Roles of insulin receptor substrates in insulin-induced stimulation of renal proximal bicarbonate absorption

Insulin resistance is frequently associated with hypertension, but the mechanism underlying this association remains speculative. Although insulin is known to modify renal tubular functions, little is known about roles of insulin receptor substrates (IRS) in the renal insulin actions. For clarifying these issues, the effects of insulin on the rate of bicarbonate absorption (JHCO ₃^-) were compared in isolated renal proximal tubules from wild-type, IRS1-deficient (IRS1^-/-), and IRS2-deficient (IRS2 ^-/-) mice. In wild-type mice, physiologic concentrations of insulin significantly increased JHCO₃^-. This stimulation was completely inhibited by wortmannin and LY-294002, indicating that the phosphatidylinositol 3-kinase pathway mediates the insulin action. The stimulatory effect of insulin on JHCO₃^- was completely preserved in IRS1^-/- mice but was significantly attenuated in IRS2^-/- mice. Similarly, insulin-induced Akt phosphorylation was preserved in IRS1^-/- mice but was markedly attenuated in IRS2 ^-/- mice. Furthermore, insulin-induced tyrosine phosphorylation of IRS2 was more prominent than that of IRS1. These results indicate that IRS2 plays a major role in the stimulation of renal proximal absorption by insulin. Because defects at the level of IRS1 may underlie at least some forms of insulin resistance, sodium retention, facilitated by hyperinsulinemia through the IRS1-independent pathway, could be an important factor in pathogenesis of hypertension in insulin resistance.