Adenylate cyclase modulation by ammonium ion: GTP-like effect on muscarinic and alpha 2-adrenergic receptors.

The stimulatory influence of ammonium sulphate on adenylate cyclase activity has been investigated. By competition binding experiments on the beta-adrenergic stimulatory receptor in rat myocardial membranes, no influence could be detected of ammonium sulphate neither in receptor coupling to the stimulatory guanine nucleotide binding protein nor in the GTP-induced uncoupling. In order to detect an impaired inhibition instead of an increased stimulation of adenylate cyclase activity by ammonium sulphate the investigation was extended to inhibitory receptors. The same type of effect by ammonium sulphate was detected on both the muscarinic cholinergic receptor in rat myocardial membranes as well as on the alpha 2-adrenergic receptor in human platelets. The influence of ammonium sulphate noted in competition binding studies and off-kinetics experiments was GTP-like, i.e. causing a decrease in agonist-receptor affinity leaving all the inhibitory receptors in the low affinity state. In conclusion, this paper indicates that the observed stimulatory effect of ammonium sulphate is exerted by the ammonium ion on the inhibitory guanine nucleotide binding protein, impairing the negative control of adenylate cyclase activity.