MEDIATION BY M(3)-MUSCARINIC RECEPTORS OF BOTH ENDOTHELIUM-DEPENDENT CONTRACTION AND RELAXATION TO ACETYLCHOLINE IN THE AORTA OF THE SPONTANEOUSLY HYPERTENSIVE RAT

1 Experiments were designed to characterize the subtype(s) of endothelial muscarinic receptor that mediate(s) endothelium-dependent relaxation and contraction in the aorta of spontaneously hypertensive rats (SHR). 2 Rings of SHR aorta with endothelium were suspended in organ baths for the measurement of isometric force. Ecothiopate (an inhibitor of acetylcholinesterase) was present throughout the experiments. Endothelium-dependent contraction to acetylcholine was studied in quiescent aortic rings in the presence of N-G-nitro-L-arginine (to prevent the formation of nitric oxide). Endothelium-dependent relaxation to acetylcholine was obtained during contraction to phenylephrine and in the presence of indomethacin (to inhibit cyclo-oxygenase activity). Responses to acetylcholine were assessed against the non-preferential muscarinic receptor antagonist, atropine, and the preferential antagonists pirenzepine (M(1)), methoctramine (M(2)) and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP; M(3)) 3 The potency of acetylcholine in inducing endothelium-dependent contraction was 6.54 +/- 0.07 (EC(50)) Atropine, pirenzepine, methoctramine and 4-DAMP displayed competitive antagonism towards the endothelium-dependent contraction to acetylcholine. The pA(2) values for these muscarinic receptor antagonists were estimated from Arunlakshana-Schild plots to be (- log M) 9.48 +/- 0.07, 6.74 +/- 0.22, 6.30 +/- 0.20 and 9.39 +/- 0.22 respectively. The potency of acetylcholine in inducing endothelium-dependent relaxation was 7.82 +/- 0.09 (IC50). Atropine, pirenzepine and 4-DAMP displayed competitive antagonism towards the endothelium-dependent relaxation to acetylcholine but methoctramine had no effect. The pA(2) values for atropine and 4-DAMP for the relaxation to acety!choline were estimated from Arunlakshana-Schild plots to be (- log M) 9.15 +/- 0.23 and 9.63 +/- 0.28, respectively. These results suggest that the muscarinic Mg receptor subtype mediates both endothelium-dependent relaxation and contraction to acetylcholine in SHR aorta.