THE EFFECT OF CYCLE FREQUENCY ON THE POWER OUTPUT OF RAT PAPILLARY-MUSCLES IN-VITRO

Papillary muscles were isolated from the right ventricles of rats and the length for maximum active force generation (L(max)) was determined isometrically, The work loop technique was used to derive the length for maximum work production (L(opt)) at the cycle frequency, strain am plitude and stimulation phase shift found to be optimal for power outp ut. L(opt) was typically 7% shorter than L(max) and within the physiol ogical length range (87.5%L(max) to L(max)). Net work and power output were measured during sinusoidal strain cycles around L(opt), over the cycle frequency range 1-9 Hz, strain amplitude and phase shift being optimised for work and power at each frequency. Experiments were perfo rmed at 37 degrees C. Distinct optima were found in both the work-freq uency and the power-frequency relationships. The optimum cycle frequen cy for net work production was lower than the frequency for maximum po wer output, The mean maximum power output at 37 degrees C was 8.62+/-0 .50 W kg(-1) (mean +/- S.E.M., N=9) and was achieved at a cycle freque ncy of approximately 6 Hz, close to the estimated resting heart rate o f 5.8 Hz for the rats used (mean mass 223+/-25 g). The cycle frequency , strain amplitude and stimulation phase shift found to be optimal for power output produced an ia vitro contraction closely simulating the basal in vivo contraction.