Role of calcium ions in the contraction of vascular smooth muscle

Abstract

The role of Ca ions in the contraction of vascular smooth muscle by norepinephrine and potassuium was studied by perfusing the vascular system of the rabbit ear. Vascular smooth muscle tone was increased with time in Ca solutions of concentrations greater than 2 mM. Complete depletion of Ca in the perfusing fluids induced a diphasic response, i.e., transient increase followed by decrease of the tone. Norepinephrine caused vasoconstriction in the rabbit ear. Contractile response to norepinephrine was increased with the increment of extracellular Ca concentration up to 2 mM, but above 2 mM the effect was reversed. Norepinephrine brought about vasoconstriction even in a Na-free, Ca-free solution, and induced its effect by a mechanism other than membrane depolarization. Norepinephrine produces vasoconstriction mostly through liberating the bound Ca in the cell without electrical excitation. Difference in the Ca fractions of the cellular store which norepinephrine and high-K solution utilize to activate contractile elements is discussed.