AORTIC BAROREFLEX CONTROL OF HEART-RATE DURING HYPERTENSIVE STIMULI -EFFECT OF FITNESS

We examined the aortic baroreflex control of heart rate (HR) in seven healthy young men of average fitness (AF) and seven of high fitness (H F). The fitness level was determined by maximal oxygen uptake (AF = 42 .9 +/- 1.1, HF = 62.3 +/- 1.8 ml . kg-1 . min-1). Aortic baroreflex co ntrol of HR was determined during a steady-state increase of mean arte rial pressure (MAP; AF, +15.0 +/- 2.1 and HF, +18.3 +/- 0.8 mmHg) with phenylephrine (PE) infusion combined with positive neck pressure (NP; AF, 18 +/- 2.0 and HF, 20 +/- 0.8 mmHg) to counteract the increased c arotid sinus pressure and with low levels of lower body negative press ure to counteract the increased central venous pressure. There was no group difference in the increased MAP or NP, nor was there stage diffe rence in MAP within either group during PE infusion. However, the isol ated cardiac-aortic baroreflex gains (i.e., DELTAHR/DELTAMAP) were sig nificantly less in the HF (0.16 +/- 0.02 and 0.14 +/- 0.03 beats . min -1 . mmHg-1) than in the AF (0.52 +/- 0.08 and 0.59 +/- 0.07 beats . m in-1 . mmHg-1) subjects at PE + NP and PE + NP + lower body negative p ressure. We concluded that during steady-state increases in MAP, the s ensitivity of aortic baroreflex control of HR was significantly less i n the HF than in the AF subjects.