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°Cacclimated 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 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

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