INFLUENCE OF CYCLE FREQUENCY, MUSCLE STRAIN AND MUSCLE LENGTH ON WORKAND POWER PRODUCTION OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) VENTRICULAR MUSCLE

This study investigates the effects of cycle frequency, strain and len gth on work and power output of isolated rainbow trout (Oncorhynchus m ykiss) ventricular preparations using the work loop technique. These e ffects are discussed in the context of the whole heart using analogies with heart rate, stroke volume and end-diastolic volume. Power output was dependent on cycle frequency, increasing threefold beween 0.3 and 1.1 Hz, The frequency for maximum power output was approximately 1.1 Hz, corresponding to the frequency for maximum power in perfused heart experiments. The length for maximum work production (L-opt) was found to be the same as the length for maximum isometric force production ( L-max) The decline in net work at lengths greater than L-opt/L-max was attributed to an increase in passive work (the work done on an unstim ulated muscle) or to hysteresis and to a large increase in lengthening work. The strain yielding maximum work decreased with increasing freq uency. This is discussed in the context of the decline in stroke volum e observed at increased heart rates in vivo. Muscle strain in intact h earts paced at 0.3 Hz was +/- 11.9 % (23.8 % peak to peak), a value si milar to the optimum strain at 0.3 Hz in vitro (+/- 12 %).