Angiotensin increases cytosolic free calcium in cultured vascular smooth muscle cells

We used the calcium-sensitive fluorescent indicator quin 2 to monitor changes in cytosolic free calcium concentration ([Ca2+]i) associated with angiotensin II receptor activation in cultured vascular smooth muscle cells isolated from rat aorta. Resting [Ca2+]i in unstimulated vascular smooth muscle cells was 198 +/- 7 nM. Angiotensin II induced concentration-dependent rapid increases in [Ca2+]i (threshold congruent to 10(-11) M; effective concentration, 50% congruent to 5 X 10(-10) M; maximum congruent to 10(-8) M); the rate of increase in [Ca2+]i also appeared to be concentration dependent. The angiotensin II-induced changes were completely blocked by the angiotensin II receptor antagonist [Sar1, Ile8]-angiotensin II. In the presence of extracellular calcium, 10(-8) M angiotensin II induced an increase in [Ca2+]i that reached peak values of five to six times the resting levels within 15 seconds, followed by a gradual decline to a plateau at two to three times the resting level. When EGTA was added to chelate external calcium, the angiotensin II-induced increases in peak [Ca2+]i were attenuated and the plateau phase was abolished. These data show that (1) quin 2 can be used in cultured vascular smooth muscle cells to study changes in calcium homeostasis induced by angiotensin II, and (2) angiotensin II acts on cultured vascular smooth muscle cells to cause a rapid increase in [Ca2+]i that appears to depend on both the mobilization of intracellular calcium and the influx of extracellular calcium.