Selective potentiation of subtetanic and tetanic contractions by the calcium-channel antagonist nifedipine in the rat diaphragm preparation.

1. Nifedipine (1.5-3.0 x 10(-5) M) potentiated the (sub)tetanic tension during 10-50 Hz indirect or direct stimulation of the rat diaphragm preparation; the twitch contractions were not potentiated. 2. The effect was antagonized in high Ca2+ (5-10 x normal) solutions. 3. A comparison with the twitch potentiators caffeine (1.0 x 10(-3) M), quinine (1.4 x 10(-5) M) and phenytoin (2.0 x 10(-5) M), showed that only phenytoin, a putative Ca-antagonist, caused a nifedipine-like frequency-dependent potentiation, indicating a Ca-antagonistic rather than an unspecific effect. 4. A similar (sub)tetanic potentiation was found in a K(+)-free solution. 5. The slow development of the potentiation during repetitive stimulation is in accordance with an effect on the slow Ca channels known to be present in mammalian skeletal muscle. 6. A delay of the fatigue-inducing accumulation of K+ in the T tubules, which may occur during a nifedipine-induced reduction of a Ca2(+)-stimulated K+ efflux, as well as in a K(+)-free solution, may explain the effect.