Search

To determine the influence of dietary calcium on spaceflight-induced alterations in calcium metabolism and blood pressure (BP), 9-wk-old spontaneously hypertensive rats, fed either high- (2%) or low-calcium (0.02%) diets, were flown on an 18-day shuttle flight. On landing, flight animals had increased ionized calcium (P < 0.001), elevated parathyroid hormone levels (P < 0.001), reduced calcitonin levels (P < 0.05), unchanged 1,25(OH)(2)D(3) levels, and elevated skull (P < 0.01) and reduced femur bone mineral density. Basal and thrombin-stimulated platelet free calcium (intracellular calcium concentration) were also reduced (P < 0.05). There was a tendency for indirect systolic BP to be reduced in conscious flight animals (P = 0.057). However, mean arterial pressure was elevated (P < 0.001) after anesthesia. Dietary calcium altered all aspects of calcium metabolism (P < 0.001), as well as BP (P < 0.001), but the only interaction with flight was a relatively greater increase in ionized calcium in flight animals fed low- compared with high-calcium diets (P < 0.05). The results indicate that 1) flight-induced disruptions of calcium metabolism are relatively impervious to dietary calcium in the short term, 2) increased ionized calcium did not normalize low-calcium-induced elevations of BP, and 3) parathyroid hormone was paradoxically increased in the high-calcium-fed flight animals after landing.

The renal cell line mIMCD3 exhibits markedly upregulated phosphorylation of the extracellular signal-regulated kinase (ERK) 1 and 2 in response to urea treatment (200 mM for 5 min). Previous data have suggested the involvement of a classical protein kinase C (cPKC)-dependent pathway in downstream events related to urea signaling. We now show that urea-inducible ERK activation requires extracellular calcium; unexpectedly, it occurs independently of activation of cPKC isoforms. Pharmacological inhibitors of known intracellular calcium release pathways and extracellular calcium entry pathways fail to inhibit ERK activation by urea. Fura 2 ratiometry was used to assess the effect of urea treatment on intracellular calcium mobilization. In single-cell analyses using subconfluent monolayers and in population-wide analyses using both confluent monolayers and cells in suspension, urea failed to increase intracellular calcium concentration. Taken together, these data indicate that urea-inducible ERK activation requires calcium action but not calcium entry. Although direct evidence is lacking, one possible explanation could include involvement of a calcium-dependent extracellular moiety of a cell surface-associated protein.

Background: Previous studies have shown that farnesol, a 15-carbon nonsterol derivative of mevalonic acid, inhibits vasoconstriction. Because of its lipophilic properties, we hypothesized that farnesol increased membrane dynamics, thus reducing uptake of Ca2+ and contraction., Objective: To characterize the effect of farnesol on cell membrane fluidity., Design: The study was conducted using A7r5 cells, a rat aortic vascular smooth muscle cell line. Inhibition of Ca2+ uptake by farnesol was first established in these cells. Then, the effect of farnesol on membrane dynamics was determined. Finally, to ascertain that activation of Ca2+ extrusion and reuptake processes by farnesol did not occur, Ca2+-ATPase activity was examined., Methods: Membrane fluidity in cell homogenates was estimated using two fluorescent dyes (1,6-diphenyl-1,3,5-hexatriene) and (1-[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene). Ca2+ uptake was determined by monitoring the changes in cytosolic Ca2+ concentration ([Ca2+]i) in fura-2-loaded cells after addition of KCI. Ca2+-ATPase activity was measured in 100000 x g cell fractions., Results: Farnesol reduced KCI-induced (Ca2+]i transients significantly (P < 0.001), but did not modify membrane dynamic properties [0.214+/-0.007 versus 0.218+/-0.007 (n = 10) and 0.142+/-0.002 versus 0.146+/-0.003 (n = 5) for 1 -[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene and 1,6-diphenyl-1,3,5-hexatriene anisotropies, respectively; NS]. Administration of up to 30 micromol/l farnesol did not affect Ca2+-ATPase activity., Conclusion: Farnesol inhibits KCI-dependent rise of [Ca2+]i in A7r5 cells. This effect of farnesol is not related to a global change in plasma membrane lipid organization or to activation of Ca2+ pumps. Other mechanisms such as direct inhibition of voltage-dependent Ca2+ channels could therefore explain the biologic action of farnesol in the vascular tissue.

The association between dietary calcium intake, calcium metabolism, and blood pressure form the basis of this review. Epidemiologic data consistently show an inverse relationship between dietary calcium and blood pressure. Clinical trials of calcium supplementation have not been as consistent in outcome. Approximately two-thirds of the supplementation studies have found a beneficial effect of calcium on blood pressure. The lack of consistency in outcome from the clinical trials relative to the epidemiological literature may be related to calcium intake. The epidemiological literature indicates an inverse relationship between calcium intake and blood pressure, with those individuals with the lowest calcium intake (< 700 mg/day) having the highest blood pressure. Clinical studies utilizing patients with high baseline calcium levels (> 700 mg/day) may not see an effect of calcium supplementation on blood pressure because of a ceiling effect. Supplemental calcium appears to correct a defect in calcium handling characterized by a renal calcium leak, increased circulating parathroid hormone, and increased intracellular calcium levels. In part, the deficit in cellular calcium homeostasis may be a consequence of abnormal calmodulin activity. Specifically, it appears that calmodulin activity is diminished in experimental hypertension and that increasing dietary calcium may improve calmodulin activity in the spontaneously hypertensive rat. The deficit in calmodulin activity has the potential to interfere with a number of cellular processes crucial to the regulation of cell function and maintenance of appropriate vascular tone. It is concluded that additional research should be directed toward understanding the ramifications of altered calmodulin activity in hypertension and the influence that dietary calcium can have on the activity of calmodulin.

