Low-Temperature Performance of Isolated Working Hearts from a Hibernator and a Nonhibernator

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Abstract
An isolated working heart preparation was used to measure the effect of low temperatures on cardiac output (CO), oxygen consumption ($\dot{V}O_{2}$), coronary flow (CF), and aortic systolic and diastolic pressure development in hearts from rats and 13-lined ground squirrels (Spermophilus tridecemlineatus). Cardiac efficiency (CE) was calculated using these data (CE = energy output [KCal/h/g]/energy input [KCal/h/ g]). Below 20 C, rat hearts became arrhythmic and were unable to produce CO. Cardiac arrest occurred at 16.5 ± 0.8 C. Squirrel heart CO decreased progressively as temperature was lowered, but these hearts produced CO at 7 C. A declining spontaneous heart rate was compensated for by increased stroke volume. Mean aortic pressure (MAP) declined in rat hearts between 37 and 20 C, but MAP was maintained in squirrel hearts regardless of temperature. Cardiac efficiency increased slightly (10%-18%) in hearts from both species as temperature was lowered from 37 to 27 C, but squirrel heart CE increased by 60% when temperature was reduced from 17 to 7 C. The proportion of CO devoted to CF was significantly increased at lower temperatures in squirrel hearts but not in rat hearts. The ground squirrel heart has an intrinsic capacity to reapportion CF, make a more efficient use of metabolic fuel, and maintain CO and MAP at low temperatures. These adaptations support survival during deep and sustained hibernation.

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