Effects of sodium depletion on contractions evoked in intact and skinned muscles of the guinea-pig mesenteric artery.

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Abstract
In the guinea pig mesenteric artery, reduction in [Na]o by 30 mM (substituted by choline or sucrose; 137 mM [Na]o in Krebs solution) generated contraction with no change in membrane potential. In NaCl-free solution (15 mM [Na]o), the amplitude of phasic contraction reached 0.8 times the contraction evoked by 118 mM [K]o with only a slight depolarization. In NaCl-free solution, the amplitude of phasic contraction evoked by noradrenaline (NA) [norepinephrine] 5 .times. 10-5 M or caffeine 5mM increased to roughly twice the amplitude of the contraction evoked in the control solution. In Ca-free solution, the N-, NaCl-free or Na-free-induced contractions rapidly ceased but NA-induced contraction ceased within 5 min and the caffeine-induced contraction persisted for > 15 min. In a skinned fiber, increase of [Na]o from 10 to 60 mM suppressed the pCa-tension relationship in the ranges of 10-7 and 10-5 M free Ca but not with a dose of 30 mM [Na]o. NA (10-5 M) had no effect on skinned fibers. Increase in Na concentration (60 mM) had no effect on Ca accumulation in the store site or on Ca release by caffeine. Possible Na-related mechanisms on the development of mechanical response are discussed in relation to Ca on the surface and the internal membrane structure. The NaCl-free-induced contraction in smooth muscles of the guinea pig mesenteric artery is postulated to be due to influx of Ca through the Na channel rather than the Ca channel.