Reciprocal modulation of agonist and antagonist binding to muscarinic cholinergic receptor by guanine nucleotide.

The ability of guanine nucleotide to decrease the binding affinity of agonists but not antagonists has been documented in a number of hormone and neurotransmitter receptor systems. By contrast, recent reports indicate that both agonist and antagonist binding to the muscarinic cholinergic receptors appear to be regulated in a reciprocal fashion by guanine nucleotide. We document two forms of the muscarinic cholinergic receptor in frog heart, which are present in approximately equal proportions and which display high-agonist/low antagonist and low-agonist/high-antagonist affinities, respectively. Guanine nucleotide appears to convert the former type of site into the latter type. These observations can be interpreted in terms of a model for two interconvertible forms of the muscarinic cholinergic receptor reciprocally favored by agonists and antagonists. This model has implications both for the understanding of neurotransmitter-receptor interactions generally and for the nature of the biological effects of receptor antagonists.