Changes in Intracellular Free Ca2+ Concentration by Activation of α-Adrenoceptors in Rat Tail Artery

Abstract

Changes in intracellular free Ca2+ concentration ([Ca2+]1) by activation of α1- and α2-adrenoceptors were microfluorometrically measured in rat tail arteries loaded with the Ca2+-sensitive fluorescent dye fura-2. Under the normal condition (1.8 m M Ca2+), stimulation with the selective α1-adrenoceptor agonist phenylephrine very rapidly raised [Ca2+]i to the peak concentration, plateauing at this level, whereas the stimulation with the selective α2-adrenoceptor agonist UK-14,304 gradually increased [Ca2+]i. In Ca2+-free solution, the stimulation of αα-adrenoceptors caused a transient increase in [Ca2+]i, whereas the stimulation of α2-adrenoceptors had no effect. These results directly prove that α1-adrenoceptor activation induces both the release of intracellular Ca2+ and the influx of extracellular Ca2+, whereas activation of α2-adrenoceptors evokes only the influx of extracellular Ca2+. Plotting the increase in [Ca2+]i against the developed contraction revealed that it was always linearly related regardless of the type of α-adrenoceptor stimulated. Therefore, the intracellular mechanisms after the increase in [Ca2+]i are probably common to the vasocontractile responses mediated by α1 - and α2-adrenoceptors.