Stimulation-induced regional alteration of Ca2+ levels in single arterial smooth muscle cells.
Contraction in vascular smooth muscle is normally triggered by an increase in the cytosolic free Ca2+ concentration. We employed the fluorescent Ca indicator, fura-2, and digital imaging microscopy to study the spatial distribution of intracellular Ca2+ in arterial myocytes and the changes elicited by norepinephrine (NE) and by alterations in the Na+ electrochemical gradient. In modified Kreb's solution containing 1.8 mM Ca2+, these bovine tail artery myocytes were relaxed and able to maintain normal membrane potentials. The cells contracted rapidly when exposed to NE or high K+ solution. They then relaxed slowly when the activator was washed away. A rise in Ca2+ concentration, [Ca2+], was elicited by NE as well as by Ca2+ entry via Na+/Ca2+ exchange. Digital analysis of images of cellular fura-2 fluorescence revealed that the intracellular [Ca2+] was relatively uniformly distributed in resting cells prior to activation. During NE-evoked contractions, intracellular [Ca2+] increased an average of two- to threefold, and the distribution of [Ca2+] became much more heterogeneous. Upon recovery from activation, the cells relaxed, usually attaining greater than 90% of their original resting length. In contrast to the relatively uniform Ca2+ distribution observed prior to NE activation, discrete regions of elevated [Ca2+] were observed throughout the recovered cells. Similarly, reducing the Na+ electrochemical gradient and, presumably, activating Na+/Ca2+ exchange elicited a rise in [Ca2+]. In cells exposed to increased [Na+]i and reduced [Na+]o, as in cells following recovery from NE stimulation, discrete areas of high [Ca2+] were apparent. The large spatial variation induced by cell activation implies that Ca2+ was sequestered at localized sites throughout the cell.
Goldman
W F
WF
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
Blaustein
M P
MP
D000818:Animals; D001158:Arteries / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D004558:Electric Stimulation; D004594:Electrophysiology; D066298:In Vitro Techniques; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D013268:Stimulation, Chemical
D012964:Sodium; D002118:Calcium; D009638:Norepinephrine