Effects of halothane on high affinity agonist binding and guanine nucleotide sensitivity of muscarinic acetylcholine receptors from brainstem of rat

The influence of halothane on muscarinic receptors with a high affinity for agonists was studied using [ 3H]oxotremorine-M. [ 3H]Oxotremorine-M bound with high affinity ( K D = 2.8 nM) to a subpopulation of muscarinic receptors in the brainstem of rat, representing 32% of the total receptor pool. Agonist affinity for binding sites for [ 3H]oxotremorine-M was not affected by a guanine nucleotide (5'-guanylylimidodidiphosphate; Gpp(NH)p), although the level of binding was decreased, presumably due to the conversion of receptors to lower affinity conformations. However, only 58% of 3 nM binding of [ 3H]oxotremorine-M was sensitive to Gpp(NH)p. Halothane had two effects on the binding of [ 3H]oxotremorine-M: halothane (1) decreased the level of binding of [ 3H]oxotremorine-M without affecting agonist affinity for the surviving sites, and (2) lowered the sensitivity of the binding of [ 3H]oxotremorine-M to Gpp(NH)p by a factor of 120. The decrease in binding of [ 3H]oxotremorine-M binding was nonselective with regard to the sensitivity of the receptors to the guanine nucleotide, insofar as Gpp(NH)p inhibited the binding of [ 3H]oxotremorine-M to the same extent in the presence and absence of halothane. These results suggest that halothane (1) converts both G protein-coupled and -uncoupled muscarinic receptors to states of lower agonist affinity and (2) lowers the affinity of receptor-G protein complexes for guanine nucleotides.