Effects of horizontal body casting on the baroreceptor reflex control of heart rate.
The purpose of this study was to investigate the effects of long-term horizontal body position on baroreceptor reflex control of heart rate. Six male rhesus monkeys (6.2-9.4 kg) were given bolus injections of 4.0 micrograms/kg phenylephrine during each of the following conditions: awake, anesthetized (10 mg/kg ketamine HCl), and after beta-blockade (1 mg/kg propranolol HCl) before, 7, 14, and 28 days after being placed in a horizontal body cast. R-R interval vs. systolic arterial pressure was plotted, and the slope was determined by least-squares-fit linear regression. Baroreceptor slope was significantly reduced by 7 days of horizontal body position and remained attenuated throughout the 28-day restraint period both before and after beta-receptor blockade. These data are consistent with the thesis that prolonged exposure to a zero-gravity environment impairs autonomic reflex regulation of the cardiovascular system.
D000818:Animals; D002370:Casts, Surgical; D006339:Heart Rate; D008253:Macaca mulatta; D008297:Male; D011187:Posture; D011311:Pressoreceptors / Q000502:physiology*; D012018:Reflex / Q000502:physiology*; D012044:Regression Analysis