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 (alpha-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<or=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.