Inhibition of ethanol-induced inositol phosphate formation and Ca(2+) mobilization by okadaic acid in rat hepatocytes: evidence for a role of protein phosphatases in the modulation of phospholipase C by ethanol.

The effect of the protein phosphatase inhibitor okadaic acid on phospholipase C (PLC)-linked signal transduction processes was investigated in intact hepatocytes. A short (5 min) pretreatment of the hepatocytes with okadaic acid (1 mu M) markedly inhibited a subsequent stimulation of PLC by ethanol as well as by receptor-mediated stimuli (vasopressin and phenylephrine). Okadaic acid inhibited the agonist-induced hydrolysis of polyphosphoinositides, the accumulation of inositol trisphosphate (InsP(3)) and the increase in cytosolic Ca(2+) concentrations. The inhibition could be overcome by high concentrations of vasopressin or ethanol, but only partly so with phenylephrine. A comparison of the sensitivity of different agonists at similar rates of InsP(3) accumulation and Ca(2+) mobilization indicated that ethanol-induced PLC activation was more resistant to the effects of okadaic acid than the hormonal agonists. Moreover, the stimulation of PtdInsP kinase by ethanol, which accompanies PLC activation, was refractory to okadaic acid treatment. These findings suggest that receptor-mediated PLC activation is subject to multiple controls by phosphorylation-dephosphorylation, not all of which affect the actions of ethanol on this signal transduction system.