Abstract
The K d values of the multiple agonist binding sites in cardiac muscarinic receptors (mAChR) and pD 2 values for negative inotropic actions were determined independently and their relations was examined. The guinea-pig cardiac mAChR is known to have three agonist binding sites (super-high (SH), high (H) and low (L) affinity agonist binding sites) for carbachol (CCh). Pilocarpine (Pilo) and oxotremorine (Oxo) distinguished two sites (higher (H o/p) with pK d of 5.88 and 8.20, respectively, and lower (L o/p) affinity agonist binding sites with pK d of 5.08 and 6.17, respectively). The effects of guanine nucleotide and sulfhydryl reagent indicated that the H o/p site corresponded with the SH site for carbachol, and the L o/p site with the H + L sites for carbachol. The pD 2 values of cCh, Pilo and Oxo for negative inotropic actions on autocontraction of right atria were 5.38, 5.30 and 6.80, respectively. The pD 2 values of CCh and Oxo on electrically stimulated contraction of left atria in the presence of isoproterenol were 5.80 and 6.46, respectively, thus being closely related to H or L o/p agonist binding sites of mAChR
Published Version
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