Influence of Na+ gradient on Ca2+ transients and contraction in vascular smooth muscle.
The effects of reducing external Na+ ([Na+]o) and increasing internal Na+ ([Na+]i) on evoked tension in arterial rings were correlated with intracellular Ca2+ transients evoked in cultured A7r5 cells. Contractions were elicited in rings of rat aorta and a small mesenteric artery branch by serotonin (5-HT), arginine vasopressin (AVP), or a high K+ concentration ([K+]o). Contractions were augmented by reduction of [Na+]o (replaced by Li+ or N-methylglucamine) or inhibition of the Na+ pump (to raise [Na+]i). These effects were dependent on external Ca2+. Ca2+ transients were measured by digital imaging of fura-2/AM-loaded A7r5 cells. The apparent free Ca2+ concentration [( Ca2+]app) in eight unstimulated cells was 110 +/- 10 nM in the peripheral cytoplasm. Activation of A7r5 cells by 5-HT, AVP, or high [K+]o evoked transient increases in [Ca2+]app. Lowering [Na+]o or raising [Na+]i elevated [Ca2+]app significantly in unstimulated cells and increased the amplitude and duration of the Ca2+ transients. These effects required external Ca2+. Caffeine (5-10 mM) reduced [Ca2+]app in resting cells and attenuated the vasoconstrictor-evoked Ca2+ transients in low [Na+]o. The data imply that Na+ gradient reduction promotes Ca2+ entry and slows Ca2+ extrusion via Na(+)-Ca2+ exchange. This increases cytosolic free Ca2+ and the intracellular Ca2+ stores so that more Ca2+ is delivered to the contractile apparatus during cell activation. Such augmented Ca2+ transients could induce the amplified vasoconstrictor-evoked contractions observed in arterial rings under reduced Na+ gradient conditions.
Bova
S
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Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
Goldman
W F
WF
Yauan
X J
XJ
Blaustein
M P
MP
D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology; D001158:Arteries; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002460:Cell Line; D066298:In Vitro Techniques; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D051381:Rats; D012701:Serotonin / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000502:physiology; D014661:Vasoconstriction / Q000187:drug effects; D014662:Vasoconstrictor Agents
D015222:Sodium Channels; D014662:Vasoconstrictor Agents; D001127:Arginine Vasopressin; D012701:Serotonin; D012964:Sodium; D002118:Calcium