Seasonal acclimation in cardiac activity: Mammals vs. fish

The dominance of vagal tonus over cardiac sympathetic tone is accentuated on acute cold exposure of mammals to a core temperature of 25 °C, leading to a bradycardia appropriate for the reduction in metabolic rate. When investigated by Power Spectral Analysis, changes in heart rate variability (HRV) can be seen as a decrease in the ratio of low frequency to high frequency (LF:HF) power in rats. In hamsters this only occurs after a period of cold acclimation to an environmental temperature of 4 °C, whilst maintaining core temperature. Interestingly, despite ∼30% cardiac hypertrophy on chronic cold exposure, the power output of acclimated hamster hearts (either rate-pressure product, or dP/dt) is less than those from euthermic controls, although less temperature-sensitive. Acclimation of common carp to similar environmental temperatures leads to a commensurate drop in core temperature, with a similar degree of cardiac hypertrophy. In this case, heart rate varied directly with oxygen consumption, although HR decreased and HRV increased in a non-linear manner between 25 and 5 °C on chronic exposure. Interestingly, acute changes in temperature resulted in a decrease in vagal tone on warming, but no significant change on cooling. Pharmacological blockade shows both chronic and acute responses to be dominated by cholinergic influences, with little role for adrenergic control. Supported by NERC.