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1 Department of Physiology, University of Maryland, School of Medicine, Baltimore, MD, USA
* To whom correspondence should be addressed. E-mail: clamo001{at}umaryland.edu.
The functions of ryanodine receptors (RyR) and inositol (1,4,5) trisphosphate receptors (InsP3R) in adrenergically activated contractions of pressurized rat mesenteric small arteries were investigated. Caffeine (20mM) but not phenylephrine (PE) (10µM), facilitated the depletion of smooth muscle SR Ca2+ stores by ryanodine (40µM). In ryanodine-treated, SRdepleted arteries, 1) Ca2+-sparks were absent, 2) low concentrations of PE failed to elicit either vasoconstriction or the normal asynchronous propagating Ca2+-waves, and 3) high [PE] induced abnormally slow oscillatory contractions (vasomotion) and synchronous Ca2+-oscillations. In ryanodine-treated SR-depleted arteries denuded of endothelium, high [PE] induced steady contraction and steady elevation of intracellular [Ca2+]. In contrast, 2- aminoethyl diphenyl borate (2-APB), a putative blocker of InsP3R, produced opposite effects to ryanodine: 1) Ca2+-sparks were present, 2) Ca2+ waves were absent, 3) caffeinereleasable Ca2+ stores were intact, and, 4) PE, even at high concentrations on endothelialdenuded arteries, failed to elicit contraction, asynchronous Ca2+ waves, synchronous Ca2+ oscillations or maintained elevated [Ca2+]. We conclude: 1) InsP3R are essential for adrenergically-induced asynchronous Ca2+-waves and the associated steady vasoconstriction, 2) RyR are not appreciably opened during adrenergic activation (since PE did not facilitate the development of the effects of ryanodine), and 3) InsP3R are not essential for Ca2+ sparks. This provides an explanation of the fact that adrenergic stimulation decreases the frequency of Ca2+-sparks (previously reported) while simultaneously increasing the frequency of asynchronous propagating Ca2+-waves; different SR Ca2+-release channels are involved.
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