Effects of Na removal and readmission on the mechanical response in the guinea-pig tracheal smooth muscle.

In the guinea-pig tracheal smooth muscle, tonic contraction, maintained in normal medium, was abolished by Ca removal or by indomethacin (10(-6) M). Removal of Na produced a transient contraction followed by a gradual decrease in tension to a level lower than the control. When Na was readmitted, a rapid relaxation was observed. The degree of the relaxation was enhanced with the exposure time to Na-free solution, and with the concentration of Na readmitted. At 5.9 mM K, the contraction produced by Ca readmission with simultaneous Na removal became smaller and slower as the external Na was reduced before Ca readmission, while at 40 mM K, the Ca-induced contraction was less dependent on the Na concentration. The rate of relaxation on Ca removal was slower when the Na concentration was reduced both at 5.9 and 40 mM K. Verapamil (10(-5) M) had a weak suppressing effect in normal solution, but suppressed markedly in 40 mM K medium. In the presence of verapamil, the difference between the effects of Na removal and of Na readmission on Ca-induced contraction was markedly potentiated, particularly at 40 mM K. It was concluded that the Na-Ca exchange mechanism may contribute to the transient response on Na removal and to the relaxation phase following Na readmission. At 5.9 mM K, removal of Na may increase the Ca conductance and also reduce the contribution of prostaglandins. On the other hand, when depolarized by excess K, the effect of Na is less significant; probably due to a high Ca conductance.