Does different thyroid state effect on the contractility of the cardiac muscle of eurythermal fish species, rainbow trout ( Oncorhynchus mykiss, Walbaum)?

Abstract

The effect of thyroid state on heart size and atrial contractility was examined in the rainbow trout (Oncorhynchus mykiss, Walbaum). Fish were randomly divided into four treatment groups: control (no treatment), sham (daily injections of drug vehicle), hyperthyroid (100 μg T 3 kg −1 day −1) and hypothyroid (20 mg methimazole kg −1 day −1) fish. Hypothyroidism induced 6% and 8% increase in body mass ( M b) and 32% and 20% increase in relative ventricular mass ( M V), compared to the values of control and sham, respectively. In hyperthyroid fish, M b and M V did not change at all. In hypothyroid and hyperthyroid fish, heart rate was 40% and 21% higher than in control fish, respectively. Thyroid state did not effect the kinetics of isometric contractions or recovery of force from inactivation (restitution). Inhibitory effect of 10 μmol l −1 ryanodine on isometric force (30–40%) and rate of restitution were similar in all treatment groups, suggesting that thyroid state did not have any effect on the function of the sarcoplasmic reticulum. These findings indicate that hypothyroidism causes clear increases in heart rate and heart size in the rainbow trout, while the effects of hyperthyroidism on heart size and function are negligible. Although the increases in heart rate and heart size in hypothyroid trout are similar as the changes induced by cold acclimation, the absence of any changes in kinetics of contraction or ryanodine sensitivity of force development in hypothyroid trout heart suggest that thyroid state alone cannot be responsible for seasonal temperature acclimatization in trout.