Responses evoked in single sympathetic nerve fibres of the rat tail artery by systemic hypoxia are dependent on core temperature.

No direct evidence exists of the changes evoked by systemic hypoxia in sympathetic nerves to the rat cutaneous circulation, and of the concomitant changes in cutaneous blood flow. Here we investigated responses evoked by two levels of systemic hypoxia (12% and 8% inspired O(2)) in single sympathetic units supplying tail caudal ventral artery (CVA) in spontaneously breathing anaesthetized rats, whilst simultaneously recording tail blood flow and vascular resistance (TVR) from the CVA, under conditions of modest hypothermia and hyperthermia. During modest hypothermia and normoxia, TVR was high and CVA unit activity was present, with marked respiratory modulation and a rhythmictiy (T-rhythm) that was often independent of respiration. Hypoxia evoked a graded fall in TVR indicating vasodilatation, but there were no consistent changes in CVA unit firing rate or T-rhythm frequency, although respiratory modulation increased. By contrast, during hyperthermia, TVR was low and CVA unit activity was absent. Systemic hypoxia evoked graded increases in TVR, indicating vasoconstriction, and in 8% O(2) there was recommencement of firing in some CVA units, at low discharge rate, with respiratory modulation but no T-rhythm. These results indicate that the changes evoked by systemic hypoxia in TVR and sympathetic nerve activity to CVA are dependent on core temperature. During modest hypothermia, hypoxia-induced cutaneous vasodilatation in the tail is independent of sympathetic activity, whereas during hyperthermia, when sympathetic activity is 'switched off', severe hypoxia initiates respiratory-related low level activity, causing cutaneous vasoconstriction.