Effects of guanine nucleotide and sulfhydryl reagent on subpopulations of muscarinic acetylcholine receptors in mammalian hearts: possible evidence for interconversion of super-high and low affinity agonist binding sites.

Multiple site models of muscarinic acetylcholine receptors (mAChR) for agonist binding were applied to curves for the inhibition of QNB binding by carbachol by using nonlinear least square regression analysis. The effects of a guanine nucleotide guanyl-5'-yl imidodiphosphate (Gpp(NH)p) and a sulfhydryl reagent 5,5'-dithiobis(2-nitrobenzoic acid (DTNB) on the curves were also analyzed. The results suggested that mAChR of dog and guinea pig heart had three types of sites with different affinities for carbachol (super-high (SH), high (H) and low (L]. In the presence of Gpp(NH)p, SH sites were eliminated and L sites increased, indicating conversion of SH sites to L sites. On the contrary, in the presence of DTNB, L sites were converted to SH sites. These results were obtained at both 37 degrees C and 0 degrees C incubation although the affinity of each site was high at 0 degrees C than at 37 degrees C. These data suggest the interconversion of SH and L sites. The possible existence of two subtypes (GTP-regulated mAChR(SH-L type) and GTP-independent mAChR (H type] is discussed.