Myocardial metabolic and electrical properties of rabbits and ground squirrels at low temperatures

When perfused in vitro with Krebs-Henseleit solution both rabbit and arctic squirrel hearts exhibit similar contraction rates in a temperature range of 35°–25°C. Below 25° the rabbit heart was more susceptible to the present effect of cold. A reduction in temperature produced a much more pronounced depression in diastolic ventricular excitability in the rabbit than in the ground squirrel. At any given temperature, both species had quite similar in vitro coronary flow rates. As the perfusion fluid was cooled, however, flow rates of both species were markedly lowered. Ventricular tissue removed from rabbits with rectal temperatures of 15°C showed that a conversion of ATP to ADP had occurred. Lowering of the body temperature to 15°C in squirrels had no effect on ventricular ATP levels. At both normal and hypothermic temperatures, the total ventricular nucleotide content was much higher in squirrels than in rabbits. At 38° and at 15° squirrels exhibited a higher glutamic but a lower ß-hydroxybutyric, malic and succinic oxidase activity than rabbits.