The influence of extracellular calcium on muscarinic receptors in vascular smooth muscle.
The present study was designed to investigate the influence of extracellular calcium (Ca++) on the affinity of cholinergic agonists and antagonists for muscarinic receptors in isolated rabbit aortic strips. Dose-response curves which demonstrate the effect of acetylcholine on the force of contraction in the presence of various Ca++ concentrations were made. The affinity constant was calculated and the intrinsic activity was determined at each of the different Ca++ concentrations used. In addition, the relationship between extracellular Ca++ and the pA2 value for atropine was determined. No apparent relationship between the affinity of acetylcholine for the cholinergic receptors and Ca++ concentration in vascular muscle was demonstrated. However, the intrinsic activity was altered when the extracellular Ca++ was varied. In addition, atropine was effective to the same degree both in the presence and complete absence of Ca++. These results are discussed in relation to the dependency of acetylcholine on extracellular Ca++ to induce contractile responses after receptor occupancy in vascular muscle.
D000109:Acetylcholine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D004347:Drug Interactions; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D011817:Rabbits; D011950:Receptors, Cholinergic / Q000187:drug effects*; D011976:Receptors, Muscarinic / Q000187:drug effects*
D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D001285:Atropine; D000109:Acetylcholine; D002118:Calcium