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 Lace'pede) hearts, follow-ing 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 moni-tored 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 lmmoll"1 andSmmoll"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 enhance-ment 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