Increased ventricular stiffness and decreased cardiac function in Atlantic cod (Gadus morhua) at high temperatures

We employed the work loop method to study the ability of ventricular and atrial trabeculae from Atlantic cod to sustain power production during repeated contractions at acclimation temperatures (10 degrees C) and when acutely warmed (20 degrees C). Oxygen tension (PO2) was lowered from 450 to 34% air saturation to augment the thermal stress. Preparations worked under conditions simulating either a large stroke volume (35 contractions/min rate, 8-12% muscle strain) or a high heart rate (70 contractions/min, 2-4% strain), with power initially equal under both conditions. The effect of declining PO2 on power was similar under both conditions but was temperature and tissue dependent. In ventricular trabeculae at 10 degrees C (and atria at 20 degrees C), shortening power declined across the full range of PO2 studied, whereas the power required to lengthen the muscle was unaffected. Conversely, in ventricular trabeculae at 20 degrees C, there was no decline in shortening power but an increase in lengthening power when PO2 fell below 100% air saturation. Finally, when ventricular trabeculae were paced at rates of up to 115 contractions/min at 20 degrees C (vs. the maximum of 70 contractions/min in vivo), they showed marked increases in both shortening and lengthening power. Our results suggest that although elevated heart rates may not impair ventricular power as they commonly do isometric force, limited atrial power and the increased work required to expand the ventricle during diastole may compromise ventricular filling and hence, stroke volume in Atlantic cod at warm temperatures. Neither large strains nor high contraction rates convey an apparent advantage in circumventing this.