Experiments were designed to characterize pre- and postjunctional muscarinic receptors in the canine saphenous vein. In rings contracted by 2-Hz electrical stimulation, acetylcholine produced concentration-dependent relaxations that result from prejunctional inhibition of norepinephrine release from adrenergic nerve endings (prejunctional effect). In quiescent preparations, acetylcholine caused concentration-dependent contractions (postjunctional effect). Both responses to acetylcholine were inhibited in a competitive manner by atropine, pirenzepine and gallamine. The affinity of atropine for pre- and postjunctional muscarinic receptors was similar (pKB = 8.7 and 9.2, respectively). However, gallamine displayed higher affinity for the prejunctional than for the postjunctional receptor (pKB = 6.1 and 4.7, respectively), whereas the converse was true for pirenzepine (pKB = 6.5 and 8.1). The data support the presence in the canine saphenous vein of two muscarinic receptor subtypes, M1 postjunctionally and M2 prejunctionally, which are distinguishable by selective antagonists.
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Robert J. Levy (1963) studied this question.
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