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

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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+]i. In Ca2+-free solution, the stimulation of α1-adrenoceptors caused a transient increase in [Ca2+]i, whereas the stimulation of α2-adrenoceptors had no effect. These results directly prove that α-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 21. Plotting the increase in [Ca2+], 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.
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