Characterization of muscarinic receptors of bovine coronary artery by functional and radioligand binding studies.
The nature of the muscarinic receptor subtype mediating contraction of the endothelium-denuded bovine coronary artery was investigated in vitro by functional measurements and radioligand binding studies. The acetylcholine (ACh)-induced isotonic contraction of circularly cut muscle strips was recorded and expressed as a percentage of the maximum contraction obtained with 80 mM K+. In order to distinguish between M1, M2 and M3 receptors, the potency of the five subtype-selective antagonists, 4-diphenylacetoxy-N-methyl-piperidine methobromide (4-DAMP), parafluor-hexahydro-siladifenidol (pFHHSiD), pirenzepine, AF-DX 116 and methoctramine, to block the ACh-induced contraction was estimated. All the antagonists competitively inhibited the responses induced by ACh, with one exception, namely, 4-DAMP, whose Schild plot had a slope greater than one. The low affinity of pirenzepine (pA2 7.14 +/- 0.14) excluded an action at the M1 subtype. The low affinity of AF-DX 116 (pA2 6.49 +/- 0.18) and methoctramine (pA2 5.88 +/- 0.07) suggest that the bovine coronary artery smooth muscle receptor is not of the M2 (cardiac) subtype. In contrast, 4-DAMP (pA2 9.04 +/- 0.03) and pFHHSiD (pA2 7.64 +/- 0.04) potently inhibited the ACh-induced contraction with affinities similar to those reported for the M3 (glandular) receptor. In addition, the muscarinic receptors mediating coronary artery contraction were characterized in antagonist/[3H]N-methyl-scopolamine ([3H]NMS) competition binding studies. With the exception of AF-DX 116, all antagonists bound to a homogeneous population of receptors with pseudo-Hill slopes not different from unity. The pKi values, albeit somewhat lower, essentially substantiated the functional affinity estimates.(ABSTRACT TRUNCATED AT 250 WORDS)
Brunner
F
F
Institut für Pharmakodynamik und Toxikologie, Universität Graz, Austria.
Kühberger
E
E
Schloos
J
J
Kukovetz
W R
WR
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D003331:Coronary Vessels / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure*; D004730:Endothelium, Vascular / Q000502:physiology; D005260:Female; D007700:Kinetics; D008297:Male; D018727:Muscarinic Antagonists; D009119:Muscle Contraction / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000502:physiology / Q000648:ultrastructure; D019832:N-Methylscopolamine; D010276:Parasympatholytics / Q000378:metabolism; D010830:Physostigmine / Q000494:pharmacology; D011869:Radioligand Assay; D011976:Receptors, Muscarinic / Q000378:metabolism / Q000502:physiology*; D012602:Scopolamine Derivatives / Q000378:metabolism; D014316:Tritium
D018727:Muscarinic Antagonists; D010276:Parasympatholytics; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D014316:Tritium; D010830:Physostigmine; D000109:Acetylcholine; D019832:N-Methylscopolamine