HIMBACINE DISCRIMINATES BETWEEN PUTATIVE MUSCARINIC M(1) RECEPTOR-MEDIATED RESPONSES

This study describes the antagonistic properties of himbacine, in comp arison with those of pirenzepine, at muscarinic receptors mediating th e depolarization of rat superior cervical ganglion, the inhibition of electrically-induced twitch contractions of rabbit vas deferens and th e contraction of dog saphenous vein, currently classified as putative muscarinic M1 sites. The affinity of himbacine for the vas deferens si te (pA2 8.08) was nearly ten times higher than those for the M1 recept ors of rat ganglion and dog saphenous vein (pA2 7.14 and 7.16, respect ively); affinity estimates for pirenzepine were similar throughout the different preparations. The present data are consistent with the allo cation of ganglion and saphenous vein receptors into the M1 subclass; the profile of the vas deferens site, conversely, appears to be differ ent, and possibly more closely related to that reported for the M4/m4 receptor.