Heart lesions in the frog at high environmental temperature

Frogs (R. pipiens) adapted to 12 degrees C were exposed to increased environmental temperature during 2 hr. At 33 degrees C gross heart lesions started to appear in a few cases and at 37 degrees C, 71% of the frogs showed ventricular aneurysms and some died. Critical thermal maximum (CTM) was around 37 degrees C in autumn, winter and early spring, whereas in late spring and summer CTM was at 39 degrees C and during these last-mentioned periods of the year gross heart lesions at high temperature did not start until at 37 degrees C. After 2 hr at 37 and 39 degrees C respectively, ECG-abnormalities occurred indicating myocardial injury. Isolated strips of the heart ventricle from frogs with gross heart lesions showed a deteriorated resistance towards cyanide anoxia as measured by an isometric procedure. The ultrastructure was deranged by the high temperature, the changes mainly involving the mitochondria and the myofibrils. These alterations were focal but depletion of glycogen particles was diffuse. As the high temperature induced changes similar to those provoked by catecholamines, propranolol was given in some frogs before the heat exposure. This beta-adrenoceptor blockade did only reduce some of the abnormal changes by 37 degrees C.