Arterial baroreflex control of sympathetic nerve activity during acute hypotension: effect of fitness

We examined arterial baroreflex control of muscle sympathetic nerve activit y (MSNA) during abrupt decreases in mean arterial pressure (MAP) and evalua ted whether endurance training alters baroreflex function. Acute hypotensio n was induced nonpharmacologically in 14 healthy subjects, of which 7 were of high fitness (HF) and 7 were of average fitness (AF), by releasing a uni lateral arterial thigh cuff after 9 min of resting ischemia under two condi tions: control, which used aortic and carotid baroreflex (ABR and CBR, resp ectively) deactivation; and suction, which used ABR deactivation alone. The application of neck suction to counteract changes in carotid sinus transmu ral pressure during cuff release significantly attenuated the MSNA response (which increased 134 +/- 32 U/14 s) compared with control (which increased 195 +/- 43 U/14 s) and caused a greater decrease in MAP (19 +/- 2 vs. 15 /- 2 mmHg; P< 0.05). Furthermore, during both trials, the HF subjects exhib ited a greater decrease in MAP compared with AF subjects despite an augment ed baroreflex control of MSNA. These data indicate that the CBR contributes importantly to the MSNA response during acute systemic hypotension. Additi onally, we suggest that an impaired control of vascular reactivity hinders blood pressure regulation in HF subjects.