Comparison of affinities of muscarinic antagonists to pre- and postjunctional receptors in the guinea-pig ileum.

The myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum was preincubated with [3H]choline, then superfused and stimulated electrically (1 Hz 120 pulses). Oxotremorine reduced the evoked outflow of [3H]acetylcholine in a concentration-dependent manner. Each of the six antagonists (scopolamine, methylatropine, trihexyphenidyl, 4-DAMP, clozapine, pirenzipine) produced parallel shifts of the concentration-response curves for the prejunctional effects of oxotremorine. Similarly, in contraction experiments, the antagonists competitively antagonized the postjunctional responses to oxotremorine. The pre- and postjunctional pA2 values did not differ significantly for any of the antagonists. It is concluded that pre- and postjunctional muscarinic receptors in the guinea-pig ileum are pharmacologically similar.