Calcium transport in basolateral plasma membranes from kidney cortex of Milan hypertensive rats.

Ca2+ transport was investigated in basolateral plasma membranes (BLM) isolated from kidney cortex of the Milan strain of genetically hypertensive rats (MHS) and their normotensive controls (MNS) during a pre-hypertensive stage (age 3-4 weeks). It was found that the Vmax of ATP-dependent Ca2+ transport (in the presence of calmodulin) was about 16% lower in MHS than in control rats. In membranes from MNS rats which had been isolated in the presence of EGTA, the ATP-dependent Ca2+ transport showed a hyperbolic Ca2+ concentration dependence, a high Km (Ca2+) and a low Vmax; upon addition of exogenous calmodulin, the kinetics became sigmoidal, the Km (Ca2+) was decreased and the Vmax was increased. In membranes from MHS rats, the Ca2+ concentration dependence of ATP-driven Ca2+ transport was sigmoidal and the Ca2+ affinity was high in the absence of added calmodulin. Addition of exogenous calmodulin to these membranes resulted in an increase in Vmax, but no change in other kinetic parameters. Low-affinity hyperbolic kinetics of Ca2+ transport could only be obtained in MHS rats if the membranes were extracted with hypotonic EDTA and hypertonic KCl. These data suggest that the plasma membrane Ca2+-ATPase, which catalyses the ATP-dependent Ca2+ transport, exists in BLM of pre-hypertensive MHS rats predominantly in an activated, high-affinity form.