Calcium transport and vitamin D in three breeds of spontaneously hypertensive rats

Regulation of parathyroid hypertensive factor secretion by vitamin D3 analogs in parathyroid cells derived from spontaneously hypertensive rats

Parathyroid hypertensive factor (PHF) is a novel substance secreted by the parathyroid gland (PTG), which is elevated in 30-40% of all hypertensive patients; specifically, the low-renin subset. However, very little is known about the regulation of PHF secretion. Since the classical parathyroid regulator, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), may be elevated concurrent with or preceding the development of low-renin hypertension and elevated plasma PHF, we hypothesized that 1,25-(OH)2D3 would stimulate PHF release. To test this hypothesis, PTG organ and cell cultures, derived from spontaneously hypertensive rats (SHR) and the normotensive genetic control Wistar Kyoto (WKY) rats, were exposed to various vitamin D3 metabolites and PHF release measured by ELISA. 1,25-(OH)2D3 rapidly stimulated PHF release with enhanced sensitivity in SHR versus WKY cultures indicated by a leftward shift in the dose-response curve, whereas 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3) had the converse effect. Vitamin D3 analog "BT," an agonist for the classical nuclear vitamin D receptor (1,25VDR(nuc)), was without effect suggesting a 1,25VDR(nuc)-independent mechanism and potential involvement of the plasma membrane-bound vitamin D receptor (1,25 D3-MARRS). Interestingly, protein expression of the 1,25 D3-MARRS was increased in SHR versus WKY parathyroid cells. In conclusion, these results support the idea that 1,25-(OH)2D3 may contribute to elevated plasma PHF in the SHR.

Characterization of six rat strains (Rattus norvegicus) by mitochondrial DNA restriction fragment length polymorphism

Restriction fragment length polymorphism (RFLP) was used to examine the extent of mtDNA polymorphism among six strains of rats (Rattus norvegicus)--Wistar, Wistar Munich, Brown Norway, Wistar Kyoto, SHR and SHR-SP. A survey of 26 restriction enzymes has revealed a low level of genetic divergence among strains. The sites of cleavage by EcoRI, NcoI and XmnI were shown to be polymorphic. The use of these three enzymes allows the 6 strains to be classified into 4 haplotypes and identifies specific markers for each one. The percentage of sequence divergence among all pairs of haplotypes ranged from 0.035 to 0.33%, which is the result of a severe population constriction undergone by the strains. These haplotypes are easily demonstrable and therefore RFLP analysis can be employed for genetic monitoring of rats within animal facilities or among different laboratories.

Ultraviolet light may contribute to geographic and racial blood pressure differences

Mean systolic and diastolic pressures and the prevalence of hypertension vary throughout the world. Published data suggest a linear rise in blood pressure at increasing distances from the equator. Similarly, blood pressure is higher in winter than summer. Blood pressure also is affected by variations in skin pigmentation. Altered calcium, vitamin D, and parathyroid hormone status is associated with hypertension and may vary with latitude and season. Since changes in UV light affect vitamin D and parathyroid hormone status and UV light intensity are influenced by seasonal change and latitude, these disparate observations suggest an association between blood pressure and ultraviolet light. This discussion presents the hypothesis that reduced epidermal vitamin D3 photosynthesis associated with high skin melanin content and/or decreased UV light intensity at distances from the equator, alone or when coupled with decreased dietary calcium and vitamin D, may be associated with reduced vitamin D stores and increased parathyroid hormone secretion. These changes may stimulate growth of vascular smooth muscle and enhance its contractility by affecting intracellular calcium, adrenergic responsiveness, and/or endothelial function. Thus, UV light intensity and efficiency of epidermal vitamin D3 photosynthesis may contribute to geographic and racial variability in blood pressure and the prevalence of hypertension.

In vivo effect of calcitriol on calcium transport and calcium binding proteins in the spontaneously hypertensive rat

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.

Impaired duodenal response to short-term dietary calcium restriction in adolescent spontaneously hypertensive rats

The response pattern of plasma calcitriol level and related intestinal adaptation to short-term moderate calcium (Ca) restriction was examined in adolescent male, spontaneously hypertensive rats (SHR) and normotensive WKY control rats. Twelve-week-old SHR and WKY fed a low (0.1%) Ca diet for 3, 6, or 12 days were compared with rats of either strain fed a normal (1.0%) Ca diet. Plasma calcitriol response was measured and duodenal adaptation to Ca restriction was investigated by evaluating active Ca transport, calbindin-D9K (CaBP9K) protein, CaBP9K mRNA, and alkaline phosphatase activity (ALP). Under the normal Ca diet, no significant difference between strains was observed for all five parameters. In response to the low Ca diet, the SHR and WKY showed a similar increase (nearly 50%) in plasma calcitriol, starting at day 3 of this diet. However, only the WKY displayed the expected duodenal adaptation: active Ca transport increased at day 6 and CaBP9K as well as ALP increased at day 3 of the low Ca diet. The stimulation of the latter three parameters was maintained until day 12 of Ca restriction. Moreover, CaBP9K mRNA was increased in WKY after 3 days of Ca restriction. In contrast, the SHR had either no or only a minor increase of duodenal parameters in response to Ca restriction. Finally, a significant and positive correlation between Ca transport and plasma calcitriol and between Ca transport and CaBP9K was found in WKY but not in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium deprivation increases NaCl intake of Fischer-344 rats

Fischer-344 rats fed Ca2(+)-deficient diet for 63 days increased intake of 0.3 M NaCl solution from control levels of approximately 8 ml/day to greater than 60 ml/day. During the same period, rats fed Na(+)-deficient diet drank approximately 11 ml/day. These results indicate that Fischer-344 rats, which generally spurn NaCl, drink large amounts of it when Ca2+ deprived.

