Middle cardiac nerve section alters ventilatory response to Pa CO 2 in the cockerel

Abstract

To determine whether afferents in the middle cardiac nerves (MCN) contributied to extrapulmonary Pa co 2 mg/kg), and cannulated the cutaneous ulmar vein, and the carotid and brachial arteries. The thorax was opened and each lung unidirectionally ventilated from separate gas delivery systems. A ligature, which temporarily occluded blood flow, was placed around the right pulmonary artery. Both cardiac sympathetic nerves were cut, as well as the left vagus just above the level of the recurrent branch. We exposed the non-perfused right lung to 105 Torr P co 2 to silence intrapulmonary cemoreceptors (IPC). We measured blood pressure, heart rate and ventilatory movements while the denervated left lung was used to fix Pa co 2 at sevel levels ranging from 7–140 Torr. As arterial P co 2 increased, ventilatory amplitude increased from 0.3 mm to 3.6 mm, while frequency decreased from 140 to 24 per min. After cutting the MCN, ventilatory movements were less responsive to Pa co 2 changes. Ventilatory amplitude was 3.0 mm at the lowest Pa co 2 and increased to 4.0 at the highest Pa co 2 . We conclude that: 1) when IPC discharge is low, afferents in the MCN inhibit ventilatory movements during hypocapnia, and 2) these afferens may contribute to systemic CO 2 sensitivity.