Endurance training alters arterial baroreflex function in dogs

The present study was designed to determine whether 12 wk of daily exercise alter autonomic neural control of the heart during baroreflex stimulation in healthy dogs. We studied 16 untrained and 12 endurance-trained anesthetized dogs which were instrumented to measure arterial blood pressure (AP), carotid sinus baroreceptor pressure (CBP), electrocardiogram (ECG), heart rate (HR), and R-R interval (RR). The arterial baroreflex was studied during hypertension caused by i.v. bolus infusion of phenylephrine, hypotension caused by i.v. bolus infusion of nitroprusside, and bilateral carotid occlusion (BCO) in which carotid sinus pressure was reduced to 41 ± 2 mm Hg (mean ± SEM). Arterial baroreflex sensitivity, which was assessed by determining the change in heart interval (i.e., change in RR) per unit change in systolic AP (ΔRR/ΔAP), was significantly lower during the hypertensive challenge in the trained dogs compared to the untrained dogs (2.2 ± 0.3 vs 6.8 ± 1.5 ms·mm Hg^-1, respectively). Similarly, the ΔRR/ΔAP was substantially lower during the hypotensive challenge in trained dogs vs the untrained dogs (1.2 ± 0.3 vs 1.8 ± 0.4 ms·mm Hg^-1, respectively). In addition, the HR response to the BCO was significantly less in trained dogs (22 ± 2 bpm) vs untrained dogs (32 ± 5 bpm). The open-loop gain (Go), which was used to quantitate the effectiveness of the carotid baroreflex to increase mean systemic AP during BCO, was similar in both untrained and trained dogs (2.9 ± 0.6 and 2.4 ± 0.5, respectively). These data indicate that, while endurance training significantly reduces the HR component of the arterial baroreflex, the arterial pressure response apparently is not altered.