Hyperoxia blunts renal sympathetic nerve activity response to acute intermittent hypercapnia in rats

Activation of the sympathetic nervous system plays an important role in the pathophysiology of sleep-related breathing disorders. The aim of the present study was to examine the effects of different levels of hypercapnia in the presence of various background oxygen levels on the magnitude of sympathoexcitation, measured by the renal sympathetic nerve activity (RSNA) in the acute intermittent hypercapnia (AIHc) rat model. The study was conducted on 56 urethane-anesthetized, vagotomized and mechanically ventilated Sprague-Dawley rats (n = 7/group). Each experimental group was subjected to a distinct AIHc protocol that varied in the applied levels of hypercapnia and background oxygen. Mean arterial pressure and RSNA were analyzed in 7 experimental time points: baseline, five hypercapnic episodes (each lasting 3 min) and 15 minutes following the last hypercapnic episode. Exposure to severe hypercapnia (FiCO₂ = 0.15) evoked an increase in RSNA, which was preserved throughout the protocol, whereas in moderate hypercapnia (FiCO₂ = 0.05) groups there was a trend of progressive diminution of RSNA magnitude following the first hypercapnic episode. Exposure to severe hypercapnia elicited significantly greater RSNA response during first hypercapnic episode and it was enhanced during subsequent episodes compared to exposure to moderate hypercapnia. Additionally, hyperoxic₂ background (50% O₂) blunted the RSNA response to AIHc compared to room air background, both in severe and moderate hypercapnia groups. Mean arterial blood pressure was preserved throughout the experimental protocol in all studied groups. These findings indicate that acute intermittent hypercapnia evokes increased renal sympathetic nerve activity that is dependent on the severity of hypercapnic exposures and the background oxygen level.