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

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.