A diminished aortic-cardiac reflex during hypotension in aerobically fit young men

We compared the aortic-cardiac baroreflex sensitivity in eight average fit (AF: VO2max = 44.7 ± 1.3 ml•kg^-1•min^-1) and seven high fit (HF: VO2max = 64.1 ± 1.7 ml • min^-1 • kg^-1) healthy young men during hypotension elicited by steady state sodium nitroprusside (SN) infusion. During SN mean arterial pressure (MAP) was similarly decreased in AF (-12.6 ± 1.0 mm Hg) and HF (^-12.1 ± 1.1 mm Hg). However, the increases in heart rate (HR) were less (P< 0.023) in HF (15 ± 3 bpm) than AF (25 ± 1 bpm). When sustained neck suction (NS, -22 ± 1 torr in AF and -20 ± 1 torr in HF, P > 0.05) was applied to counteract the decreased carotid sinus transmural pressure during SN, thereby isolating the aortic baroreceptors, the increased HR remained less (P < 0.021) in HF (8 ± 2 bpm) than AF (16 ± 2 bpm). During both SN infusion and SN+NS, the calculated gains (i.e., AHR/AMAP) were significantly greater in AF (2.1 ± 0.3 and 1.3 ± 0.2 bpm•mm Hg^-1) than HF (1.2 ± 0.2 and 0.6 ± 0.2 bpm•mm Hg^-1). However, the estimated carotid-cardiac baroreflex sensitivity (i.e., the gain difference between the stage SN and SN+NS) was not different between AF (0.7 ± 0.2 bpm•mm Hg^-1) and HF (0.6 ± 0.1 bpm•mm Hg^-1). These data indicated that the aortic-cardiac baroreflex sensitivity during hypotension was significantly diminished with endurance exercise training.