Relationship between sympathetic and phrenic nerve responses to peripheral chemoreflex in the cat

To test the hypothesis that peripheral chemoreflex effect on the preganglionic cervical sympathetic nerve (PSN) activity is entirely mediated by the central respiratory drive (CRD), as expressed in the phrenic nerve (PN) activity, we studied the relationship between PN and PSN activities under controlled conditions of carotid chemosensory excitation in the anesthetized cats. The cats were vagotomized, paralyzed and artificially ventilated. Tracheal pO2 and pCO2, systemic blood pressure, activities of single or a few PSN and PN fibers and a PN bundle were simultaneously recorded. The PSN preparations, which were responsive to hypoxia and showed PN rhythm, were selected for the study. Carotid chemoreceptor excitation, produced by hypoxia (end-tidal pO2 approximately equal to 50 Torr) or by sodium cyanide injection (50-100 micrograms, i.v.), elicited 3 types of responses: (1) the PSN discharged during the silent period of PN activity, although the PSN peak activity was still coupled to the PN peak activity, (2) PSN discharged only during PN activity, and (3) during the absence of PN discharge induced by hyperventilation hypocapnia, cyanide injection stimulated the PSN without PN activity. These observations suggest a model of chemoreflex control of sympathetic nerve activity which consists of two parts: one is dependent on PN activity and the other is not. Accordingly, all PSN chemoreflex responses may not be integrated with all inspiratory chemoreflexes.