Novel Effect of Insulin: Insulin-Stimulated Na+Transport Is Mediated by Hydrolysis of Phosphoinositides

Abstract Previous studies showed that insulin stimulation of electrogenic Na + transport in renal epithelial cells is mediated by a calcium-dependent signal transduction mechanism. The present study was performed to determine whether the insulin-induced increase in intracellular Ca 2+ (Ca 2+ i ) was mediated by hydrolysis of phosphatidylinositol and release of inositol trisphosphate. Experiments were conducted with cultured A6 cells, derived from Xenopus laevis, grown on permeable supports. Addition of insulin resulted in 2 to 3 fold increases in inositol trisphosphate and a 50% increase in 1,2 diacylglycerol within 10s, which corresponded to the time-course, previously reported, of insulin stimulated increases in Na + transport and Ca 2+ i . Further studies showed that aldosterone, previously shown to stimulate an increase in 1,4,5-inositol trisphosphate at onset of the rise in Na + transport, also increased DAG levels during the initial phase of stimulation of Na + transport. These studies provide the first evidence that a biological response induced by insulin is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) which results in two products, inositol trisphosphate which causes the release of Ca 2+ from intracellular stores and 1,2 diacylglycerol. In addition this study provides further support for the proposal that a common signal transduction mechanism mediates electrogenic Na + transport by multiple agonists.