Hypercapnia selectively attenuates the somato-sympathetic reflex

The effects of hyperoxic hypercapnia (5, 10 or 15% CO₂ in O ₂) on splanchnic sympathetic nerve activity (sSNA) and sympathetic reflexes such as the somato-sympathetic reflex or baroreflex were studied in urethane anaesthetised, paralysed, artificially ventilated and vagotomized Sprague-Dawley rats. Hypercapnia caused a small increase in mean arterial blood pressure (MAP) in the 10% CO₂ group and a fall in heart rate (HR) in all three groups. sSNA increased in all three groups. Phrenic frequency and amplitude increased during hypercapnia, with frequency adapting back towards baseline during the CO₂ exposure. The somato-sympathetic reflex was attenuated in the 5% CO₂ group and abolished in the 10 and 15% CO₂ groups, whereas there was little effect on the sSNA baroreflex. Hypercapnia significantly affects phrenic nerve activity (PNA), sSNA and selectively inhibits the somato-sympathetic reflex with little effect on the sSNA baroreflex.