Redistribution of intracellular Ca²⁺ stores after β-adrenergic stimulation of rat tail artery SMC

β-Adrenereric agonists induce the relaxation of vascu-lar 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 β₂-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 μM phenylephrine(PE) or norepinephrine produced phasic and tonic increases in cytoplasmic intracellular Ca²⁺ concentration([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 after stimulation by Iso. Exposure to 1 μM 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°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.