Interaction of himbacine with carbachol at muscarinic receptors of heart and smooth muscle

The interaction of himbacine with carbachol was studied and compared with that of homatropine in guinea-pig atria and rat stomach fundus. Himbacine was a competitive antagonist of carbachol in both preparations. In atria, himbacine had similar affinity for the muscarinic receptors associated with force (pA2 = 8.33) and rate of contraction (pA2 = 8.42). In stomach, himbacine had a pA2 value of 7.29 which corresponds to a 10-fold less affinity than for atria. In contrast, homatropine had similar affinities for muscarinic receptors in stomach (pA2 = 7.13) and for those in atria mediating force (pA2 = 7.21) and rate (pA2 = 7.07) responses. These results indicate that himbacine distinguishes between cardiac and smooth muscle muscarinic receptors whereas it is unable to distinguish between possible subtypes of cardiac receptors associated with force and rate responses.