The inhibitory GTP-binding protein (Gi) regulates the agonistic property of beta-adrenergic ligands in isolated rat adipocytes. Evidence for a priming effect of cyclic AMP.

Prenalterol, an allegedly beta 1-selective adrenergic agonist with high intrinsic sympathomimetic activity (ISA), was shown to be weakly lipolytic in rat adipocytes. However, in pertussis-toxin-treated adipocytes, the ISA of prenalterol was markedly increased (from 10-20% to approx. 100% of that of isoprenaline). The cellular sensitivity was also increased (EC50 approx. 60 nM and approx. 3 microM in pertussis-toxin-treated and control cells respectively). A similar effect was seen for other partial agonists such as the beta 2-selective agonist terbutaline and for beta-adrenergic antagonists with some intrinsic activity (metoprolol, pindolol). There was no clear change in sensitivity to isoprenaline's ability to stimulate adenylate cyclase in adipocyte membranes from pertussis-toxin-treated animals but the cyclase activity was increased approx. 4-fold in the presence of 1 microM-GTP. Prenalterol stimulated lipolysis by only small increases in intracellular cyclic AMP (cAMP) levels (less than 10% of that seen with isoprenaline). Basal lipolysis was increased in cells from pertussis-toxin-treated rats and the cellular sensitivity to the non-degradable cAMP analogue, N6-monobutyryl-cAMP, was increased. In control cells, a submaximal concentration of prenalterol (0.1 microM) increased the sensitivity to the cAMP analogues, N6-monobutyryl-cAMP and 8-bromo-cAMP. A low concentration (1 mM) of 8-bromo-cAMP also increased the effect of prenalterol. Similar effects were seen when the phosphodiesterase was inhibited. Thus (1) lipolysis is extremely sensitive to small increases in intracellular cAMP; (2) the degree of activation of adenylate cyclase and thus cAMP formation is the rate-limiting step for the biological response of partial agonists; (3) the inhibitory GTP-binding protein, Gi, is an important modulator ('tissue factor') of the beta-adrenergic agonistic property; (4) low levels of cAMP exert a priming effect on protein kinase A.