Influence of cycle frequency, muscle strain and muscle length on work and power production of rainbow trout (Oncorhynchus mykiss) ventricular muscle.

This study investigates the effects of cycle frequency, strain and length on work and power output of isolated rainbow trout (Oncorhynchus mykiss) ventricular preparations using the work loop technique. These effects 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 (Lopt) was found to be the same as the length for maximum isometric force production (Lmax). The decline in net work at lengths greater than Lopt/Lmax was attributed to an increase in passive work (the work done on an unstimulated muscle) or to hysteresis and to a large increase in lengthening work. The strain yielding maximum work decreased with increasing frequency. This is discussed in the context of the decline in stroke volume observed at increased heart rates in vivo. Muscle strain in intact hearts paced at 0.3 Hz was +/-11.9 % (23.8 % peak to peak), a value similar to the optimum strain at 0.3 Hz in vitro (+/-12 %).