Acceleration of the adipocyte aden: by inhibitory hormonal factors (antilipolytic agents/cyclic AMP/guanine nucleotide site/guanosine t

Adrenergic agonists such as isoproterenol stimulate hamster adipocyte adenylate cyclase by a GTP-depen- dent process, whereas prostaglandin E1, a-adrenergic agonists, and nicotinic acid inhibit the enzyme by a process also dependent on GTP and amplified by sodium ions. We have determined the first-order rate constant describing the decay of isoproterenol plus GTP-stimulated adenylate cyclase and the modulation of this off rate constant by sodium and inhibitory hormonal factors. With 1 IxM GTP and 0.2 mM isoproterenol, the off rate constant was 5.0 min-1 at 25'C. Addition of NaCl (100 mM), which increased basal and isoproterenol-stimulated cyclase activities, decreased the rate constant of the hormone-stimulated enzyme to 1.4 min-1. Pros- taglandin E1 (10 ,M) and nicotinic acid (30 /tM), which decreased basal and hormonally stimulated enzyme activities, increased the NaCl-suppressed off rate constant to 6.1 and 6.0 min-1, respec- tively. Similar data were obtained with 1 mM isoproterenol with MnATP and MgATP as the adenylate cyclase substrate. On the other hand, the turn-on reaction of adipocyte adenylate cyclase, measured with the stable GTP analogue 5'-guanylyl imidodiphos- phate (30 jiM), was accelerated by isoproterenol (1 mM) and NaCl (100 mM). Under all of these conditions, inhibitory hormonal agents did not cause any delay in the turn-on reaction. These data indicate that, in hamster adipocyte membranes, inhibitory hor- monal factors inhibit adenylate cyclase by increasing the enzyme's turn-off reaction.