Contracture and change in membrane potential produced by sodium removal in the dog trachea and bronchiole

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Abstract
Mechanical responses and changes in membrane potential induced by Na removal were investigated in dog tracheal and bronchiolar smooth muscles. In both muscles, reduction of the external Na concentration ([Na]o) to less than 70 mM produced a sustained contracture, dose dependently. The relative amplitude of the Na-free contracture was greater than that induced by excess [K]o in the trachealis. Readmission of 1-10 mM Na, after exposure to Na-free solution, relaxed the contracture evoked by Na removal, and the degree of relaxation was dependent on [Na] readmitted. In the absence of both Na and Ca, some tension remained, and readmission of Ca increased the muscle tone. Even after pretreatment with Ca-free ethylene glycol-bis (beta-aminoethylether)-N,N,N,N′-tetraacetic acid- (0.2 mM) containing solution for 30 min, removal of Na caused some mechanical response in both muscles. D 600 (10(-7) to 10(-4) M), a blocker of voltage-dependent Ca2+ influx, suppressed the response to Na removal, but 10(-4) M D 600 did not completely block the contracture. Na removal depolarized the smooth muscle membrane to a greater extent in the bronchiole than in the trachealis. It was concluded that an increase in Ca permeability across the membrane and inhibition of the Na-Ca exchange mechanism in the absence of Na are responsible for the generation of Na-free contracture in both muscles.