Effect of insulin, catecholamines and calcium ions on phospholipid metabolism in isolated white fat-cells

Abstract

The incorporation of [32P]Pi into phosphatidylinositol by rat fat-cells was markedly increased in the presence of adrenaline. Phosphatidic acid labelling was also increased, but to a lesser extent. These effects are due to α1-adrenergic stimulation since they were unaffected by propranolol, blocked by α-blockers in the potency order prazosin«phentolamine<yohimbine and mimicked by methoxamine. The α-adrenergic stimulation of phosphatidylinositol labelling did not require extracellular Ca2+, which supports the hypothesis that an increased turnover of phosphatidylinositol is involved in α-adrenergic activation of Ca2+ entry. Insulin and the ionophore A23187 gave a small increase in 32P labelling of phosphatidylinositol in Ca2+-free medium containing 1mm-EGTA. The increases due to insulin or ionophore A23187 were abolished if 2.5mm-Ca2+ was added to medium containing EGTA. However, the increases in labelling of phosphatidylinositol due to α-adrenergic amines were still evident in medium containing EGTA and Ca2+. Lipolytic agents such as corticotropin, dibutyryl cyclic AMP, adrenaline in the presence of phentolamine and isoproterenol decreased [32P]Pi incorporation into phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. This inhibitory effect may be secondary to accumulation of intracellular unesterified fatty acids, since it was decreased by incubating fewer cells in medium with 6 rather than 3% albumin and was restored by the addition of oleate to the medium. The incorporation of [32P]Pi into phosphatidylcholine was unaffected by lipolytic agents. The data suggest that there is an inhibition of the synthesis of certain phospholipids in the presence of lipolytic agents, which may be secondary to intracellular accumulation of unesterified fatty acids.