Micromolar calcium decreases affinity of inositol trisphosphate receptor in vascular smooth muscle

The mechanism by which Ca2+ inhibits InsP3-induced Ca2+ release from sarcoplasmic reticulum of vascular smooth muscle was investigated. InsP3 binding to sarcoplasmic-reticulum vesicles from dog aortic smooth muscle was inhibited by 51 +/- 6% by 2 microM Ca2+ in the presence of 10 nM [3H]InsP3. Scatchard analysis indicated the presence of two InsP3-binding sites in the absence of Ca2+ (Kd = 2.5 +/- 0.9 and 49 +/- 8 nM InsP3), though the low-affinity site was more prevalent (representing 92 +/- 3% of the total number of binding sites). Ca2+ (2 microM) did not alter InsP3 binding to the high-affinity site (P > 0.05), but increased the Kd of the low-affinity site 3-fold (Kd = 155 +/- 4 nM InsP3; P < 0.001). The possibility that the apparent decrease in InsP3 affinity was caused by Ca(2+)-dependent activation of an endogenous phospholipase C could be excluded, because the Ca(2+)-dependent inhibition of InsP3 binding was completely reversible and insensitive to an inhibitor of phospholipase C. Moreover, Ca2+ did not inhibit InsP3 binding to InsP3 receptor partially purified by heparin-Sepharose chromatography, though another fraction (devoid of InsP3 receptor) restored Ca(2+)-sensitivity of the partially purified InsP3 receptor. Thus Ca2+ binding to a Ca(2+)-sensitizing factor associated with the InsP3 receptor decreases the affinity of the receptor complex for InsP3. This Ca(2+)-sensitizing factor may provide a negative-feedback mechanism for regulating the rise in cytosolic Ca2+ concentration in vascular smooth muscle after hormone activation of the phosphoinositide cascade.