CHANGES IN INTRACELLULAR FREE CA-2+ CONCENTRATION BY ACTIVATION OF ALPHA-ADRENOCEPTORS IN RAT TAIL ARTERY

Changes in intracellular free Ca2+ concentration ([Ca2+]i) 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 mM 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+]j. In Ca2+-free solution, the stimulation of -adrenoceptors caused a transient increase in [Ca2+]i, whereas the stimulation of α1-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. © 1990, The Japanese Pharmacological Society. All rights reserved.