Cardiovascular alterations by chronic intermittent hypoxia: importance of carotid body chemoreflexes

1. Humans experiencing intermittent hypoxia (IH) owing to recurrent apnoea syndromes exhibit serious cardiovascular morbidity, including high blood pressure, increased sympathetic nerve activity, cardiac arrhythmia and myocardial infarction. Although apnoeas are accompanied by a simultaneous decrease in arterial O(2) (hypoxia) and an increase in CO(2) (hypercapnia), studies on experimental animals suggest that hypoxia, rather than hypercapnia, is the primary stimulus for developing hypertension and enhanced sympathetic nerve activity. Enhanced hypoxic-sensing ability of the carotid bodies and the ensuing reflex activation of the sympathetic nervous system have been suggested to play a critical role in cardiorespiratory alterations resulting from recurrent apnoeas. 2. The purpose of the present review is to highlight recent studies demonstrating the effects of IH on carotid body sensory activity and its consequences on sympathetic activation in a rodent model of chronic IH. Adult rats exposed to chronic IH (15 s of 5% O(2) followed by 5 min of 21% O(2), nine episodes per h, 8 h/day for 10 days) exhibited selective enhancement of carotid body sensory response to hypoxia. In addition, chronic IH induced a novel form of sensory plasticity in the carotid body, manifested as sensory long-term facilitation (LTF). Functional changes in the carotid body occurred in the absence of morphological changes in the chemoreceptor tissue. 3. Acute hypoxia increased expiratory modulated splanchnic nerve activity (SNA) and acute IH-induced LTF in SNA. Hypoxia-induced SNA activation was prevented by bilateral sectioning of the sinus nerves. Rats exposed to chronic IH exhibited enhanced hypoxia-induced sympathetic activation and augmented LTF of the SNA. Bilateral sectioning of the sinus nerves abolished these responses, suggesting chronic IH-induced alterations in carotid body sensitivity contribute to LTF in SNA and the subsequent cardiovascular alterations.