Direct stimulation of phosphatidylinositol degradation by addition of vasopressin to purified rat liver plasma membranes

Rat liver plasma membranes were incubated in Ca2+-free buffer plus 0.5 mM ethyleneglycol-bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) containing 0.5 mg/ml deoxycholate or 50 mU/ml vasopressin or both compounds. The membrane phospholipids were extracted, separated by two-dimensional chromatography and quantitated by phosphorus analysis. Vasopressin significantly enhanced degradation of phosphatidylinositol in the presence of deoxycholate (21% with deoxycholate alone versus 32% in the presence of vasopressin plus deoxycholate). However, no significant effects of deoxycholate or vasopressin were seen on the degradation of phosphatidylcholine, phosphatidylethanolamine or phosphatidylserine. These results indicate that specific degradation of phosphatidylinositol can be seen after direct addition of vasopressin to a plasma membrane preparation.

Forskolin as an activator of cyclic AMP accumulation and lipolysis in rat adipocytes

Forskolin increased cyclic AMP accumulation in isolated adipocytes and markedly potentiated the elevation of cyclic AMP due to isoproterenol. In adipocyte membranes, forskolin stimulated adenylate cyclase activity at concentrations of 0.1 microM or greater. Forskolin did not affect the EC50 for activation of adenylate cyclase but did increase the maximal effect of isoproterenol. Neither the soluble nor particulate low-Km cyclic AMP phosphodiesterase activity was affected by forskolin. Low concentrations of forskolin (0.1-1.0 microM), which significantly elevated cyclic AMP levels, did not increase lipolysis, whereas similar increases in cyclic AMP levels due to isoproterenol elevated lipolysis. Forskolin did not inhibit the activation of triacylglycerol lipase by cyclic AMP-dependent protein kinase or the subsequent hydrolysis of triacylglycerol. Higher concentrations of forskolin (10-100 microM) did increase lipolysis. Both the increased cyclic AMP production and lipolysis due to forskolin were inhibited by the antilipolytic agents insulin and N6-(phenylisopropyl)adenosine. Hypothyroidism reduced the ability of forskolin to stimulate cyclic AMP production and lipolysis. These results indicate that forskolin increases cyclic AMP production in adipocytes through an activation of adenylate cyclase. Lipolysis is activated by forskolin but at higher concentrations of total cyclic AMP than for catecholamines.

Effects of melittin on adipocyte metabolism unrelated to lysophospholipid accumulation

Melittin addition to rat or hamster adipocytes resulted in inhibition of lipolysis, cyclic AMP accumulation and glucose oxidation. Low concentrations of melittin were not insulin-like with respect to either stimulation of glucose metabolism or inhibition of lipolysis. Higher concentrations of melittin lysed adipocytes. In the presence of melittin, cellular phospholipids were released to the medium and hydrolyzed with little accumulation of lysophospholipids. Only in adipocytes incubated with melittin contaminated with phospholipase A2 was any appreciable accumulation of lysophospholipids seen and this was in the medium. These data suggest that the toxic effects of melittin on adipocytes are not due to the accumulation of lysophospholipids but rather to the loss of membrane phospholipids or alterations in membrane proteins.