The hibernating mammalian heart

Abstract

Decapitation during hibernation and observation of the beating heart, or observation of the isolated heart of the hibernating thirteen-lined ground squirrel displayed survival of this heart in the cold for periods of time far in excess of that seen in nonhibernating mammals (the rat) subjected to the same experimental methods. This phenomenon has been studied and data have been collected, about which speculation is made and which is summarized as follows: 1. 1. Heart in hibernation: A hibernating heart is relatively temperature-insensitive throughout the range of hibernating temperatures (0° to 20°C.). Contrarily, it is very sensitive to tactile stimulation. The mechanical contractions of an isolated hibernating heart may be extremely faint (to a point at which they cannot be observed visually), whereas electrical beating may continue for hours after visible contractions are no longer observable. This latter property poses the question whether in the intact hibernating mammal the heart actually moves blood with each electrical record of a heartbeat. 2. 2. Heart in arousal from hibernation: stimulation to initiation of arousal of the hibernating heart is triggered by the action of a sympathetico-adrenal mechanism on a “physiologically vagotomized” heart. Initiation of arousal is further strengthened by respiratory muscle contractions in proximity to the heart at rates which act as “pacemaker” for the entire arousal process. Thus, the early arousal process of the heart seems to be geared to nervous and mechanical mechanisms, but not to a change in body temperature. Later in the arousal process, further stimulation of the heart rate is certainly modified in the direction of still further increases in rate by a rise in body temperature. There is evidence for changes in metabolic stores within the heart musculature as arousal proceeds. 3. 3. Heart in induction to hibernation: The stimulus to induction of the heart into hibernation seems to have four elements: (a) a preparatory phase (possibly hormonal) during which the musculature of the heart is readied (this may be “seasonal”, in the sense that the animal “readies itself”, not in the sense of the “four seasons”); (b) a fall in body temperature during which the heart rate of the animal entering hibernation follows a cooling curve similar to that in Nembutal anesthesia plus cold for such an animal; (c) a “physiological vagotomy” with possible maintenance of sympathetico-adrenal stimulation at a very low level; and (d) a shift from one metabolic scheme to another (probably enzymatic) when the body temperatures falls below 20°C.