Pirenzepine does not discriminate between pre- and postsynaptic muscarine receptors in the guinea-pig small intestine.
The effects of pirenzepine and scopolamine on presynaptic muscarine receptors (mediating inhibition of acetylcholine release) and postsynaptic receptors (mediating smooth muscle contraction) were compared in the myenteric plexus-longitudinal muscle preparation of the guinea pig. The preparation was preincubated with 3H-choline and subsequently superfused with Tyrode's solution. Field stimulation (1 Hz; 2 min) caused an outflow of 3H-acetylcholine that was depressed by the muscarinic agonist oxotremorine. Both pirenzepine and scopolamine produced shifts to the right of the concentration-response curve for the inhibitory effect of oxotremorine. Similarly, in contraction experiments, pirenzepine and scopolamine competitively antagonized the responses to oxotremorine. From the dose ratios pA2 values for pre- and postsynaptic effects were calculated. The presynaptic pA2 values for pirenzepine (6.90) and scopolamine (9.07) did not differ significantly from the respective postsynaptic pA2 values (6.66 and 9.40). Thus, pirenzepine like scopolamine does not distinguish between muscarine receptors that mediate contraction and those which mediate inhibition of acetyl-choline release.
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001570:Benzodiazepinones / Q000494:pharmacology*; D002794:Choline / Q000378:metabolism; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000378:metabolism*; D010095:Oxotremorine / Q000494:pharmacology; D010890:Pirenzepine; D011950:Receptors, Cholinergic / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012601:Scopolamine / Q000378:metabolism; D013569:Synapses / Q000378:metabolism
D001570:Benzodiazepinones; D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D010890:Pirenzepine; D010095:Oxotremorine; D012601:Scopolamine; D002794:Choline; D000109:Acetylcholine