The abnormal intestinal Ca2+ transport reported in spontaneously hypertensive rats (SHR) has been attributed to decreased responsiveness to calcitriol. We reexamined this hypothesis by studying the calcitriol regulation of SHR duodenal calbindin-D9K and calmodulin and the relation of calcitriol to Ca2+ uptake by isolated enterocytes. SHR and normotensive Wistar-Kyoto (WKY) rats were injected with either 50 ng/d calcitriol (vit-D) or vehicle alone (control) for 3 days. Decreased calbindin-D9K (P < .001) and cellular Ca2+ flux (P < .001) were observed in control SHR. Calcitriol increased total cell and brush border calbindin-D9K (P < .0001); this variation paralleled plasma calcitriol levels in both strains. In contrast, Ca2+ flux, which increased in vit-D animals, remained lower in SHR for plasma calcitriol levels similar to those in WKY rats. Immunoreactive calmodulin was similar in both strains whether assayed in total cell or brush border membranes. In contrast, when measured by ligand blotting (45Ca), calmodulin was lower in SHR than in WKY rats (P < .01), suggesting the existence of a calmodulin pool with reduced Ca2+ binding capacity in the hypertensive strain. Calcitriol had no effect on calmodulin in either strain. In conclusion, Ca2+ binding protein regulation by calcitriol is normal in the SHR, and decreased hormone responsiveness cannot account for the defective duodenal calcium transport of this experimental model of hypertension.

The urinary excretion of various substances involved in kidney stone formation was evaluated in 67 patients with hypercalciuric lithiasis (HCl), 36 lithiasis patients with normal calciuria (NCl) and 21 controls without urinary stones. All subjects were hospitalized for 3 days and given a calcium, phosphorous and sodium-controlled diet. The 24-hour urine volume was significantly larger in the HCl and NCl groups than in controls. The 24-hour Ca, Na and uric acid excretion was significantly greater in the HCl group than in the NCl and control groups. Oxalate and pyrophosphate excretion was the same in all three groups. Urinary Ca correlated with urinary creatinine in the HCl and control groups, but the slope and ordinate of the regression line were significantly higher in the former group. Similarly, urinary Na correlated with urinary creatinine in the HCl and control groups with a significantly steeper slope in the HCl group. These data are suggestive of abnormalities in the tubular reabsorption of Ca and Na in HCl patients. Finally, there was no correlation between the values obtained and the activity of the disease, as evaluated by the finding of at least 3 urinary stones or one staghorn calculus during the 5 years preceding the study. It is concluded that measurements of Ca, Na, uric acid, creatine, oxalates and phosphates during a stay in hospital provide pathophysiological information but cannot be taken as indices of urolithiasis activity.

Intestinal calcium (Ca2+) transport was examined at the cellular level using duodenal enterocytes isolated from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Compartmental analysis of 45Ca2+ uptake was performed on enterocytes isolated from young (12- to 14-wk-old) and mature animals (24- to 26-wk-old) fed either normal (1%) or high (2%) calcium diets. Intracellular Ca2+ flux (Jc) was reduced in SHR compared with WKY for both young (0.67 +/- 0.05 vs. 1.08 +/- 0.08 nmol Ca2+.mg protein-1.min-1 P less than 0.01) and mature (0.39 +/- 0.03 vs. 0.71 +/- 0.05 nmol Ca2+.mg protein-1.min-1; P less than 0.001) animals on a normal calcium diet. On a high-calcium diet, this strain difference of Jc disappeared in the young rats (0.87 +/- 0.09 vs. 1.06 +/- 0.06 nmol Ca2+.mg protein-1.min-1, NS). In mature SHR, the high-calcium diet stimulated Jc, whereas it lowered it in mature WKY resulting in a similar flux for both strains (0.56 +/- 0.05 vs. 0.49 +/- 0.05 nmol Ca2+.mg protein-1.min-1, NS). Young SHR had a lower intracellular Ca2+ pool compared with WKY. This defect was corrected by a high-calcium diet. The membrane Ca2+ flux (Jm) was lower in mature SHR than WKY fed a normal calcium diet (P less than 0.02); Jm increased to the control value (P less than 0.05) in the SHR on a high-calcium diet. Diet-induced changes of plasma 1,25(OH)2 vitamin D levels in the SHR did not parallel the observed changes of intestinal Ca2+ fluxes. Thus duodenal enterocytes from SHR appear to have an intrinsic alteration of Ca2+ transport that can be corrected in part by a higher calcium diet.

Eleven patients with kidney stone disease and idiopathic hypercalciuria (urinary calcium above 4 mg/kg/j), without phosphorus renal leak and 6 control subjects have been put for 3 days on a diet containing 1 g calcium and 1 phosphorus daily (period A), and then for 4 days on a diet containing 1 g calcium, 450 mg phosphorus and 3 g aluminium hydroxyde daily (period B). During period A, no significant difference in blood calcium, phosphorus and magnesium, not in phosphaturia, rate of phosphorus reabsorption (RPR) and ratio maximum RPR/creatinine clearance was found between the two groups. After 2 days on a low phosphate diet (period B) the blood phosphorus decreased significantly in the hypercalciuric patients but not in the control subjects, thus revealing among the forme a latent abnormality in the retention of phosphates. This abnormality could play an important role in the pathogenesis of hypercalciuria.