A comparison of simultaneously recorded muscle and skin vasoconstrictor population activities in the rat using frequency domain analysis

In anaesthetized rats, an apparently autonomous sympathetic rhythm (T-rhythm, frequency range 0.4–1.2 Hz), has been observed in nerve activity controlling thermoregulatory circulations but not renal nerves. To further explore the differential control of sympathetic activity here, we investigate whether the so-called T-rhythm is a feature of muscle vasoconstrictor (MVC) population activity. Population activity was studied in vagotomised anaesthetised rats (α-chloralose or urethane maintenance, after barbiturate or halothane induction, respectively). Some rats were additionally sino-aortic denervated (SAD) and/or given a pneumothorax and neuromuscular blocked. Animals were held in central (hypocapnic) apnoea (ventilated at 2 Hz, tidal volume ≤ 2 ml) so that the T-rhythm could be studied without the confounding influence of central respiratory drive. In all animals (34; 17 with SAD) a peak in autospectra at T-rhythm frequency (T-peak: approximately 0.75 Hz) was a characteristic feature of activity supplying a thermoregulatory circulation (hind foot cutaneous vasoconstrictor activity, CVC), but not of simultaneously recorded MVC (gastrocnemius) activity. Percentage power at T-peak frequency was 4–5 times greater in CVC than MVC autospectra and at heart rate frequency approximately 14 fold greater in MVC than CVC autospectra: no peak was present at heart rate frequency in CVC autospectra. No peaks were present in MVC autospectra in SAD preparations. MVC–CVC coherence at both frequencies was low (approximately 0.2) in all types of preparation; i.e., most of the activity recorded from the two nerves was not linearly related. We conclude that under the experimental conditions of this study the T-rhythm is not a robust feature of MVC activity and SAD does not increase MVC–CVC coherence: observations which are consistent with fundamentally different neural substrates regulating MVC and CVC activities under the conditions of these experiments.