Magnesium Uptake By Intestinal Brush-border Membranes of Spontaneously Hypertensive Rats

Magnesium uptake by intestinal brush-border membranes (BBM) was studied in duodenal and jejunal vesicles of the spontaneously hypertensive rat (SHR) and normotensive control, the Wistar-Kyoto (WKY) rat. In the duodenum, no statistical difference was evidenced between the two types of rats. By contrast, initial rates of magnesium uptake in jejunal vesicles were lower in SHR (5.4 +/- 2.1 nmol/mg protein x 10 sec) in comparison to WKY rats (11.0 +/- 2.5 nmol/mg protein x 10 sec) at a magnesium concentration of 1 mM (P < 0.01). In jejunal BBM, kinetic analysis of magnesium uptake showed three components in WKY rats, with one being diffusional. In SHR, only two components were seen, with the diffusional one being absent. The two saturable components showed V(max) of 6.5 +/- 1.3 and 26.2 +/- 6.0 nmol/mg protein x 10 sec and apparent K(m) of 0.22 +/- 0.12 mM and 1.9 +/- 0.4 mM in WKY rats, and V(max) of 10.9 +/- 3.5 and 14.8 +/- 5.9 nmol/mg protein x 10 sec and apparent K(m) of 0.43 +/- 0.23 mM and 1.3 +/-0.2 mM in SHR. Only the component with the lowest apparent affinity appeared statistically different in SHR as compared with WKY rats for both V(max) and apparent K(m) (P < 0.05). Time course evolution of magnesium uptake in jejunal BBM indicated, by extrapolation at zero time, that 2.5 and 5.1 nmol magnesium/mg protein in SHR and WKY rats, respectively, would be in the bound state. The study of the influence of medium osmolarity on 60-min magnesium uptakes was also indicative of a smaller binding compartment in jejunal BBM of SHR (3.70 and 8.26 nmol/mg protein in SHR and WKY rats, respectively); at the four osmolarities assayed, the 60-min uptakes were significantly lower in SHR as compared with WKY rats (P < 0.01). From 60-min glucose uptakes, a smaller volume of jejunal BBM vesicles was determined for SHR as compared with WKY rats (0.34 +/- 0.06 and 0.63 +/- 0.17 mul/mg of protein in SHR and WKY rats respectively, P < 0.05), this volume being significantly augmented by the presence of 1 mM MgCl2 (0.48 +/- 0.05 and 1.27 +/- 0.02 mul/mg of protein in SHR and WKY rats respectively, P < 0.01). These results suggest that magnesium uptake and binding by jejunal BBM are altered in SHR in comparison to WKY rats, implying a possible role of the small intestine in the abnormalities of magnesium metabolism in genetic hypertension