Hypercapnia induces long-term changes in postganglionic renal nerve activity in the piglet

Abstract

In developing swine, time and frequency domain analyses were used to compare changes in discharge features of efferent phrenic and postganglionic renal nerve activities evoked by prolonged (1 h) exposure to severe hypercapnia (10% CO 2, balance O 2), before and after combined carotid sinus and aortic depressor nerve (CSN–AOD) sectioning. With intact CSN–AOD innervation, respiration-related activity in renal nerve discharge was rare (3 of 11 animals) during baseline periods with intact innervation, but was observed in most cases (10 of 11 animals) during baseline following denervation. Renal nerve respiration-related activity was recruited by hypercapnic stimulation in animals with intact CSN–AOD innervation, and was augmented in denervated animals with ongoing respiratory activity. Phrenic nerve discharge was markedly augmented during hypercapnia, whether CSN–AOD innervation was intact or not, and it did not exhibit a post-hypercapnic depression. Autopower spectra of renal nerve activity revealed the presence of two coexisting rhythms, 2–6 and 7–13 Hz, which were present whether CSN–AOD innervation was intact or not. The hypercapnic-induced increases of activity in the 2–6 and 7–13 Hz bands were not comparable, with the latter region exhibiting a much more robust response to hypercapnia, especially following CSN–AOD denervation. Thus, prolonged exposure to hypercapnia evoked changes in renal nerve discharge that involved increased coupling to neuronal ensembles shaping central inspiratory activity and those generating central sympathetic outflows, especially to networks generating 7–13 Hz rhythm. Such changes may permit more efficient modulation of innervated structures during exposure to stressors.