Multiple effects of guanosine triphosphate on beta adrenergic receptors and adenylate cyclase activity in rat heart, lung and brain
Hegstrand, L.R.; Minneman, K.P.; Molinoff, P.B.
Journal of Pharmacology and Experimental Therapeutics 210(2): 215-221
1979
ISSN/ISBN: 0022-3565 PMID: 222893 Document Number: 140252
The effects of GTP on .beta.-adrenergic receptors and adenylate cyclase activity were examined in homogenates of heart and lung and in 5 regions of rat brain. The inhibition by catecholamine agonists [epinephrine, norepinephrine and isoproterenol] of specific [125I]iodohydroxybenzylpindolol (IHYP) binding to .beta.1-adrenergic receptors in rat heart and .beta.2-receptors in rat lung showed an apparent negative cooperativity with Hill coefficients of 0.5-0.7. High concentrations of GTP (300 .mu.M) caused a 5-fold decrease in the potency of these agonists as measured by inhibition of specific IHYP binding and increased the Hill coefficients toward 1.0. In rat cerebellum homogenates, GTP also decreased the affinity and increased the Hill coefficient for the inhibition of specific IHYP binding by isoproterenol. Neither of these effects was observed in the 4 other brain regions studied (cortex, caudate, hippocampus and diencephalon). In these tissues GTP did not cause a change in the Kd value for isoproterenol and the Hill coefficients were close to 1.0 either in the presence or absence of GTP. This difference does not appear to be due to the persistence of GTP in the washed membrane fractions used for these experiments. Although GTP was required for stimulation of adenylate cyclase activity by isoproterenol in membranes prepared from rat heart, it had no effect on basal enzyme activity. In rat lung GTP markedly increased basal activity but was not necessary for isoproterenol stimulation. Isoproterenol caused a significant stimulation of adenylate cyclase activity in hypotonic homogenates of the cerebellum, but it had little or no effect in cortex, caudate, hippocampus or diencephalon. GTP can apparently affect either .beta.1- or .beta.2-adrenergic receptors. Its effects on binding of agonists to .beta.-adrenergic receptors, on basal adenylate cyclase activity and on the coupling of the .beta.-adrenergic receptor to adenylate cyclase appear to occur independently.