Redistribution of intracellular Ca2+ stores after beta-adrenergic stimulation of rat tail artery SMC.
beta-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 inositol trisphosphate-mediated channels. To determine if smooth muscle relaxation induced by beta 2-adrenergic agonists involves the redistribution of intracellular Ca2+, we studied the effects of isoproterenol (Iso) on freshly isolated, single rat tail artery smooth muscle cells loaded with fura 2, using digital ratiometric fluorescence imaging. Stimulation with 1 microM phenylephrine (PE) or norepinephrine produced phasic and tonic increases in cytoplasmic intracellular Ca2+ concentration ([Ca2+]i) associated 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 after stimulation by Iso. Exposure to 1 microM Iso induced a time-dependent decrease in PE-activated peak and tonic [Ca2+]i without any change in 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+ that remained in the SR after prolonged exposure to Iso (30% of the pre-Iso level by 80 min at 22 degrees C) failed to elicit a contractile response. The cells, perfused 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.
Miyashita
Y
Y
Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, USA.
Sollott
S J
SJ
Cheng
L
L
Kinsella
J L
JL
Koh
E
E
Lakatta
E G
EG
Froehlich
J P
JP
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D001158:Arteries / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D016257:Fura-2; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010656:Phenylephrine / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013623:Tail / Q000098:blood supply; D014018:Tissue Distribution
D000318:Adrenergic beta-Agonists; D010656:Phenylephrine; D007545:Isoproterenol; D002118:Calcium; D016257:Fura-2