Changes in intracellular free Ca2+ concentration by activation of alpha-adrenoceptors in rat tail artery.

Changes in intracellular free Ca2+ concentration ([Ca2+]i) by activation of alpha 1- and alpha 2-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 alpha 1-adrenoceptor agonist phenylephrine very rapidly raised [Ca2+]i to the peak concentration, plateauing at this level, whereas the stimulation with the selective alpha 2-adrenoceptor agonist UK-14,304 gradually increased [Ca2+]i. In Ca2(+)-free solution, the stimulation of alpha 1-adrenoceptors caused a transient increase in [Ca2+]i, whereas the stimulation of alpha 2-adrenoceptors had no effect. These results directly prove that alpha 1-adrenoceptor activation induces both the release of intracellular Ca2+ and the influx of extracellular Ca2+, whereas activation of alpha 2-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 alpha-adrenoceptor stimulated. Therefore, the intracellular mechanisms after the increase in [Ca2+]i are probably common to the vasocontractile responses mediated by alpha 1- and alpha 2-adrenoceptors.