium and not decreased absorption as observed by some investigators in SHR. 1 Previous studies have shown increased serum levels of PTH and increased urinary camp in patients with essential hypertension. 2 Resnick et al 23 found increased levels of PTH in patients with low but not in those with high renin. McCarron et al found a significant and direct correlation between serum PTH levels and mean arterial pressure in males but not in females with essential hypertension. 24 In our small group of hypertensive patients we were unable to show any significant difference in serum PTH between normotensive and hypertensive subjects nor between patients with low and those with normal-high PRA. Some investigators have also shown that serum phosphorus is decreased in patients with essential hypertension and is inversely correlated with arterial pressure in males but not in females. 25 In our study, we found no significant difference in serum phosphorus among normotensive and hypertensive patients. The data on serum levels of l,25(oh) 2 D levels in hypertensive patients are also conflicting. McCarron et al found lower levels of l,25(oh) 2 D in the hypertensive subjects. 1 On the other hand, Resnick et al found increased levels in patients with low PRA but normal levels in patients with normal or high PRA. 26 We did not find any significant difference in serum levels of 25(OH)D, l,25(oh) 2 D, and 24,25(OH) 2 D between normotensive and hypertensive patients. Serum levels of l,25(oh)d in our low-pra hypertensive patients were slightly higher than in normal subjects, but the difference did not reach statistical significance. Our hypertensive patients manifested increased urinary excretion of calcium, confirming the observations of several other investigators Resnick et al, 17 on the other hand, found no difference in urinary excretion of calcium among normotensives and hypertensives, even when the latter were stratified according to PRA levels. The mechanisms for hypercalciuria in essential hypertension remain obscure. This abnormality does not appear to be due to increased dietary calcium intake. McCarron et al 1 have instead shown that the urinary excretion rate of calcium was higher even though dietary intake of calcium was lower in hypertensive compared with normotensive subjects. Our estimate of di-

5 408 GADALLAH ET AL AJH-MAY 1991-VOL 4, NO. 5, PART 1 etary calcium intake obtained through dietary records for 3 days preceding the study showed no difference in dietary calcium intake among normotensive and hypertensive subjects. McCarron et al 1 have proposed that a "renal calcium leak" may be responsible for the high urinary calcium excretion. Whether this is a primary or secondary renal defect remains to be ascertained. Decreased renal calcium reabsorption has been observed in several models of experimental hypertension such as the SHR, 29 the Milan strain of SHR, 32 the Dahl salt-sensitive rats, 3334 and the DOCA-saline rat model of hypertension. 34,35 The universal appearance of this disturbance in models of hypertension with different initiating events would suggest that impairment in renal tubular reabsorption of calcium might be secondary to the development of hypertension rather than primary. REFERENCES 1. McCarron DA: Calcium metabolism and hypertension. Kidney Int 1989;35: Campese VM: Calcium, parathyroid hormone and blood pressure. Am J Hypertens 1989;2:34S-44S. 3. Schedl HP, Miller DL, Pape JM, Horst RL: Calcium and sodium transport and vitamin D metabolism in the spontaneously hypertensive rat. J Clin Invest 1984,73: Lucas PA, Brown RC, Drueke T, et al: Abnormal vitamin D metabolism, intestinal calcium transport, and bone calcium status in the spontaneously hypertensive rat compared with its genetic control. J Clin Invest 1986;78: Lau Κ, Langman LB, Gafter V, et al: Increased calcium absorption in prehypertensive spontaneously hypertensive rat. Role of serum 1,25-Dihydroxy vitamin D 3 levels and intestinal brush border membrane fluidity. J Clin Invest 1986;78: Stem N, Lee DBN, Silis V, et al: Effects of high calcium intake on blood pressure and calcium metabolism in young SHR. Hypertension 1984;6: Brickman AS, Cobum JW, Rowe PH, et al: Impaired calcium absorption in uremic man: Evidence for defective absorption in proximal small