1 The properties of the muscarinic receptors in the rabbit saphenous artery were determined from electrical and mechanical responses of smooth muscle cells produced by acetylcholine (ACh). The inhibitory action of atropine and pirenzepine on the ACh-induced responses was also studied. 5 The excitatory junction potential (ej.p.) evoked by perivascular nerve stimulation was inhibited by ACh (above 10-8 M). The ACh-induced inhibition of the ej.p. was antagonized by atropine more preferentially than by pirenzepine (the ID5o values were 3 x 10-8M for atropine and 6 x 10-6 M for pirenzepine).6 It is concluded that in the rabbit saphenous artery, two subtypes of muscarinic receptor (M, and M2) are located on the endothelial cells. Stimulation of each subtype releases a different substance, i.e., a hyperpolarizing substance (M,-subtype) or a relaxant substance (M2-subtype). In prejunctional nerve terminals, the muscarinic receptors responsible for inhibiting the release of transmitter substances are of the M2-subtype.
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