DIFFERENT MODULATION OF HEPATOCELLULAR NA+/H+ EXCHANGE ACTIVITY BY INSULIN AND EGF

Indirect evidence suggests that insulin, like epidermal growth factor (EGF), stimulates liver cell Na+/H+ exchange. We directly studied the effect of insulin on intracellular pH (pH(i)) and the Na+/H+ exchanger in isolated rat hepatocytes with the fluorescent probe, 2',7'-bis(2-carboxyethyl)-5 (6)-carboxyfluorescein (BCECF). The effects of insulin were compared with those of EGF. Studies were carried out in the absence of HCO3- and in the presence of acetazolamide to isolate the Na+/H+ exchanger from other pH regulatory mechanisms. Insulin (9 nM) caused a reversible acidification of baseline pH(i), whereas no significant effect was observed with EGF (30 nM). pH(i) was acidified by two different methods (NH4C] pulse and external Na+ removal) to assess liver cell Na+/H+ exchange activity. In the NH4Cl pulse experiments, insulin had no significant effect on the Na+/H+ exchanger compared with the control (0.141 +/- 0.009 pH units/min, n = 14, and 0.122 +/- 0.023 pH units/min, n = 16, respectively). However, in the same conditions, EGF nearly doubled the rate of the Na+/H+ exchange activity (0.193 +/- 0.015 pH units/min, P < 0.05, n = 6). In the Na+ removal experiments, EGF again significantly increased the pH(i) recovery rate (0.542 +/- 0.032 pH units/min, n = 3) compared with the control (0.227 +/- 0.028 pH units/min, n = 5) and insulin (0.245 +/- 0.053 pH units/min, n = 5). Compared with control conditions, a subchronic administration of insulin (9 nM) in vitro had no significant effect on the Na+/H+ exchanger, nor did it affect baseline pH(i). Results show that insulin, unlike EGF, does not stimulate liver cell Na+/H+ exchange, suggesting different modulation of the membrane transporter by different classes of receptor tyrosine kinase.