Inotropic and electrophysiological effects of dantrolene on guinea-pig papillary muscle

Abstract

1. The mechanism of the inotropic effects of dantrolene, an inhibitor of intracellular Ca release in skeletal muscle, was analysed in isolated guinea-pig papillary muscles contracting isometrically at a frequency of 1 Hz. 2. In muscles from reserpine-pretreated animals and in muscles exposed to 30 nmol/l TTX, but taken from nonreserpinized animals, dantrolene produced a monophasic negative inotropic effect if applied at 1, 10, or 100 μmol/l. Within 24 min this effect amounted to ca. 30% of control force of contraction and was reached faster at the higher concentrations. It was not reversed during a subsequent 75-min superfusion with drug-free solution. 3. In muscles from non-reserpinized guinea pigs, the effects of 1 or 10 μmol/l dantrolene were identical to those described above, but 100 μmol/l induced a slight positive inotropic effect in 5 of 6 preparations. 4. The reduction of peak force of contraction in catecholamine-depleted muscles was accompanied by reductions in the rate of force development, time to peak force, and relaxation time. Resting potential, \(\dot V_{\max } \), and overshoot of the action potential were unaffected, and action potential duration (90% repol.) was prolonged slightly. Slow action potentials elicited at 24 mmol/l [K]0 at a frequency of 0.5 Hz were likewise unaffected with respect to \(\dot V_{\max } \) and overshoot and also showed a prolonged duration. Equilibration with 50 μmol/l dihydroouabain at 0.6 mmol/l [Ca]0 did not alter the extent of the negative inotropic dantrolene effect. The negative inotropic effect persisted in the presence of and after equilibration with caffeine (20 mmol/l) or ryanodine (2 μmol/l). 5. We conclude that the direct negative inotropic effect of dantrolene cannot be attributed to overt effects on the sarcolemma such as an inhibition of fast Na or slow Ca channels. The experiments with dihydroouabain argue against an impairment of the Nai-dependent regulator of contractility (Na−Ca exchange). While this evidence is compatible with an intracellular site of action of dantrolene, such as the sarcoplasmic reticulum, the drug acts differently from two other presumed inhibitors of this organelle, caffeine and ryanodine.