Differential baroreflex control of heart rate in sedentary and aerobically fit individuals

Purpose: We compared arterial, aortic, and carotid-cardiac baroreflex sensitivity in eight average fit (maximal oxygen uptake, VO(2max) = 42.2 ± 1.9 mL·kg^-1·min^-1) and eight high fit (VO(2max) = 61.9 ± 2.2 mL·kg^-1·min^-1) healthy young adults. Methods: Arterial and aortic (ABR) baroreflex functions were assessed utilizing hypo- and hyper-tensive challenges induced by graded bolus injections of sodium nitroprusside (SN) and phenylephrine (PE), respectively. Carotid baroreflex (CBR) sensitivity was determined using ramped 5-s pulses of both pressure and suction delivered to the carotid sinus via a neck chamber collar, independent of drug administration. Results: During vasoactive drug injection, mean arterial pressure (MAP) was similarly altered in average fit (AF) and high fit (HF) groups. However, the heart rate (HR) response range of the arterial baroreflex was significantly attenuated (P < 0.05) in HF (31 ± 4 beats·min^-1) compared with AF individuals (46 ± 4 beats·min^-1). When sustained neck suction and pressure were applied to counteract altered carotid sinus pressure during SN and PE administration, isolating the ABR response, the response range remained diminished (P < 0.05) in the HF population (24 ± 3 beats·min^-1) compared with the AF group (41 ± 4 beats·min^-1). During CBR perturbation, the HF (14 ± 1 beats·min^-1) and AF (16 ± 1 beats·min^-1) response ranges were similar. The arterial baroreflex response range was significantly less than the simple sum of the CBR and ABR (HF, 38 ± 3 beats·min^-1 and AF, 57 ± 4 beats·min^-1) in both fitness groups. Conclusions: These data confirm that reductions in arterial-cardiac reflex sensitivity are mediated by diminished ABR function. More importantly, these data suggest that the integrative relationship between the ABR and CBR contributing to arterial baroreflex control of HR is inhibitory in nature and not altered by exercise training.