Cytosolic free calcium levels in monolayers of cultured rat aortic smooth muscle cells. Effects of angiotensin II and vasopressin.

A rise in cytosolic free calcium (Ca2+i) is thought to be the principal mediator in vascular smooth muscle contraction. Quantitative changes of Ca2+i in response to two vasoconstrictor peptide hormones, angiotensin II and vasopressin, were directly measured in monolayers of adherent cultured rat aortic smooth muscle cells loaded with the fluorescent calcium indicator Quin 2. Angiotensin II induced rapid, concentration-dependent rises in Ca2+i from 1.53 +/- 0.27 X 10(-7) (n = 16) up to 1.2 X 10(-6) M, with ED50 of 0.45 X 10(-9) M, an effect which was blocked by the antagonist analogue Sar1, Ala8angiotensin II. Vasopressin also elicited transient rises in Ca2+i to peak levels of about 8 X 10(-7) M, with ED50 of 1.05 X 10(-9) M, and this response was completely abolished by a vasopressor antagonist. In calcium-free medium, basal Ca2+i levels fell to 0.92 +/- 0.24 X 10(-7) M (n = 4), and both hormones were still able to raise Ca2+i, although to a lesser extent. Readdition of extracellular calcium following the Ca2+i transient induced a second, slower Ca2+i rise. In calcium-containing medium, lanthanum ion (2 X 10(-5) M) reduced peptide-evoked Ca2+i rises to the values observed in calcium-free medium. Stimulation with each peptide completely desensitized the smooth muscle cells to a subsequent identical challenge, with little crosstachyphylaxis. Potassium ion (50 mM) only minimally affected Ca2+i levels. The calcium channel blocker nifedipine (10(-6) M) did not prevent the Ca2+i rises induced by angiotensin II, vasopressin, or potassium. These findings indicate that the two physiologically important vasoconstrictor hormones angiotensin II and vasopressin rapidly raise Ca2+i in cultured vascular smooth muscle cells, in part by mobilizing calcium from intracellular pools and in part through activation of receptor-operated calcium channels.