Abstract

Ca 2+ 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 V max of ATP-dependent Ca 2+ 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 Ca 2+ transport showed a hyperbolic Ca 2+ concentration dependence, a high K m (Ca 2+) and a low V max ; upon addition of exogenous calmodulin, the kinetics became sigmoidal, the K m (Ca 2+ was decreased and the V max was increased. In membranes from MHS rats, the Ca 2+ concentration dependence of ATP-driven Ca 2+ transport was sigmoidal and the Ca 2+ affinity was high in the absence of added calmodulin. Addition of exogenous calmodulin to these membranes resulted in an increase in V max , but no change in other kinetic parametrs. Low-affinity hyperbolic kinetics of Ca 2+ 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 Ca 2+-ATPase, which catalyses the ATP-dependent Ca 2+ transport, exists in BLM of pre-hypertensive MHS rats predominantly in an activated, high-affinity form.

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