Calcium uptake kinetics into brush-border membrane vesicles: higher Vmax in the spontaneously hypertensive rat than in normotensive control

Arterial hypertension in the spontaneously hypertensive rat (SHR) is associated with an abnormal Ca2+ homeostasis, compared with its normotensive control, the Wistar Kyoto rat (WKY). In particular, epithelial Ca2+ transport is perturbed, with intestinal absorption and renal tubular reabsorption being decreased in the adolescent and adult SHR. In the present study we examined Ca2+ uptake into isolated duodenal brush-border membrane vesicles (BBMV) in 12-14 week-old male rats. This uptake can be separated in a nonsaturable and a saturable component. The latter follows Michaelis-Menten kinetics. Vmax of this component was found to be significantly higher (p less than 0.05) in SHR than in WKY (0.58 +/- 0.19 versus 0.35 +/- 0.06 nmol/mg protein x 10 sec, mean +/- SD) whereas Km did not differ. Thus, the defect in the intestinal Ca2+ absorption previously identified in the SHR of this age is not due to a decrease in Ca uptake at the level of the duodenal brush-border membrane, but is most likely located in the baso-lateral membrane.

Ultrastructural and functional abnormalities of intestinal and renal epithelium in the SHR

Intestinal calcium transport, renal tubular calcium reabsorption, and plasma 1.25 (OH)2 vitamin D3 (calcitriol) levels have all been reported to be diminished in the spontaneously hypertensive rat (SHR) compared with its genetic control the Wistar Kyoto rat (WKY). In the present study, absorptive duodenal and renal tubular epithelia of 12- to 14-week-old male SHR and WKY were examined by electron microscopy to determine whether such disturbances could be related to structural abnormalities. Patchy loss of microvilli in both duodenal and proximal tubular epithelia was observed in the SHR, whereas brush border membrane was entirely normal in the WKY. Irregular spaces were observed between the basal aspects of SHR intestinal epithelial cells and their basement membrane. In addition, the average height of duodenal and renal microvilli was reduced in the SHR. Two specific markers of the brush border membrane, alkaline phosphatase and villin, as well as the cytoplasmic vitamin-D dependent calcium-binding proteins, CaBP9K and CaBP28K were determined. Duodenal alkaline phosphatase activity was reduced in the SHR, compared with the WKY: 0.145 +/- 0.002 vs. 0.186 +/- 0.002 IE/min.microns 3 x 10(3) brush border, mean +/- SEM, N = 10 pairs, P less than 0.001. However, duodenal villin expression was not different from that of the WKY. Duodenal CaBP9K and renal CaBP28K content was diminished in the SHR: 21.0 +/- 0.80 vs. 29.9 +/- 2.19 micrograms/mg protein, N = 6 pairs, P less than 0.01 for duodenum, and 4.47 +/- 0.39 vs. 7.67 +/- 0.54 micrograms/mg protein, N = 6 pairs, P less than 0.001 for kidney. These data showing structural and functional abnormalities of intestinal and kidney cells in the SHR appear to reflect a disorder of transporting epithelia which may be either intrinsic or related to reduced circulating calcitriol.

Molecular evidence of genetic heterogeneity in Wistar-Kyoto rats: implications for research with the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto ("WKY") rats are frequently employed in experimental studies of hypertension. Although both SHR and WKY rat strains have been presumed to be fully inbred, recent studies have revealed important biologic variability in WKY rats from different commercial sources. Genealogic evidence suggests that, in the United States, breeding stocks of WKY rats may have been distributed to major commercial suppliers as early as the F10 generation. To test the hypothesis that commercially available WKY rats are genetically heterogeneous, we performed deoxyribonucleic acid (DNA) "fingerprint" analysis on genomic DNA of WKY rats from two of the largest vendors in the United States, Taconic Farms and Charles River Laboratories. We found molecular evidence of genetic variability not only among WKY rats from two different breeding facilities, but also among WKY rats within a single breeding facility (Taconic Farms). Although some studies have suggested the possibility of biologic variability in SHR from different sources, preliminary studies have not revealed molecular evidence of genetic heterogeneity in SHR from these vendors. In demonstrating genetic variability in WKY rats from different sources, the current study provides compelling evidence that rats designated WKY do not constitute an inbred strain. Accordingly, the results of studies in which SHR and WKY rats are compared might vary because of genetic heterogeneity in "the WKY rat control strain."