Proceedings of The Physiological Society

Trinity College Dublin (2003) J Physiol 551P, PC36


Comparison of gastrocnemius and tibial nerve sympathetic discharge frequency components in anaesthetised rats

C. Huang and M.P. Gilbey

Department of Physiology, University College London, London NW3 2PF, UK

Previously we have demonstrated that population and single unit cutaneous vasoconstrictor discharges have a dominant rhythm (~0.4-1.2 Hz: T-rhythm). We propose that it arises from a family of oscillators whose graded and dynamic synchronisation may be important in sympathetic control (see Gilbey, 2001). Currently we are analysing muscle sympathetic activity to examine the idea that our concepts have equal application to mechanisms underlying muscle vasoconstrictor rhythms. As an initial stage we have compared population gastrocnemius (GN) (muscle) and tibial nerve (TN) (recorded at a point distal to the main muscle branches - 'plantar' branch) sympathetic activity.

Thirteen male Sprague-Dawley rats (290-350 g) were anaesthetised (60 mg kg-1 sodium pentobarbitone I.P.; supplemented with 5-10 mg α-chloralose I.V. as required) vagotomised, ventilated artificially, given a pneumothorax and neuromuscularly blocked (pancuronium bromide, 1 mg kg-1). Anaesthesia and neuromuscular block were monitored and maintained (both I.V.) until humane killing with anaesthetic (I.V.) (see Johnson & Gilbey, 1996). Some animals additionally underwent sino-aortic denervation (SAD; ibid.). Using glass suction electrodes, recordings were made from either TN or a GN (medial or lateral head) simultaneously with that from an ipsilateral GN (filtered, 80-1000 Hz; rectified and smoothed, t{special} = 20 ms). Phrenic nerve discharge was recorded as an index of central respiratory drive. Auto and coherence spectra were calculated as described by Smith & Gilbey (2000) (300 s data sets; sampling rate 100 Hz; FFT block size 2048, 50 % overlap, 28 blocks).

During central apnoea in baroreceptor-intact animals (mmHg, means ± S.E.M.: Pa,CJ{special} 32 ± 2, Pa,J{special} 157 ± 17, BP 106 ± 5, n = 8) GN spectra (11 recordings in 8 animals including 3 pairs) had a peak at heart rate frequency (fHR) whereas TN spectra did not (n = 5). Conversely, in these same recordings TN spectra had a peak in the range 0.6-0.9 Hz (T-rhythm) whereas GN spectra did not. Consistent with these differences in spectral profile GN-GN (3 pairs) coherence was significant (>0.1) at all frequencies examined (< 10 Hz), whereas GN-TN coherence at T-rhythm frequency reached significance (>0.1) in 1/5 cases. In SAD rats (n = 5, GN-TN pairs, Pa,CJ{special} 26 ± 4, Pa,J{special} 146 ± 17, BP 101 ± 7) a peak at fHR was absent and GN-TN coherence was significant (>0.1) in 1/5 cases (at T-rhythm frequency).

Our data show that the coherence spectrum of GN and TN sympathetic activities rarely reach significance (>0.1) at T-rhythm frequency. We conclude that at this frequency the drive to these nerves is not obligatorily linearly coupled.

This work was supported by BHF grant no. PG/2001054.

Where applicable, experiments conform with Society ethical requirements