[2 different populations of adrenergic alpha receptors in the splenic capsule in rats]

Using the contraction of the smooth muscle of rat spleen capsula, studies of the character of the alpha-adrenergic receptors involved in this reaction where performed in vitro. By computer analysis, the flat dose-response curve of noradrenaline shows the presence of two different subpopulations of alpha-receptors, each being responsible for approximately 50% of the maximal overall contraction. The two subpopulations differ roughly one hundred times by their apparent affinity for noradrenaline (pD2 cca 8 and pD2 cca 6). Solely the high-affinity component is competitively antagonized by the alpha 1-blocker prazosin, needs necessarily extracellular Ca2+ for evoking the contraction and is desensitized by previous contact with a high concentration of noradrenaline. Neither of the two components is influenced in its action by calcium-channel blockers nifedipine and verapamil. The needed exogenous calcium thus enters the cell by different routes than by those affected by the mentioned blockers.