Myocardial norepinephrine turnover during induced hypothermia and rewarming

Norepinephrine (NE) turnover in myocardial tissue was measured in male golden hamsters (Mesocricetus auratus) during 1) continuous hypothermia, 2) rewarming from hypothermia initiated by exposure to 22 degrees C ambient, and 3) normothermic control state. Hypothermia was induced by exposure to 80% He-20% O2 atmosphere at 0 to -10 degrees C. At sequential periods after tritiated norepinephrine ([3H]NE) infusion, hamsters were killed by cervical transection and hearts were removed and analyzed for NE and [3H]NE content. Rate constants, turnover times, and turnover rates were determined from regression analysis of [3H]NE/micrograms NE tissue decay. Myocardial concentrations of NE were constant during NE-turnover measurements in each group. However, myocardial NE levels were reduced by 37% in both continuous hypothermia and rewarming from hypothermia compared with normothermic controls. NE turnover was highest during rewarming from hypothermia (0.34 micrograms.g-1.h-1), but no decay in myocardial [3H]NE was detectable during continuous hypothermia. Control animals had turnover values of 0.15 micrograms.g-1.h-1. Turnover data indicate severe depression in myocardial sympathetic nerve activity during hypothermia but a significant increase above normothermic control levels during rewarming from hypothermia.