The development of the force-velocity relation in normal and dantrolene-treated frog single muscle fibres.

The force-velocity (P-V) relation for single fibres isolated from the semitendinosus muscle of the frog was determined at preset times during the rise of tension and the plateau of isometric tetani. The controlled-velocity release method was used. Experiments were performed at a sarcomere length of about 2.25 micrometer. Addition of dantrolene-sodium (DaNa) to Ringer's solution resulted in a large reduction of the peak twitch tension and of the speed of rise of twitch and tetanic tensions. The plateau tetanic tension was either unaffected or only slightly reduced by DaNa. In all fibres the speed of rise of isometric tension after a quick release imposed at the tetanus plateau was not affected by DaNa. DaNa did not significantly affect the observed value of V0 and the calculated values of Hill's constants V0, P0, a/P0 and b for the P-V relation determined at the tetanus plateau. In accordance with previous work, during the tetanus rise the P-V relation gradually attained its final characteristics, but there was a significant delay of the development of the isometric tension with respect to the development of the P-V relation. Treatment of the fibre with DaNa increased both the development time of the P-V relation and the level of isometric tension at which during the tetanus rise the P-V relation attained its final characteristics. It is concluded that DaNa, which inhibits the release of Ca2+ from the sarcoplasmic reticulum, also depresses the rate of development of activation.