LOW AMBIENT-TEMPERATURE INCREASES BAROREFLEX-GOVERNED SYMPATHETIC OUTFLOW TO MUSCLE VESSELS IN HUMANS

Microelectrode multi-unit recordings of muscle nerve sympathetic activity, primarily involved in blood pressure regulation, were made from the right peroneal nerve in 10 healthy subjects during exposure to cold inside a box used for hypothermic surgery. Blood pressure was monitored by an automatic cuff applied to the left arm. Heart activity (ECG) and the temperature inside the box were monitored. Muscle nerve sympathetic activity outflow and blood pressure were stable during 15 minutes initial rest at 22.7 +/- 0.3-degrees-C (mean +/- SEM). All subjects increased muscle nerve sympathetic activity and blood pressure with lowering of ambient temperature. The box temperature was 10.5 +/- 0.3-degrees-C at the end of the cooling period. With rewarming the changes in muscle nerve sympathetic activity and blood pressure were reversed. Mean outflows of muscle nerve sympathetic activity during initial rest, final phase of cooling period, and post-rewarming were 20.3 +/- 2.7, 26.6 +/- 3.2 and 20.2 +/- 2.8 burst min-1, respectively. Systolic blood pressure during the same periods was 118 +/- 3, 131 +/- 4 and 120 +/- 4 mmHg, respectively, whereas corresponding values for diastolic blood pressure were 73 +/- 2, 82 +/- 2 and 75 +/- 2 mmHg (P < 0.001 for all variables; (ANOVA). It is concluded that muscle nerve sympathetic activity, while not regarded as being involved in body temperature regulation, is under the influence of ambient temperature and contributes to blood pressure elevation in a cold environment. The increase in muscle nerve sympathetic activity may play a role in body temperature regulation by preserving heat within the central core or by inducing heat production by small muscles in the vessel walls, the latter possibility being in agreement with recent observations suggesting that smooth muscles in limb vasculature may contribute to thermogenesis.