The activity of Na–Ca-exchange and ATP-dependent Na and Ca transport by heart sarcolemmal membranes from male 3–4-week-old spontaneously hypertensive rats (SHR) and their normotensive controls (WKY) ...were compared. Differences in active Ca and Na transport between the two substrains were suppressed by addition of exogenous calmodulin. Calmodulin was active only in the presence of Ca. The rate of the Na-dependent Ca efflux, reflecting the activity of the Na–Ca exchange, was significantly higher in SHR than in WKY vesicles. An alteration of the intracellular calmodulin activity or content might thus be responsible for the modifications in Ca handling, and limit the activity of the Na pump in SHR membranes. The platelet cytosolic free Ca concentration of young SHR and WKY was measured by using the fluorescent indicator Quin-2/AM, In the absence or presence of added external Ca, no difference in the intracellular concentration of Ca was observed between platelets of either origin. The intraerythrocytic sodium content, measured by flame spectrophotometry, was similar in SHR and WKY. This study, performed before the onset of hypertension, shows that membrane mechanisms controlling the intracellular ion content were already modified, whereas the resulting intracellular concentrations remained within the normal range.
