The SH-H subgroup of cardiac M2 receptors (M2 alpha) inhibits adenylate cyclase activity.

Muscarinic receptors in the guinea-pig heart seem to consist entirely of M2 receptors, but are coupled with several responses including inhibition of adenylate cyclase activity. On the other hand three affinity states (SH, H and L) can be distinguished in cardiac membranes with muscarinic agonists such as carbachol. We showed previously that the three agonist binding states were the sum of two equilibria (SH-H and H-L subgroup), both regulated by GTP-binding protein(s). In this study we determined which subgroup was responsible for the inhibitory effect of muscarinic M2 receptors on adenylate cyclase activity. The ED50 values for this response of four muscarinic agonists, acetylcholine, carbachol, pilocarpine and oxotremorine corresponded with the binding KD values of H (acetylcholine and carbachol) and LO/P (pilocarpine and oxotremorine) sites. After alkylation of spare receptors, the ED50 value of carbachol was changed from 4.3 to 5.6 microM, which corresponded with the KD value of the H site. Furthermore, the four agonists were almost fully active when membrane preparations were pretreated with propylbenzilylcholine mustard (PrBCM) in the presence of carbachol to destroy the H-L subgroup, whereas after pretreatment with PrBCM and atropine, which alkylated both types of subgroups evenly, the decrease in the number of receptors was proportional to the decrease in the inhibitory effect on adenylate cyclase activity. These results suggest that only the SH-H subgroup (M2 alpha) is responsible for the inhibitory action of muscarinic receptors on adenylate cyclase activity in the heart.