APStracts 3:0239H, 1996.

Redistribution of intracellular calcium stores following [beta] -adrenergic stimulation of rat tail artery smooth muscle cells. Miyashita, Yoshiyuki, Steven J. Sollott, Linda Cheng, James L. Kinsella, Eio Koh, Edward G. Lakatta, and Jeffrey P. Froehlich. Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224

APStracts 3:0239H, 1996.

b-adrenergic agonists induce the relaxation of vascular smooth muscle by a mechanism that activates the extrusion of Na+ and Ca2+ from the cell. A primary source of contractile Ca2+ resides in the sarcoplasmic reticulum (SR) which releases Ca2+ in response to vasoactive agents through IP3-mediated channels. To determine if smooth muscle relaxation induced by [beta]2-adrenergic agonists involves the redistribution of intracellular Ca2+ ([Ca2+]i), we studied the effects of isoproterenol (ISO) on freshly-isolated single, rat tail artery smooth muscle cells (SMC) loaded with fura-2 using digital ratiometric fluorescence imaging. Stimulation with 1 [mu]M phenylephrine (PE) or norepinephrine (NE) produced phasic and tonic increases in [Ca2+]i associated with cell shortening. Exposure to caffeine and to Ca2+-free solutions eliminated the phasic and tonic components, respectively, from the Ca2+ signal. Intermittent superfusion with PE or caffeine was used to evaluate SR Ca2+ stores following stimulation by ISO. Exposure to 1 [mu]M ISO induced a time -dependent decrease in PE-activated peak and tonic [Ca2+]i without changing resting [Ca2+]i. Intermittent stimulation with 10 mM caffeine revealed a similar decline in peak [Ca2+]i, indicating ISO -dependent depletion of SR Ca2+ stores. The Ca2+ remaining in the SR after prolonged exposure to ISO (30% of the pre-ISO level by 80 min at 21oC) failed to elicit a contractile response. Perfusing the cells with a Na+- and Ca2+-free medium to block Na+/Ca2+ exchange prevented depletion of the SR Ca2+ stores by ISO. We propose that ISO inhibits agonist-mediated Ca2+ influx through sarcolemmal Ca2+ channels and activates Ca2+ redistribution from storage sites in the SR to the extracellular compartment by a mechanism that involves Na+/Ca2+ exchange. These combined effects of ISO facilitate smooth muscle relaxation (and reduce vascular tonus) by reducing the increase in cytoplasmic Ca2+ evoked by vasoconstrictors.

Received 29 June 1995; accepted in final form 31 May 1996.
APS Manuscript Number H601-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 17 June 96