Insulin attenuates agonist-mediated calcium mobilization in cultured rat vascular smooth muscle cells

Insulin has been shown to attenuate pressor-induced vascular contraction, but the mechanism for this vasodilatory action is unknown. This study examines the effect of insulin on angioten-sin II (ANG II)-induced increments in cytosolic calcium in cultured rat vascular smooth muscle cells (VSMC). 20-min incubations with insulin (10,U/ml to 100 mU/ml) did not alter basal intracellular calcium concentration (ICa2I,), but inhibited the response to 100 nM ANGII in a dose-dependent manner (ANG II alone, 721±54 vs. ANG II+ 100 mU/ml insulin, 315±35 nM, P < 0.01). A similar effect of insulin on ANGII action was observed in calcium poor buffer. Moreover, insulin did not effect calcium influx. ANG II receptor density and affinity were not affected by 24-h incubation with insulin. To further clarify the mechanisms of these observations, we measured ANG II-induced production of inositol 1,4,5-triphos-phate (IP3), and IP3-releasable 45Ca. Insulin treatment did not alter ANG Il-stimulated IP3 production. However, IP3-stimu-lated release of45Ca in digitonin permeabilized cells was signifi-cantly reduced after 5-min incubations with 100 mU/ml insu-lin. Thapsigargin induced release of calcium stores was also blocked by insulin. Thus, insulin attenuates ANG 11-stimulated ICa2"I, primarily by altering IP3-releasable calcium stores. In-sulin effects on ANG II-induced ICa2`Ii were mimicked by preincubation of VSMC with either sodium nitroprusside or 8-bromo-cGMP. As elevations in cGMP in vascular tissue lower ICa2I1j, it is possible that insulin affects IP3 release of calcium by a cGMP-dependent mechanism that would contrib-ute to its vasodilatory effects. (J. Clin. Invest. 1993. 92:1161-1167.) Key words: cyclic nucleotides * hypotensive * inositol triphosphate * intracellular calcium stores * signal transductio