Aortic baroreflex control of heart rate after 15 days of simulated microgravity exposure.

To determine the effects of simulated microgravity on aortic baroreflex control of heart rate, we exposed seven male subjects (mean age 38 +/- 3 yr) to 15 days of bed rest in the 6 degrees head-down position. The sensitivity of the aortic-cardiac baroreflex was determined during a steady-state phenylephrine-induced increase in mean arterial pressure combined with lower body negative pressure to counteract central venous pressure increases and neck pressure to offset the increased carotid sinus transmural pressure. The aortic-cardiac baroreflex gain was assessed by determining the ratio of the change in heart rate to the change in mean arterial pressure between baseline conditions and aortic baroreceptor-isolated conditions (i.e., phenylephrine + lower body negative pressure + neck pressure stage). Fifteen days of head-down tilt increased the gain of the aortic-cardiac baroreflex (from 0.45 +/- 0.07 to 0.84 +/- 0.18 beats.min-1.mmHg-1; P = 0.03). Reductions in blood volume and/or maximal aerobic capacity may represent the underlying mechanism(s) responsible for increased aortic baroreflex responsiveness after exposure to a ground-based analogue of microgravity.