Insulin treatment enhances AT1 receptor function in OK cells

Increased renal sodium retention is considered a major risk factor contributing to hypertension associated with chronic hyperinsulinemia and obesity. However, the molecular mechanism involved is not understood. The present study investigates the effect of insulin treatment on AT(1) receptor expression and ANG II-induced stimulation of Na/H exchanger (NHE) and Na-K-ATPase (NKA) in opossum kidney (OK) cells, a proximal tubule cell line. The presence of the AT(1) receptors in OK cells was confirmed by the specific binding of I-125-sar-ANG II and by detecting similar to 43-kDa protein on Western blot analysis with AT(1) receptor antibody and blocking peptide as well as by expression of AT(1) receptor mRNA as determined by RT-PCR. Insulin treatment (100 nM for 24 h) caused an increase in I-125-sar-ANG II binding, AT(1) receptor protein content, and mRNA levels. The whole cell lysate and membrane showed similar insulin-induced increase in the AT(1) receptor protein expression, which was blocked by genistein (100 nM), a tyrosine kinase inhibitor, and cycloheximide (1.5 mu g/ml), a protein synthesis inhibitor. Determination of ethyl isopropyl amiloride-sensitive Na-22(+) uptake, a measure of the NHE activity, revealed that ANG II (1-100 pM)-induced stimulation of NHE in insulin-treated cells was significantly greater than in the control cells. Similarly, ANG II (1-100 pM)-induced stimulation of ouabain-sensitive Rb-86(+) uptake, a measure of NKA activity in insulin-treated cells, was significantly greater than in the control cells. ANG II stimulation of both the transporters was blocked by AT(1) receptor antagonist losartan, suggesting the involvement of AT(1) receptors. Thus chronic insulin treatment causes upregulation of AT(1) receptors, which evoked ANG II-induced stimulation of NHE and NKA. We propose that insulin-induced increase in the renal AT(1) receptor function serves as a mechanism responsible for the increased renal sodium reabsorption and thus may contribute to development of hypertension in conditions associated with hyperinsulinemia.