G-protein mediates voltage regulation of agonist binding to muscarinic receptors: effects on receptor-Na+ channel interaction

Our previous experiments in membranes prepared from rat heart and brain led us to suggest that the binding of agonists to the muscarinic receptors and to the Na+ channels is a coupled event mediated by guanine nucleotide binding protein(s) [G-protein(s)]. These in vitro findings prompted us to employ synaptoneurosomes from brain stem tissue to examine (i) the binding properties of [3H]acetylcholine at resting potential and under depolarization conditions in the absence and presence of pertussis toxin; (ii) the binding of [3H]batrachotoxin to Na+ channel(s) in the presence of the muscarinic agonists; and (iii) muscarinically induced 22Na+ uptake in the presence and absence of tetrodotoxin, which blocks Na+ channels. Our findings indicate that agonist binding to muscarinic receptors is voltage dependent, that this process is mediated by G-protein(s), and that muscarinic agonists induce opening of Na+ channels. The latter process persists even after pertussis toxin treatment, indicating that it is not likely to be mediated by pertussis toxin sensitive G-protein(s). The system with its three interacting components--receptor, G-protein, and Na+ channel--is such that at resting potential the muscarinic receptor induces opening of Na+ channels; this property may provide a possible physiological mechanism for the depolarization stimulus necessary for autoexcitation or repetitive firing in heart or brain tissues.