Phosphatidylinositol breakdown induced by vasopressin and epinephrine in hepatocytes is calcium-dependent.

The hormonal regulation of phosphatidylinositol breakdown in isolated rat hepatocytes was studied using isotopic and chemical means and showed that the effect of epinephrine was mediated by al-adrenergic receptors.

Abstract

The hormonal regulation of phosphatidylinositol breakdown in isolated rat hepatocytes was studied using isotopic and chemical means.Phospholipids extracted from hepatocytes prepared from rats injected 18-20 h previously with [3H]myo-inositol showed radioactivity almost exclusively confined to phosphatidylinositol. Subsequent incubation of these hepatocytes resulted in the release of trichloroacetic acid-soluble radioactivity, 95% of which was shown to be myo-inositol.M) angiotensin I1 (lo-' M), and ATP to M) stimulated the release of trichloroacetic acid-soluble radioactivity, whereas glucagon M), the ionophore A23187 (lo-' M), and the uncouplers 2,4-dinitrophenol M) and carbonyl cyanide m-chlorophenylhydrazone M) were without effect.Studies with prazosin and yohimbine showed that the effect of epinephrine was mediated by al-adrenergic receptors.Vasopressin and epinephrine were also shown to cause the release of myo-inositol which was identified by gas-liquid chromatography/mass spectrometry and measured by gas-liquid chromatography.Both hormones produced no significant changes before 5 min, then caused a rapid release for 5 min, followed by a slower release through 45 min.At 30and 45 min, there were associated decreases in lipid radioactivity.Measurements of phospholipids by high performance liquid chromatography showed a decrease in phosphatidylinositol at 30 min with vasopressin.Depletion of Ca2+ in hepatocytes using ethylene glycol bis(&aminoethyl ether)-N,iV,N',N'-tetraacetic acid completely abolished the effects of vasopressin, epinephrine, angiotensin 11, and ATP on phosphatidylinositol breakdown, whereas readdition of Ca2+ rapidly restored these effects.Depletion and repletion of Ca2+ alone did not alter this breakdown.Epinephrine, angiotensin 11, and ATP were less effective than vasopressin.Glucagon did not modify the effects of vasopressin and epinephrine, but A23187 and 2,4-dinitrophenol were inhibitory.Two major conclusions are drawn from these data: a) the breakdown of phosphatidylinositol induced by vasopressin and epinephrine in hepatocytes is too slow to be responsible for the changes in cell Ca2+ which