Agonist specific effects of guanine nucleotides on muscarinic cholinergic receptors in rat anterior pituitary membranes.
The effects of guanine nucleotides on the binding affinity of muscarinic cholinergic receptors for muscarinic agents were studied in rat anterior pituitary membranes using direct ligand binding methods with [3H]quinuclidinyl benzylate, GTP and Gpp(NH)p at a concentration of 0.1 mM markedly decreased the binding affinity of the agonist, oxotremorine, for the receptors but had no effect on the binding of the antagonist, atropine. Mg2+ (1 mM) on the other had markedly increased the binding affinity of oxotremorine but not that of atropine. Thus, it is conceivable that the release of the growth hormone or the inhibition of prolactin release by acetylcholine, which we and others have previously shown, is modulated by the opposite actions of guanine nucleotides and divalent metal ions such as Mg2+.