Acute heat tolerance of cardiac excitation in the brown trout (Salmo trutta fario).

The upper thermal tolerance and mechanisms of heat-induced cardiac failure in the brown trout (Salmo trutta fario) was examined. The point above which ion channel function and sinoatrial contractility in vitro, and electrocardiogram (ECG) in vivo, started to fail (break point temperature, BPT) was determined by acute temperature increases. In general, electrical excitation of the heart was most sensitive to heat in the intact animal (electrocardiogram, ECG) and least sensitive in isolated cardiac myocytes (ion currents). BPTs of Ca2+ and K+ currents of cardiac myocytes were much higher (>28°C) than BPT of in vivo heart rate (23.5±0.6°C) (P