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.