Proceedings of The Physiological Society

University of Leeds (2002) J Physiol 544P, S005


Changes in sympathetic nerve activity recorded from skeletal muscle arteries of the anaesthetised rat during graded levels of systemic hypoxia

Steven Hudson*, Christopher D. Johnson†, Andrew M. Coney* and Janice M. Marshall*

*Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TT and Medical Biology Centre, Queen's University, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland

We recently demonstrated the ability to record sympathetic nerve activity from the surface of arterial vessels of 80-120 mm within skeletal muscle; this activity has characteristic baroreceptor modulation (Hudson et al. 2002). We have now investigated the changes in muscle sympathetic nerve activity (MSNA) produced by graded levels of systemic hypoxia.

Experiments were performed on eight spontaneously breathing male Wistar rats (256-322 g) anaesthetised with Saffan (7-12 mg kg-1 h-1, I.V.). The animal was killed with an anaesthetic overdose at the end of the experiment. MSNA was recorded directly from the surface of arterial blood vessels of the right spinotrapezius muscle using the focal recording technique (Hudson et al. 2002). Hypoxic gas mixtures (12, 10 and 8 % O2 in N2) given consecutively via the tracheal cannula for 2 min each over a 6 min period evoked graded falls in mean arterial blood pressure (MABP) graded increases in heart rate (HR) and respiratory rate (Marshall & Metcalfe, 1988). Concomitantly there were graded increases in MSNA.

In four animals which experienced the complete protocol of 12, 10 and 8 % O2, MABP fell from a baseline of 118 ± 10 to 99 ± 12, 81 ± 6 and 67 ± 8 mmHg (means ± S.E.M.), respectively, while HR increased from 391 ± 28 to 401 ± 23, 416 ± 16 and 419 ± 8 beats min-1 and respiratory rate increased from 88 ± 7 to 110 ± 7, 115 ± 9 and 113 ± 9 breaths min-1. Concomitantly, MSNA (2-3 units in each recordings) increased by 81 ± 15, 125 ± 11 and 193 ± 9 % during 12, 10 and 8 % O2. Cross-correlation histogram analysis showed MSNA to have strong cardiac- (6.5-7 Hz) and respiratory- (1.5-1.9 Hz) related rhythms at rest and during each period of hypoxia.

These results indicate that graded systemic hypoxia evokes graded increases in MSNA as a consequence of peripheral chemoreceptor stimulation and/or baroreceptor unloading, even though there is an accompanying graded vasodilatation in skeletal muscle (Marshall & Metcalfe, 1988).

We gratefully acknowledge the support of the British Heart Foundation.

All procedures accord with current UK legislation.

Where applicable, experiments conform with Society ethical requirements