Effect of Insulin, Catecholamines and Calcium Ions on Phospholipid Metabolism in Isolated White Fat-cells

The incorporation of [(32)P]P(i) 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 alpha(1)-adrenergic stimulation since they were unaffected by propranolol, blocked by alpha-blockers in the potency order prazosin<<phentolamine<yohimbine and mimicked by methoxamine. The alpha-adrenergic stimulation of phosphatidylinositol labelling did not require extracellular Ca(2+), which supports the hypothesis that an increased turnover of phosphatidylinositol is involved in alpha-adrenergic activation of Ca(2+) entry. Insulin and the ionophore A23187 gave a small increase in (32)P labelling of phosphatidylinositol in Ca(2+)-free medium containing 1mm-EGTA. The increases due to insulin or ionophore A23187 were abolished if 2.5mm-Ca(2+) was added to medium containing EGTA. However, the increases in labelling of phosphatidylinositol due to alpha-adrenergic amines were still evident in medium containing EGTA and Ca(2+). Lipolytic agents such as corticotropin, dibutyryl cyclic AMP, adrenaline in the presence of phentolamine and isoproterenol decreased [(32)P]P(i) 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 [(32)P]P(i) 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.