Enhanced maximum frequency and force development of fish hearts following temperature acclimation

Inotropic and chronotropic responsiveness of yellow perch (Perca flavescens Mitchill) and smallmouth bass (Micropterus dolomieui Lacépède) hearts, following low temperature acclimation, was assessed with ventricle strips mounted for isometric force recording. Animals were acclimated to 20° and 5°C, and the performance of ventricle strips from both acclimation temperatures was monitored at 20° and 5 °C. Ventricle strips from yellow perch acclimated to 20 °C showed an increase in resting tension when tested at 5 °C in the presence of high levels of extracellular calcium. An increase in resting tension did not occur in preparations from 5°C-acclimated fish tested at 5 °C. This suggests failure of intracellular calcium regulation which may be ameliorated following an acclimation period. Ventricle strips were subjected to a force-frequency challenge over the range of 12–48 contractions min−1 at 1 mmol l−1 and 3 mmol l−1 extracellular calcium. Time to 50 % relaxation of ventricle strips tested at 5 °C was significantly lower for hearts from perch acclimated at 5 °C than from those acclimated at 20 °C. This was associated with an ability to maintain function at higher pacing rates. Similar trends were exhibited by hearts from smallmouth bass. As calcium extrusion is a prime determinant of relaxation time, these findings further suggest an enhancement in calcium handling capabilities following acclimation to low temperature. Ventricle strips from both species acclimated to 20 °C and tested at 20 °C were able to maintain force development to the highest contraction frequencies. Hearts from specimens acclimated to 5 °C and tested at 20 °C showed a negative force–frequency relationship at low extracellular calcium levels. In yellow perch, this effect was minimized by an increase in calcium availability. This is considered to be a potential mechanism by which animals acclimated to low temperature could make transient excursions into warm water.