Functional characteristics and responses to adrenergic stimulation of isolated heart preparations from hypothermic and hibernating subjects

Contemporary studies of isolated hearts from hibernators and nonhibernators are presented. Original experiments with isolated perfused hamster hearts are reported. Such hearts can maintain left ventricular function at temperatures as low as 7 °C. Generated left ventricular pressure was 40 ± 9 mm Hg and heart rate was 7 ± 1 beats/min. During cooling heart rate dropped dramatically, coronary flow increased, and ventricular pressure decreased initially, plateaued, and then fell as 7 °C was approached. Norepinephrine can cause increased heart rate and left ventricular pressure at 22 and 7 °C. This positive inotropic and chronotropic response was associated with increased cAMP at 30 sec after stimulation at 22 °C but not at 7 °C. Furthermore. cAMP was also not changed at peak response at 7 °C. Isoproterenol increased cAMP content in 37 °C ventricular slices but not at hypothermic temperatures. Possible mechanisms of nonadenylate cyclase mediation of inotropic and chronotropic responses at 7 °C are discussed.