Antarctic fish Pagothenia borchgrevinki in McMurdo Sound, Antarctica, inhabit one of the coldest and most thermally stable of all environments. Sea temperatures under the sea ice in this region remain a fairly constant -1.86 degrees C year round. This study examined the thermal plasticity of cardiac function in P. borchgrevinki to determine whether specialisation to stable low temperatures has led to the loss of the ability to acclimate physiological function. Fish were acclimated to - 1 degrees C and 4 degrees C for 4 - 5 weeks and cardiac output was measured at rest and after exhaustive exercise in fish acutely transferred from their acclimation temperature to - 1, 2, 4, 6 and 8 degrees C. In the - 1 degrees C acclimated fish, the factorial scope for cardiac output was greatest at - 1 degrees C and decreased with increasing temperature. Increases in cardiac output with exercise in the - 1 degrees C acclimated fish was achieved by increases in both heart rate and stroke volume. With acclimation to 4 degrees C, resting cardiac output was thermally independent across the test temperatures; furthermore, factorial scope for cardiac output was maintained at 4, 6 and 8 degrees C, demonstrating thermal compensation of cardiac function at the higher temperatures. This was at the expense of cardiac function at - 1 degrees C, where there was a significant decrease in factorial scope for cardiac output in the 4 degrees C acclimated fish. Increases in cardiac output with exercise in the 4 degrees C acclimated fish at the higher temperatures was achieved by changes in heart rate alone, with stroke volume not varying between rest and exercise. The thermal compensation of cardiac function in P. borchgrevinki at higher temperatures was the result of a change in pumping strategy from a mixed inotropic/chronotropic modulated heart in - 1 degrees C acclimated fish at low temperatures to a purely chronotropic modulated heart in the 4 degrees C acclimated fish at higher temperatures. In spite of living in a highly stenothermal cold environment, P. borchgrevinki demonstrated the capacity to thermally acclimate cardiac function to elevated temperatures, thereby allowing the maintenance of factorial scope and the support of aerobic swimming at higher temperatures.
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