Proceedings of The Physiological Society

Kings College London (2005) J Physiol 565P, C167


Different mechanisms underlie respiratory rhythms in muscle and thermoregulatory vasoconstrictor sympathetic activity

Huang, Chunhua ; Marina, Nephtali ; GILBEY, Michael P;

1. Physiology, UCL, London, United Kingdom.

The pattern and rhythm in sympathetic activity regulating many targets indicates cardiorespiratory coupling (Numao et al. 1987; Smith & Gilbey, 2000). Smith & Gilbey (2000) concluded that central respiratory drive (CRD)-related rhythms in renal nerve activity arose directly from central respiratory networks, whereas activity supplying thermoregulatory circulations (CVC) resulted from entrainment of autonomous sympathetic rhythm generators (T-rhythm: Chang et al. 1999). Here we investigate the mechanism(s) underlying cardiorespiratory interactions in sympathetic muscle vasoconstrictor (MVC) activity. S-D rats (male, 280-330g, n=6) were anaesthetised (sodium pentobarbitone 60 mg kg-1 I.P.; supplemented with 5-10 mg α-chloralose I.V., as required until humane killing with anaesthetic), vagotomised, sino-aortic denervated and pneumothorax. During positive pressure ventilation (2.0 Hz, CRD (index phrenic nerve (PN) activity) was adjusted by adding COnull2 to the inspired gas mixture. Arterial blood gases and pH were assessed after each data collection. A neuromuscular blocker (gallamine triethiodide 16mg kg-1 h-1; Smith & Gilbey, 2000) was administered. Autospectra were computed from rectified and smoothed (τ=20ms) population recordings from a gastrocnemius nerve (GN, MVC activity), the plantar aspect of the ipsilateral tibial nerve (TNp, activity of typical CVC activity) and PN (Huang & Gilbey, 2003). Whereas at all levels of CRD tested a dominant peak in the autospectra of MVC activity was observed at rhythmic PN discharge frequency (fPN: linear regression analysis, 21 pts, Y-intercept=0, slope=1, r2=1), a dominant peak in powerspectra of CVC activity was observed at fPN only when CRD was at an enhanced level (PnullaCOnull2 59±3): at normal levels of CRD and in central apnoea the T-rhythm dominated (linear regression analysis, 21 pts, Y-intercept=0.75, slope=-0.24, p=0.0006, r2=0.47). PN-triggered averages of MVC and CVC activities showed that during enhanced CRD both nerve activities reached a peak in early expiration, however peak MVC activity was sustained (duration 0.46±0.04 of phrenic cycle), whereas peak CVC activity subsided rapidly (duration 0.08±0.02; durations significantly different P<0.003, unpaired t test). Thus, two distinct mechanisms underlie the CRD-related MVC and CVC rhythms. We provide evidence that CRD entrains autonomous sympathetic rhythm (T-rhythm) generators associated with CVC activity. No such evidence was obtained concerning sympathetic networks controlling MVC activity.

Where applicable, experiments conform with Society ethical requirements