Attenuation of postexertional hypotension by cardiac afferent blockade

Naloxone eliminates postexertional hypotension (PEH) in human and animal models. The effect of naloxone on sympathetic activity during hemorrhage and generation of arterial baroreflex function curves can be stimulated by blockade of cardiac afferent receptors. We tested the hypothesis that cardiac afferent blockade would eliminate PEH in eight spontaneously hypertensive rats (SHR). Rats were instrumented with a Silastic-tipped catheter inserted into the pericardial space. Four weeks later, a Teflon catheter was placed in the descending aorta via the left common carotid artery for measurement of mean arterial pressure (MAP) and heart rate (HR). MAP and HR were examined before (preexercise) and after (postexercise) a single bout of dynamic treadmill exercise (9-12.0 m/min, 10-18% grade for 30-40 min) under three experimental conditions: control, cardiac efferent blockade, and combined cardiac efferent and afferent blockade. MAP significantly decreased (29 +/- 5 and 25.6 +/- 4 mmHg) in the control and cardiac efferent blockade conditions after exercise. However, when cardiac afferents were blocked, the hypotensive response to mild dynamic exercise was significantly attenuated (-6 +/- 3 mmHg). Thus blockade of cardiac afferents eliminated PEH in the SHR. These data suggest that inhibitory influence of cardiac afferents on the circulation may be enhanced after exercise.