Intestinal Calcium Transport in Spontaneously Hypertensive Rats (SHR) and Their Genetically Matched WKY Rats

Abstract

Calcium transport across the basolateral membranes of the enterocyte represents the active step in calcium translocation. This step occurs by two mechanisms, an ATP-dependent pump and a Ca2+/Na+ exchange process. These studies were designed to investigate these two processes in jejunal basolateral membrane vesicles (BLMV) of the spontaneously hypertensive rats (SHR) and their genetically matched controls, Wistar-Kyoto (WKY) rats. The ATP-dependent calcium uptake was stimulated several-fold compared with no ATP condition in both SHR and WKY, but no differences were noted between rate of calcium uptake in SHR and WKY. Kinetics of ATP-dependent calcium uptake at concentrations between 0.01 and 1.0 microM revealed a Vmax of 0.67 +/- 0.03 nmol/mg protein/20 sec and a Km of 0.2 +/- 0.03 microM in SHR and Vmax of 0.69 +/- 0.12 and a Km of 0.32 +/- 0.14 microM in WKY rats. Ca2+/Na+ exchange in jejunal BLMV of SHR and WKY was investigated in two ways. First, sodium was added to the incubation medium (cis-Na+). Second, Ca2+ efflux from BLMV was studied in the presence of extravesicular Na+ (trans-Na+). Both studies suggest a decreased exchange of calcium and Na+. Kinetic parameters of Na(+)-dependent Ca2+ uptake at concentrations between 0.01 and 1.0 microM exhibited Vmax of 0.05 +/- 0.01 nanmol/mg protein/5 sec and a Km of 0.21 +/- 0.13 microM in SHR and Vmax of 0.11 +/- 0.02 nanmol/mg protein/5 sec and a Km of 0.09 +/- 0.05 in WKY, respectively. These results confirm that the intestinal BLMV of SHR and WKY rats have two mechanisms for calcium extrusion, an ATP-dependent Ca2+ transport process and a Na+/Ca2+ exchange process. The ATP-dependent process appears to be functional in SHR; however, the Ca2+/Na+ exchange mechanism appears to have a marked decrease in its maximal capacity. These findings suggest that calcium extrusion via Ca2+/Na+ is impaired in the SHR, which may lead to an increase in intracellular calcium concentration. These findings may have relevance to the development of hypertension.

Reversal of vinblastine transport by chlorpromazine in membrane vesicles from multidrug-resistant human CCRF-CEM leukaemia cells

“…Calcium uptake by membrane vesicles was measured essentially as described by Shibata and Ghishan (1990 (1965) where the acceptor (P-glycoprotein) has two independent ligand (vinblastine)-binding sites (see Scheme 1).…”

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

“…Enzymes and substrates were obtained from Sigma Chemical Co. Cellulose nitrate filters were obtained from Sartorius Filters (Hayward, CA). Blood pressure measurements were carried out as described previously by us ( 17). Briefly, a tail cuff was used, which is connected to a recorder in adult rats, whereas a carotid cannula was used in the suckling rats.…”

Magnesium Uptake by Intestinal Brush-Border Membranes of Spontaneously Hypertensive Rats

“…However, a reduced uptake of Ca2+ by duodenal mucosa (1 l), by isolated duodenal enterocytes (8), and by duodenal and jejunal brush-border membranes of SHR ( 12,13), together with an impaired Ca2+ extrusion via the Ca2+/ Na+ exchanger by jejunal basolateral membranes, has been evidenced in SHR, the ATP-dependent pump being unaffected (10). A lower intracellular Ca2+ concentration found in duodenal cells of young SHR (8) and in jejunal cells of adult SHR (14) also points to an alteration of Ca2+ handling by the SHR intestine.…”