Heat-injured rats: pathochemical indices and survival time.

Rats were exercised on a treadmill (9.14 m/min) in a hot environment (34.5 degrees C, 30% rh) until a rectal temperature of 42.0--42.5 degrees C was reached. Analysis of plasma constituents in subsequent serial blood samples demonstrated a highly significant inverse correlation between lactate concentration (P less than 0.001) and potassium levels (P less than 0.005) in blood samples taken immediately postexercise when both of these were correlated to survival time. Alternatively, plasma creatine phosphokinase (CPK) activity in these samples, although significantly elevated over control levels (P less than 0.001), was not correlated with either survival time or lactate-potassium concentrations. When fluid was administered prior to the run and immediately thereafter to repress pathological effects, there occurred no changes in plasma lactate and potassium levels between the postrun sample and a second sample taken 60 min later, while CPK levels were significantly reduced (P less than 0.01) in the second sample. However, levels of all three indices were significantly elevated (P less than 0.01) in a third sample taken terminally despite the fact that the animals were restrained and sedentary during this interval. These findings indicate that the hyperthermic injury may have had fundamental pathological effects on metabolism and membrane integrity producing lactacidemia and hyperkalemia of sufficient magnitude to compromise cardiovascular performance.