The effect of the dihydropyridine, nifedipine, on excitation-contraction coupling was compared in toad and rat skeletal muscle, using the mechanically skinned fibre technique, in order to understand better the apparently disparate results of previous studies and to examine recent proposals on the importance of certain intracellular factors in determining the efficacy of dihydropyridines. In twitch fibres from the iliofibularis muscle of the toad, 10 μM nifedipine completely inhibited depolarization-induced force responses within 30 s, without interfering with direct activation of the Ca2+-release channels by caffeine application or reduction of myoplasmic [Mg2+]. At low concentrations of nifedipine, inhibition was considerably augmented by repeated depolarizations, with half-maximal inhibition occurring at <0.1 μM nifedipine. In contrast, in rat extensor digitorum longus (EDL) fibres 1 μM nifedipine had virtually no effect on depolarization-induced force responses, and 10 μM nifedipine caused only ∼25% reduction in the responses, even upon repeated depolarizations. In rat fibres, 10 μM nifedipine shifted the steady-state force inactivation curve to more negative potentials by <11 mV, whereas in toad fibres the potent inhibitory effect of nifedipine indicated a much larger shift. The inhibitory effect of nifedipine in rat fibres was little, if at all, increased by the absence of Ca2+ in the transverse tubular (t−) system, provided that the Ca2+ was replaced with sufficient Mg2+. The presence of the reducing agents dithiothreitol (10 mM) or glutathione (10 mM) in the solution bathing a toad skinned fibre did not reduce the inhibitory effect of nifedipine, suggesting that the potency of nifedipine in toad skinned fibres was not due to the washout of intracellular reducing agents. The results are considered in terms of a model that can account for the markedly different effects of nifedipine on the two putative functions of the dihydropyridine receptor, as both t-system calcium channel and a voltage-sensor controlling Ca2+ release.
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Posterino, G.S., Lamb, G.D. Effect of nifedipine on depolarization-induced force responses in skinned skeletal muscle fibres of rat and toad. Journal of Muscle Research and Cell Motility 19, 53–65 (1998). https://doi.org/10.1007/BF03257390
- Extensor Digitorum Longus
- Force Response
- Skeletal Muscle Fibre
- Voltage Sensor