Carotid body function in animal models of obstructive sleep apnoea

Abstract

[Objectives]: Hypoxic hypoxia (HH) occurs when O2 supply from atmosphere to blood is inadequate to meet the metabolic needs of the organism. It can be acute (as in transient ascensions to high altitude or as in breath holding) or chronic (as in long lasting sojourning or residence at high altitude or as in chronic pulmonary disease). In both instances HH is sustained, with a unique onset and end. In all situations of sustained hypoxia carotid body (CB) is the first line of defense: it detects the diminished blood PO2 and elicits cardiorespiratory reflexes aimed to secure adequate O2 provisioning and tissue distribution. If sustained hypoxia is chronic (CSH) CBs sensitize causing a hyperventilation greater than expected to any given PO2. Another type of HH is chronic intermittent hypoxia (CIH), as it occurs in obstructive sleep apnea; it is repetitive (60 episodes/h) during sleep time and may last for years. The behavior of the CB to CIH seems different to that of CSH. It is agreed that CB also sensitizes in CIH, but the reflex ventilatory responses elicited are differently appreciated: different authors see hyperventilation elicited by CIH normal, lower, or higher than expected from the prevailing PO2. Moreover, in CIH the CB triggers an increase in sympathetic tone causing cardiovascular alterations (systemic hypertension). [Materials]: We have studied, comparatively, the effects of CSH and CIH on several parameters in the rat: a) CB catecholamine (CA) content and rate of synthesis and release; b) activity in the CB sensory nerve; c) ventilatory responses to HH; d) sympathetic activity assessed as plasma CA content and rate of synthesis in renal artery; e) arterial blood pressure; f) c-fos expression in brainstem nuclei involved in the integration of respiratory and cardiovascular systems. [Conclusions]: Findings indicate the specificity of each type of hypoxia to activate the CB, the brainstem nuclei and to elicit integrated responses.