A POSSIBLE MECHANISM FOR THE INSULIN STIMULATION OF ALANINE UPTAKE BYISOLATED RAT HEPATOCYTES

The mechanism underlying the insulin-induced stimulation of alanine up take by isolated rat hepatocytes was studied by testing the involvemen t of the Na+-K+ pump and the Na+-H+ exchanger. Insulin was found to in hibit the in vitro activity of the Na+-K+ ATPase in a liver homogenate . Ouabain, a specific inhibitor of the pump, enhanced alanine transpor t, as insulin did, and no additive effects were observed when it was a dded simultaneously with the hormone. When the membrane was depolarize d by sodium removal from the incubation medium or by an increase in it s K+ concentration, a stimulation of amino acid uptake was observed. A miloride, an inhibitor of the Na+-H+ transporter enhanced also alanine uptake. The increase in transport observed when amiloride and insulin were added together was not significantly different from the one prod uced by amiloride alone. The stimulatory effect of the drug and the ho rmone disappeared, however, in presence of bicarbonate or when the mem brane was hyperpolarized by potassium removal from the incubation medi um. It was concluded that depolarization per se, increased alanine tra nsport and that the effects of amiloride and insulin were mediated by intracellular acidification followed by membrane depolarization, which in rum activated the transporters. (C) 1997 Elsevier Science Inc.