Role of Calcium in Contractile Excitation of Vascular Smooth Muscle by Epinephrine and Potassium

Abstract

A technique of constant-flow perfusion of isolated segments of dog intestinal arteries was used to investigate the role of calcium in contractile excitation of vascular smooth muscle by epinephrine and potassium. Contractile responses of the arterial muscle to epinephrine, perfused at constant concentration, and to potassium, perfused at high concentration, were both promptly prevented by calcium deionizing accomplished by simultaneous perfusion with solutions containing ethylenediaminetetraacetate (EDTA) for the chelation of available calcium ions. Potassium-induced muscular contracture was removed in a few minutes and epinephrine-induced contractions of potassium-depolarized muscle were soon prevented by perfusion with solutions containing ionic species of EDTA which chelated calcium ions. In muscle depolarized by potassium and calcium deionized by perfusion with solution containing MgK₂EDTA, the contractile responses to epinephrine were immediately restored by combined perfusate injection of calcium ion and epinephrine. Injections of isosmotic calcium chloride solution of subthreshold or threshold contractile strength, when the artery had been perfused with sodium chloride-rich solution and the muscle had been relaxed, produced much greater contractions during both potassium and epinephrine excitation. The contractions from injected calcium chloride solution were increased by epinephrine excitation of muscle already depolarized by the external application of potassium sulfate, as they were in the previously polarized or resting muscle excited by epinephrine. The thesis is advanced that the relative muscular contractions induced by standard injected volumes of isosmotic calcium chloride solution were a relative index of the calcium influx and calcium permeability of the arterial smooth muscle cells in the various experimental conditions prevailing. Equimolar injections of isosmotic calcium iodide solution produced greater muscular contractions than did injections of isosmotic calcium chloride solution. The possibility is discussed that transmembranous passage of calcium into vascular muscle cells occurs significantly in the form of associated ion pairs. A dual effect of potassium on vascular smooth muscle behavior was observed in that a previously contractile-inducing amount of injected potassium exerted a relaxing effect on the arterial segment perfused with low-potassium solution when the muscle was contracted by epinephrine. It is concluded that: (a) calcium is essentially involved in excitation-contraction coupling of vascular smooth muscle; (b) the calcium permeability and calcium influx of vascular smooth muscle is greatly increased during both epinephrine and potassium excitation of the muscle cells, and epinephrine accomplishes these calcium changes even in muscle already depolarized by potassium; (c) adrenergic neurohormones exert their vasoexcitatory contractile effect by a membrane reaction which is basically nonelectrical and which primarily triggers an increased permeability and influx of calcium into the vascular myoplasm; and (d) the migrated calcium activates intracellularly the myoplasmic events of vascular smooth muscle contraction.