intestine. J Lab Clin Med 1974;84: Rutherford WE, Blondin J, Hruska K, et al: Effects of 25-hydroxy-cholecalciferol on calcium absorption in chronic renal disease. Kidney Int 1975;8: Curtis FK, Fellows H, Rich C: Estimation of human calcium absorption by external radioisotope counting. J Lab Clin Med 1967;69: Cobum JW, Kopple ΜΗ, Brickman AS, Massry SG: Study of intestinal absorption of calcium in patients with renal failure. Kidney Int 1973;3: Arnaud CD, Tsao HS, Lihledike T: Radioimmunoassay of human parathyroid hormone in serum. J Clin Invest 1971;50: Reinheart TA, Horst RL: Simplified assays for the determination of 25(OH)D, 24,25(OH) 2 D and l,25(oh) 2 D, in Norman PW et al (eds): Vitamin D: Molecular, Cellular and Clinical Endocrinology. Proceedings of the Seventh Workshop on Vitamin D, Berlin, W. degruyter, 1988, pp Eggena P, Barrett J, Wiedman CE, Sambhi MP: The validity of comparing the measurements of angiotensin I generated in human plasma by radioimmunoassay and bioassay. J Clin Endocrinol Metab 1974;39: McCarron DA: Low serum concentrations of ionized calcium in patients with hypertension. Ν Engl J Med 1982;307: Kesteloot H, Geboers J, Math L, Van Hoof R: Epidemiological study of the relationship between calcium and blood pressure. Hypertension 1983;5(suppl 2): Strazzullo P, Nunziata V, Cirillo M, et al: Abnormalities of calcium metabolism in essential hypertension. Clin Sei 1983;65: Resnick JM, Laragh JH, Sealey JE, Alderman MH: Divalent cations in essential hypertension: Relations between serum ionized calcium, magnesium and plasma renin activity. Ν Engl J Med 1983;309: Kanis JA, Yates AJ: Measuring serum calcium. Br Med J 1985;290: Arkwright PD, Beilin LJ, Vandongen R, et al: Plasma calcium and Cortisol as predisposing factors to alcoholrelated blood pressure elevation. J Hypertens 1984;2: West DW, Ash O: Adult reference intervals for 12 chemistry analytes: Influences of age and sex. Am J Clin Pathol 1984;81: Ljunghall S, Joborn H, Lundin L, et al: Regional and systemic effects of short-term intense muscular work on plasma concentration and content of total and ionized calcium. Eur J Clin Invest 1985;15: Young EW, Morris CD: Increased urinary calcium excretion in essential hypertension. Twenty-Second Annual Meeting of the American Society of Nephrology, Washington, D.C., December 3-6, 1989, ρ 217A. 23. Resnick LM, Muller FB, Laragh JH: Calcium-regulating hormones in essential hypertension: relation to plasma renin activity and sodium metabolism. Ann Intern Med 1986;105: McCarron DA, Pingree PA, Rubin RJ, et al: Enhanced parathyroid function in essential hypertension: a homeostatic response to a urinary calcium leak. Hypertension 1980;2: McCarron DA: Calcium metabolism and hypertension. Kidney Int 1989;35: Resnick LM, Nicholson JP, Laragh JH: The effects of calcium channel blockade on blood pressure and calcium metabolism. Am J Hypertens 1989;2: Kestelott H, Geboers J: Calcium and blood pressure. Lancet 1982;i: Staessen J, Bulpitt C, Fagard R, et al: Four urinary cations and blood pressure: a population study in two Belgian towns. Am J Epidemiol 1983;117: McCarron DA, Yung NN, Ugoretz BA, Krutzik S: Disturbances of calcium metabolism in the spontaneously hypertensive rat. Hypertension 1981;3(suppl):I162-I Ayachi S: Increased dietary calcium lowers blood pressure in the spontaneously hypertensive rat. Metabolism 1979;28:

6 AJH-MAY 1991-VOL 4, NO. 5, PART 1 INTESTINAL ABSORPTION OF CALCIUM IN HYPERTENSION Lau K, Zikos D, Spirnak J, Eby B: Evidence for an intestinal mechanism in hypercalciuria of spontaneously hypertensive rats. Am J Physiol 1984;247:E625-E633. Bianchi G, Ferrari P, Salvati P, et al: A renal abnormality in the Milan hypertensive strain of rats and in humans predisposed to essential hypertension. J Hypertens 1986;4(suppl 3):S33-S36. Kurtz TW, Morris RC: Dietary chloride as a determinant of disordered calcium metabolism in salt-dependent hypertension. Life Sei 1985;36: Umemura S, Smythe DD, Pettinger WA: Renal adesylate cyclase in DAHL and DOC-Na hypertensive rats: Defective response to parathyroid. J Hypertens 1986;4:S291 - S Massry SG, Coburn JW, Chapman LW, Kleeman CR: The effect of long-term desoxycorticosterone acetate administration on the renal excretion of calcium and magnesium. J Lab Clin Med 1968;71: