Growth Hormone Promotes Ca2+-Induced Ca2+Release in Insulin-Secreting Cells by Ryanodine Receptor Tyrosine Phosphorylation

Elevation in cytoplasmic free Ca2+ concentration ([Ca2+]i) is a common mechanism in signaling events. An increased [Ca2+]i induced by GH, has been observed in relation to different cellular events. Little is known about the mechanism underlying the GH effect on Ca2+ handling. We have studied the molecular mechanisms underlying GH-induced rise in [Ca2+]i in BRIN-BD11 insulin-secreting cells. GH (500 ng/ml, 22 nm) induced a sustained increase in [Ca2+]i. The effect of GH on [Ca2+]i was prevented in the absence of extracellular Ca2+ and was inhibited by the ATP-sensitive K(+)-channel opener diazoxide and the voltage-dependent Ca(2+)-channel inhibitor nifedipine. However, GH failed to induce any changes in Ca2+ current and membrane potential, evaluated by patch-clamp recordings and by using voltage-sensitive dyes. When the intracellular Ca2+ pools had been depleted using the Ca(2+)-ATPase inhibitor thapsigargin, the effect of GH was inhibited. In addition, GH-stimulated rise in [Ca2+]i was completely abolished by ruthenium red, an inhibitor of mitochondrial Ca2+ transport, and caffeine. GH induced tyrosine phosphorylation of ryanodine receptors. The effect of GH on [Ca2+]i was completely blocked by the tyrosine kinase inhibitors genistein and lavendustin A. Interestingly, treatment of the cells with GH significantly enhanced K(+)-induced rise in [Ca2+]i. Hence, GH-stimulated rise in [Ca2+]i is dependent on extracellular Ca2+ and is mediated by Ca(2+)-induced Ca2+ release. This process is mediated by tyrosine phosphorylation of ryanodine receptors and may play a crucial role in physiological Ca2+ handling in insulin-secreting cells.