Diminished forearm vasomotor response to central hypervolemic loading in aerobically fit individuals

The aim of this study was to test the hypothesis that cardiopulmonary baroreflex control of forearm vascular resistance (FVR) during central hypervolemic loading was less sensitive in exercise trained high fit individuals (HF) compared to untrained average fit individuals (AF). Eight AF (age: 24 ± 1 yr and weight: 78.9 ± 1.7 kg) and eight HF (22 ± 1 yr 79.5 ± 2.4 kg) voluntarily participated in the investigation. Maximal aerobic power (determined on a treadmill), plasma volume and blood volume (Evans blue dilution method) were significantly greater in the HF than AF (60.8 ± 0.7 vs. 41.2 ± 1.9 ml · kg^-1 · min^-1, 3.96 ± 0.17 vs. 3.36 ± 0.08 l, and 6.33 ± 0.23 vs 5.28 ± 0.13 l). Baseline heart rate (HR), central venous pressure (CVP), mean arterial pressure (MAP, measured by an intraradial catheter or a Finapres finger cuff), forearm blood flow (FBF, plethysmography), and FVR, calculated from the ratio (MAP-CVP)/FBF, were not different between the HF and the AF. Lower body negative pressure (LBNP, -5, -10, -15, and -20 torr) and passive leg elevation (LE, 50 cm) combined with lower body positive pressure (LBPP, +5, +10, and +20 torr) were utilized to elicit central hypovolemia and hypervolemia, respectively. Range of CVP (from LBNP to LE + LBPP) was similar in the AF (from -3.9 to +1.9 mm Hg) and HF (from -4.0 to +2.2 mm Hg). However, FVR/CVP was significantly less in the HF (-1.8 ± 0.1 unit · mm Hg^-1) than AF (-3.4 ± 0.1 unit · mm Hg^-1). The FVR decrease in response to increase in CVP was significantly diminished in the HF (-1.46 ± 0.45 unit · mm Hg^-1) compared to the AF (-4.40 ± 0.97 unit · mm Hg^-1), and during LBNP induced unloading the FVR/CVP of the HF (-2.01 ± 0.49 unit · mm Hg^-1) was less (P < 0.08) than the AF (-3.28 ± 0.69 unit · mm Hg^-1). We concluded that the cardiopulmonary baroreceptor mediated FVR reflex response was significantly less sensitive to changes in CVP in individuals who practice exercise training.