Effects of Sodium Depletion on Contractions Evoked in Intact and Skinned Muscles of the Guinea-pig Mesenteric Artery

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) 5 X 10(-5) M or caffeine 5 mM increased to roughly twice the amplitude of the contraction evoked in the control solution. In Ca-free solution, the K-, NaCl-free- or Na-free-induced contractions rapidly ceased, but NA-induced contraction ceased within 5 min and the caffeine-induced contraction persisted for more than 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 in 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.