Comparative study of pA2 values of pirenzepine and atropine for guinea-pig gastric fundus and gastric smooth muscle

Abstract

Concentration-response curves of bethanechol were obtained from isolated guinea pig gastric fundus and gastric circular smooth muscle strips before and after exposure to pirenzepine or atropine. In the presence of pirenzepine and atropine these curves were shifted parallel to the right with no change in maximum response. Against bethanechol, mean pA2 values were 7.06 for pirenzepine and 8.16 for atropine on gastric fundus, with slopes of Schild plots close to unity for both antagonists, whereas in the case of gastric smooth muscle pA2 values were 6.52 for pirenzepine and 8.52 for atropine, with the slope of the Schild plot significantly less than -1 in the case of pirenzepine. The values of the Schild slope seem to indicate that pirenzepine interacts with a single, apparently homogeneous population of muscarinic receptors on gastric mucosa, at variance with the behaviour of pirenzepine on receptors of gastric smooth muscle.