Multiple sites of action of ethanol on adenylate cyclase

The mechanisms through which ethanol increases basal adenylate cyclase activity and synergistically increases hormone-stimulated activity have been investigated. In membranes prepared from wild-type and UNC S49 lymphoma cells, ethanol caused a dose-related increase in adenylate cyclase activity stimulated by GTP, NaF, MnCl2 or isoproterenol. An effect of ethanol on adenylate cyclase activity in CYC- cells was only observed in the presence of MnCl2. In striatal tissue, ethanol (300 mM) elicited a 22% increase in maximum activity (Vmax) under basal conditions, but increased the Vmax of the dopamine-stimulated enzyme by 71%. Ethanol did not alter the Km of the enzyme for Mg-ATP or the concentration of GTP or free magnesium ion required for half-maximal activation. Activation of striatal adenylate cyclase by guanyl-5'-yl-imidodiphosphate occurred after a characteristic lag and inclusion of ethanol decreased the T1/2 of activation by guanyl-5'-yl-imidodiphosphate from 16.9 to 11.2 min. Stimulation of adenylate cyclase by dopamine was abolished by high concentrations of magnesium, but ethanol (300 mM) still increased the Vmax of the enzyme by 54%. Ethanol also increased adenylate cyclase activity after pretreatment of striatal membranes with cholera toxin. The data suggest that at least part of the activation of adenylate cyclase by ethanol involves an increase in the enzymatic activity of the catalytic subunit. However, because an increase in the turnover number of the enzyme would not explain the synergistic interaction between ethanol and dopamine, a second site of action, possibly on the rate of activation of the regulatory subunit or on the coupling of the catalytic and regulatory subunits, is also suggested.