Extracellular Na+ dependence of changes in free Ca2+, 45Ca2+ efflux, and total cell Ca2+ produced by angiotensin II in cultured arterial muscle cells

Abstract

Angiotensin II (ANG II) evoked a rapid efflux of 45Ca2+ which was largely, but not completely, prevented by the removal of extracellular Na+. ANG II had little, if any, effect on a fast component of 45Ca2+ uptake, although the hormone increased a slow component of 45Ca2+ uptake by 2.5-fold. ANG II decreased total cell Ca2+ by about 40% within 1 min of hormone addition. The replacement of extracellular Na+ with N-methyl-D-glucamine or K+ nearly prevented ANG II from decreasing total cell Ca2+. ANG II caused a 5-fold increase in free Ca2+ which reached a peak about 10 s after hormone addition. Free Ca2+ decreased rapidly from the peak to a plateau phase which lasted for several min. Free Ca2+ during the plateau phase was about 1.5 times the basal level. Removing extracellular Na+ caused a more than 2-fold increase in the free Ca2+ concentration that was maintained during the plateau phase, and Na+, added during the plateau phase, rapidly decreased free Ca2+. These findings suggest that the transient nature of the increase in free Ca2+ produced by ANG II is due in part to a net efflux of Ca2+ which is largely dependent on external Na+ and probably mediated by Na+/Ca2+ exchange.