Intestinal Brush Border Calcium Uptake in Spontaneously Hypertensive Rats and Their Genetically Matched WKY Rats

The current studies were designed to characterize calcium transport by intestinal brush border membrane in the spontaneously hypertensive rat (SHR) and normo-tensive control, the Wistar-Kyoto (WKY) rat. The biochemical and functional purity of the intestinal brush border membranes in SHR and WKY rats was validated by marker enzymes and the ability to transiently transport d-glucose in the presence of Na⁺ gradient. Calcium transport into duodenal and jejunal vesicles represented a minor binding component and transmembrane movement as evident by initial rate studies, A23187 studies, and lanthanum displacement experiments. Initial rate and time course of calcium uptake was lower in SHR compared with WKY rats. Kinetic analysis of calcium uptake by the jejunum (total uptake minus binding component) showed a V_max of 6.98 ± 0.2 and 1.8 ± 0.2 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P < 0.001), whereas K_m values were 0.76 ± 0.04 and 0.87 ± 0.1 mM for WKY rats and SHR, respectively. Similar kinetic analysis of calcium uptake by the duodenal segments showed a V_max of 10.3 ± 0.8 and 2.8 ± 0.2 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P < 0.01). K_m values were 0.7 ± 0.2 and 0.3 ± 0.06 mM (P > 0.05). V_max of calcium uptake in the 2-week-old rats (prehypertensive period) was 6.0 ± 0.3 and 3.53 ± 0.3 nmol/mg protein/7 sec in WKY rats and SHR, respectively (P < 0.001), whereas K_m values were 0.60 ± 0.07 and 0.5 ± 0.01 mM, respectively. These results suggest that calcium binding and uptake by duodenal and jejunal intestinal brush border membranes of SHR is significantly decreased compared with WKY rats. The decrease in transmembrane calcium uptake is secondary to decrease in V_max and is present before the appearance of hypertension, implying a genetically determined defect in calcium uptake in intestinal brush border membranes of the SHR.