| Abstract: | Human astrocytoma cells (EH118MG) respond to catecholamines and prostaglandins with a marked increase in the rate of formation of cyclic AMP. Treatment of EH118MG cells with cholera toxin (10 to 100 ng/ml) for 45 to 60 min caused an increase in cellular cyclic AMP content (5- to 10-fold over basal). Cholera toxin also decreased the K0.5 for isoproterenol 10- to 50-fold and decreased the K0.5 for prostaglandin E1 (PGE1)30- to 100-fold, while increasing the maximal response to PGE1 by 1.5- to 3-fold. Treatment with cholera toxin did not change the K1 values for beta-adrenergic receptor antagonists such as propranolol, alprenolol, and sotalol. Direct binding studies using [125I]iodohydroxybenzylpindolol indicated no significant changes in the number of beta-receptors or in the kinetics of the interaction of the radioligand with receptors after treatment of cells with the toxin. Competition binding studies with propranolol and sotalol revealed no toxin-induced change in Kd values for these antagonists. Treatment with cholera toxin caused only small decreases (2- to 3-fold) in the Kd values for binding of isoproterenol and norepinephrine. It is concluded that cholera toxin has little direct effect on the binding of agonists or antagonists to beta-receptors, but instead increases the efficiency of coupling of receptor and catalytic moieties of adenylate cyclase. |
