PLASMA-MEMBRANE CALCIUM PUMP-MEDIATED CALCIUM EFFLUX AND BULK CYTOSOLIC-FREE CALCIUM IN CULTURED AORTIC SMOOTH-MUSCLE CELLS FROM SPONTANEOUSLY HYPERTENSIVE AND WISTAR-KYOTO NORMOTENSIVES RATS

Objective To compare the efficacy of the calcium pump-mediated calcium efflux pathway in spontaneously hypertensive rats (SHR) with that in Wistar-Kyoto normotensive rats (WKY), at rest and after angiotensin II stimulation, Design The intracellular free calcium concentration and calcium-45 efflux were measured in parallel, in cultured aortic smooth muscle cells isolated from 10-week-old male SHR and WKY rats, Methods The intracellular free calcium concentration and calcium-45 efflux we re studied in confluent vascular smooth muscle cells in culture, Exper iments were performed in the absence of added extracellular calcium an d sodium. Fura-2 was used to measure basal and angiotensin II-stimulat ed intracellular free calcium concentration. Effluxed calcium-45 was m easured over 55 intervals to determine basal and angiotensin II-stimul ated calcium efflux rates in SHR and in WKY rats, Results No significa nt difference between SHR and WKY rats was observed in basal intracell ular free calcium concentration or 100 nmol/l angiotensin II-stimulate d peak intracellular free calcium concentration. However, significantl y elevated basal and 100 nmol/l angiotensin II-stimulated calcium-45 e fflux rates were found in SHR. The calcium-45 efflux rates in SHR were elevated when the efflux was normalized with respect to the bulk intr acellular free calcium concentration. The time taken to reach the maxi mum calcium-45 efflux rate after angiotensin II stimulation was reduce d in SHR compared with that in WKY rats and was dose-dependent in both rat strains. Conclusion The calcium-pump mediated calcium efflux path way appears to be more efficient in SHR. This may be the result of pos t-translational modification, enhanced calcium pump sites in a critica l region of the membrane, or the presence of a pool of calcium near th e plasma membrane that is not readily detected by cytosolic Fura-2 but is higher in SHR both before and after angiotensin II stimulation.