Calcium is rapidly exchangeable in cultured vascular smooth muscle cells from rabbit aorta

Abstract

Calcium exchange was measured in enzymatically isolated, cultured smooth muscle cells from the rabbit thoracic aorta. Cells were grown to confluence in monolayer on scintillator discs which were then inserted into a modified flow cell/spectrometer to measure 45Ca exchange in a continuous, on-line manner. This also allowed us to measure rapid movements of cellular Ca 2+ without solubilizing the cells. Two components of Ca 2+ exchange were detected: a La 3+-displaceable, rapidly exchangeable fraction ( t 1 2 = 15.6 s ) and a slowly exchangeable fraction ( t 1 2 = 34.5 min ). The La 3+ displaceable, rapidly exchangeable Ca 2+ fraction represented 57 to 61% of the total exchangeable Ca 2+. Multiple passaging of cells did not alter the Ca 2+ flux characteristics. Low Na + perfusion increased smooth muscle cell Ca 2+ flux by 6.71 mmol/kg dry weight. This increase was localized to both the rapidly and slowly exchangeable Ca 2+ compartments. Ouabain, an inhibitor of the plasmalemmal Na +-K + pump, also increased net uptake of 45Ca. The results demonstrate that approximately 60% of exchangeable Ca 2+ of smooth muscle cells is very rapidly exchangeable ( t 1 2 < 16 s ). The results stress the importance of measuring 45Ca 2+ movements in an on-line, continuous manner in order to ensure detection of the majority of total exchangeable Ca 2+ in smooth muscle cells.