Force‐velocity relation in deuterium oxide‐treated frog single muscle fibres during the rise of tension in an isometric tetanus.

The force-velocity (P-V) relation from a single fibres isolated from the semitendinosus muscle of the frog was determined at pre-set times during the rise of tension and the plateau of isometric tetani. The controlled-velocity release method was used. Experiments were performed at about 2.25 micrometers sarcomere length and at 3-4 degrees C or at 19-21 degrees C. 2. Replacing H2O with D2O resulted in a rapid large reduction of the peak twitch tension and of the speed of development of twitch and tetanic tensions. The tetanic tension (P0) was usually reduced, in certain fibres to as low as 5% of the value in H2O-Ringer solution. 3. The depression of twitch and tetanus characteristics was followed by a recovery, the duration of which varied greatly in different fibres. During the recovery period previous conditioning activity potentiated the tetanus characteristics. 4. After the end of the recovery period in D2O-Ringer solution both the peak twitch tension and the speed of development of tetanic tension was still greatly depressed, whereas the value of P0 was slightly greater than in H2O-Ringer. The speed of rise of isometric tension after a quick release imposed at the tetanus plateau was reduced in D2O-Ringer, usually to about 50% of the value in H2O-Ringer. 5. D2O increased the development time of the P-V relation and produced a conspicuous increase in the degree of its curvature. The value of V0 (the velocity of shortening at zero load) was not significantly depressed by D2O and it was the same independent both of the time after the beginning of stimulation and of the isometric tension at which the measurement was made. The P-V relation attained its final characteristics before the isometric tension reached the plateau. During the recovery period in D2O-Ringer, at the plateau of isometric tetani of different size, the relative force exerted at a given velocity of shortening was constant. 6. In D2O-treated fibres, NO3- and caffeine (i) potentiated the peak twitch tension and the speed of development of tetanic tension without affecting significantly the speed of the redevelopment of tension after a quick release imposed at the tetanus plateau and (ii) reduced the development time of the P-V relation, but did not affect either the degree of its curvature or the value of V0 and P0. 7. The results are discussed by assuming that the release of Ca2+ from the sarcoplasmic reticulum is a rate-limiting process for the development of activation and in turn for the development of isometric tension. In terms of the cross-bridge model of Huxley (1957), the time or Ca2+-dependent factor of activation appears to be the recruitment of actin sites for cross-bridge formation, whereas the value of the rate constants regulating the cross-bridge kinetics appears to be time and Ca2+-independent.