Intermittent Hypoxic Challenge in Conscious Rats Elevates Heart Rate But Does Not Increase Lumbar Sympathetic Nerve Activity

Abstract

In humans, moderate duration hypoxia (20 min) stimulates well‐described increases in muscle sympathetic nerve activity (SNA) that persist after withdrawal of the hypoxic challenge. However, episodes of naturally occurring apnea in humans and animals are of shorter duration, and the effects of such brief exposures to hypoxia on SNA have been little studied. To test this, male Sprague‐Dawley rats were instrumented with radiotelemetry devices to record mean arterial pressure (MAP), heart rate (HR) and lumbar SNA. After 3 days of recovery from surgery, the animals were exposed to four consecutive episodes of hypoxia (10% O2, 90% N2), that were 5 min in duration and given 10 min apart, all while recording cardiovascular and ventilatory responses. In ongoing studies, we found that the repeated hypoxic episodes produced expected increases in tidal volume, breathing frequency, and minute veltilation. Hypoxia also activated increases in HR (+57 ± 21 bpm) accompanied by mild elevations in MAP (+6 ± 5 mmHg). Unexpectedly, the hypoxic challenges caused no change in lumbar SNA (from 100% to 106 ± 14%), and did not increase SNA following offset of the challenges. At study end, a testing of lumbar SNA quality revealed that the nerves responded appropriately to sodium nitroprusside, phenylephrine and to hexamethonium bromide. These data show that repeated short‐term hypoxic challenges did not did not elevate lumbar SNA in conscious rats.Support or Funding InformationGalaxoSmithKline Pharmaceuticals