Responses of autonomic cardiovascular systems induced by hypoxia: rebound phenomenon and species difference.

The rebound tachycardia, transient increase in heart rate (HR) that was observed immediately after the cessation of hypoxia was investigated. Whether the cardiovascular responses induced by hypoxia depend on species difference is also discussed. Wistar rats were chronically instrumented for measurements of arterial blood pressure (ABP), HR and renal sympathetic nerve activity (RSNA), and then subjected to hypocapnic (Hypo), isocapnic (Iso) or hypercapnic (Hyper) hypoxia. Iso did not affect mean ABP (MAP) or HR, whereas Hypo decreased MAP and increased HR, and Hyper increased MAP and decreased HR. RSNA increased in each hypoxia. After the cessation of Iso and Hyper, HR transiently increased and then returned to that of the control. This HR response did not parallel the changes in RSNA, suggesting that "rebound" tachycardia is not due to a sympathetic but rather a parasympathetic mechanism. Although it has been generally believed that cardiovascular responses to systemic hypoxia depend on the species, there was little difference between the results observed in this study and those in previous studies under similar experimental conditions. Accordingly, we suggest that the cardiovascular responses to systemic hypoxia do not depend on species difference but rather on differences in experimental design.