Different effects of N-ethylmaleimide on the tension components of bullfrog atrium

The effects of N-ethylmaleimide (NEM) on the tension components of bullfrog atrium were studied under voltage clamp conditions using the double sucrose gap technique. NEM at a dilution of 10(-3) M inhibited the ICa-independent tonic tension elicited by large and long depolarizing pulses (140 mV, 2 sec) time-dependently, whereas the drug produced an initial transient increase and a later decrease of the ICa-dependent phasic tension elicited by small and short depolarizing pulses (60 mV, 100 msec). The effect of NEM was facilitated when the atrial muscles were exposed to a solution having either a reduced Na or an increased Ca concentration. In the presence of Mn ions, 10(-3) M NEM did not evidently inhibit tonic tension, and it led to a slight transient increase in phasic tension. NEM at 10(-3) M, which is known to inhibit sarcolemmal Na, K-ATPase, enhanced both the phasic tension and the tonic tension in the presence of ouabain and when perfused with K-free solution. Even under these conditions (10(-3) M NEM caused an initial transient decrease of ICa-independent tonic tension. This suggests that NEM has a different action on the atrial muscle from those of cardiac glycosides such as ouabain.