1. Ca2+-adenosine 5′-triphosphatase (Ca2+ −Mg2+ – ATPase) activity was studied simultaneously in calmodulin-deficient erythrocyte ghost membranes and inside-out vesicles (IOVs) from 12-week-old female spontaneously hypertensive rats (SHR) and their matched controls: [Wistar-Kyoto normotensive rats (WKY)], and in detergent extracts of ghost membranes.
2. Both adenosine 5′-triphosphate (ATP)-dependent Ca2+ uptake by IOVs and Ca2+-dependent ATP hydrolysis activity of ghost membranes were reduced significantly in the SHR compared with WKY, when either the calmodulin-independent or calmodulin-stimulated activities were compared.
3. The ratios between Ca2+ uptake and ATP hydrolysis activities in the SHR remained approximately 1.0, showing a proportional reduction in both activities.
4. No difference in affinity for calmodulin was observed between SHR and WKY.
5. No significant difference in Ca2+-dependent ATP hydrolysis activity was observed between SHR and WKY after detergent solubilization of erythrocyte ghost membranes.
6. These results suggest that the number of Ca2+-Mg2+-ATPase units are similar in SHR and WKY and that the reduced activity in the intact SHR membrane is due to altered membrane environment.