Central command suppresses pressor-evoked bradycardia at the onset of voluntary standing up in conscious cats.

New Findings: What is the central question of this study? Standing up can cause hypotension and tachycardia. Accumulated evidence poses the simple question, does the cardiac baroreflex operate at the onset of standing up? If the cardiac baroreflex is suppressed, what mechanism is responsible for baroreflex inhibition? What is the main finding and its importance? In cats, we found blunting of cardiac baroreflex sensitivity in the pressor range at the onset of voluntary hindlimb standing, but not of passive hindlimb standing. This finding suggests that central command suppresses pressor-evoked bradycardia at the onset of standing up, probably in advance, to prevent or buffer orthostatic hypotension.

Abstract: It remains unclear whether cardiac baroreflex function is preserved or suppressed at the onset of standing up. To answer the question and, if cardiac baroreflex is suppressed, to investigate the mechanism responsible for the suppression, we compared the sensitivity of the arterial cardiac baroreflex at the onset of voluntary and passive hindlimb standing in conscious cats. Cardiac baroreflex sensitivity was estimated from the maximal slope of the baroreflex curve between the responses of systolic arterial blood pressure and heart rate to a brief occlusion of the abdominal aorta. The systolic arterial blood pressure response to standing up without aortic occlusion was greater in the voluntary case than in the passive case. Cardiac baroreflex sensitivity was clearly decreased at the onset of voluntary standing up compared with rest (P = 0.005) and the onset of passive standing up (P = 0.007). The cardiac baroreflex sensitivity at the onset of passive standing up was similar to that at rest (P = 0.909). The findings suggest that central command would transmit a modulatory signal to the cardiac baroreflex system during the voluntary initiation of standing up. Furthermore, the present data tempt speculation on a close relationship between central inhibition of the cardiac baroreflex and the centrally induced tachycardiac response to standing up.