Partial inhibition of skeletal muscle contraction by dantrolene sodium and its modification with perchlorate and Bay K 8644

Frequency-dependent effects of Bay K 8644 on tetanic contractions of frog skeletal muscle

1. The effects of the calcium channel agonist, Bay K 8644 (1-100 microM), on tetanic contractions, elicited by the stimulation frequencies of 100 Hz, 50 Hz and 25 Hz for 2 s, were investigated on frog skeletal muscle fibers. 2. Although the area under the tetanic force versus time curve was greatly reduced at the stimulation frequency of 100 Hz, this effect was significantly reversed at the lower stimulation frequencies of 50 and 25 Hz, at all concentrations tested. 3. During the intracellular recordings, it was revealed that the sodium action potentials elicited with the stimulation frequency of 100 Hz for 2 s were significantly blocked. 4. Similar to mechanical recordings, the blockade of repetitively elicited action potentials was also significantly reversed at lower stimulation frequencies of 50 and 25 Hz for all concentrations of Bay K 8644 tested. 5. In conclusion, the results indicate that Bay K 8644 depresses both tetanic contractions and action potentials in a frequency-dependent manner in frog skeletal muscle fibers.

Dantrolene blocks the tonic contraction and calcium influx evoked by K+ in ileal longitudinal smooth muscle

1. Dantrolene (10(-5)-10(-4) M) reduced the K+ (60 mM) tonic response in accordance with decreased Ca2+ uptake, as determined by La method, more than the phasic in ileal muscle. 2. After saponin-treatment of the fibers, which leaves the Ca2+ storage sites intact, dantrolene had only a slight effect on the caffeine-induced contraction. In Triton-X-100-treated fibers, in which the Ca2+ release sites are destroyed, dantrolene did not affect the Ca(2+)-induced contraction. 3. The results suggest that dantrolene inhibited the high-K(+)-induced tonic response mainly by inhibiting Ca2+ influx via voltage-dependent Ca2+ channels in ileum without affecting contractile elements. In contrast, dantrolene had only a slight effect on the intracellular Ca2+ release from caffeine-sensitive release sites.

Effects of nifedipine and Bay K 8644 on contractile activities in single skeletal muscle fibers of the frog

The effects of dihydropyridine derivatives on contractile activity were examined in single fibers of frog skeletal muscles. Both nifedipine and Bay K 8644 enhanced twitch responses regardless of the presence or absence of Ca2+ without any effects on the resting membrane potential, evoked action potential and Ca2+ sensitivity of the contractile machinery. Twitch responses enhanced by Bay K 8644, but not by nifedipine, were partially reduced by adenine, an inhibitor of Ca(2+)-induced Ca2+ release from sarcoplasmic reticulum, suggesting the involvement of Ca(2+)-induced Ca2+ release in Bay K 8644-induced potentiation of the twitches. Both nifedipine and Bay K 8644 caused a dose-dependent potentiation of K+ contractures without any effects on the membrane potential in the presence of Ca2+, but inhibited the contractures in the absence of Ca2+. These results suggest that dihydropyridine derivatives modify contractile activities of skeletal muscles by modulating the dihydropyridine receptors on the surface of T-tubules acting as voltage sensors.