Interactions between the muscarinic receptors, sodium channels, and guanine nucleotide-binding protein(s) in rat atria.

The effects of the voltage-sensitive sodium channel activator batrachotoxin (BTX) on the binding properties of muscarinic receptors were studied in homogenates of rat atria. Also studied were the effects of muscarinic ligands on the binding of tritium-labeled batrachotoxin ([3H]BTX) to the same preparation. BTX (1 microM), which induces an open state in sodium channels, enhanced the affinity of binding of several agonists to the muscarinic receptors. Analysis of the data indicated that the effect of BTX was to increase the affinity of the agonists toward the high-affinity sites. Binding of antagonists was not affected by BTX. At higher concentrations of toxin, the density of the high affinity muscarinic sites was also affected. The binding of agonists (but not of antagonists) to muscarinic receptors in turn enhanced the specific binding of [3H]BTX to sodium channels. These effects on the muscarinic receptors and on the sodium channels were inhibited in the presence of Gpp(NH)p at concentrations lower than those bringing about conversion of binding sites from the high affinity to the low affinity conformation. On the basis of these findings we suggest that the opening of sodium channels and the binding of agonists to muscarinic receptors in rat atrial membranes are coupled events which are mediated by guanine nucleotide-binding protein(s). Such a hypothesis is consistent with previously proposed models for signal transduction in the membrane.