Stimulatory effect of insulin on renal proximal tubule sodium transport is preserved in type 2 diabetes with nephropathy

Our previous study indicates that hyperinsulinemia in metabolic syndrome in the absence of nephropathy may promote hypertension by stimulating renal proximal tubule (PT) sodium transport via insulin receptor substrate (IRS) 2/phosphoinositide 3-kinase pathway. In the present study we showed that the stimulatory effect of insulin on the Na(+)-HCO3(-) cotransporter NBCe1 in isolated PTs was completely preserved in type 2 diabetic rats with overt nephropathy. Furthermore, the IRS2 expression and insulin-induced Akt phosphorylation in kidney cortex were preserved in these rats. By contrast, the IRS1 expression in kidney cortex was markedly reduced, which might be relevant to enhanced renal gluconeogenesis consistently reported in diabetes. The stimulatory effect of insulin on NBCe1 was preserved also in a human type 2 diabetic patient with advanced nephropathy. These results revealed that insulin can stimulate PT sodium transport even in type 2 diabetes with overt nephropathy. In addition to hypoglycemia, insulin-induced renal sodium retention might also play a role in increased cardiovascular risk associated with intensive glycemic control in type 2 diabetic patients with nephropathy.