Enhanced cardiopulmonary reflex inhibition of heart rate during exercise.

We tested the hypothesis that the reflex inhibition of heart rate (HR) during mechanical (acute bolus injection of 0.5% and 2% of estimated blood volume) and chemical (phenylbiguanide, PBG, 2.5 and 5 micrograms.kg-1) stimulation of cardiopulmonary receptors would be enhanced during exercise. Rats were instrumented with arterial and venous catheters. The reflex response to mechanical (N = 7) and chemical (N = 8) stimulation of cardiopulmonary receptors was examined at rest and during exercise (6 m.min-1, 10% grade). A two-way analysis of covariance (ANCOVA) with repeated measures was used to test for differences in the reflex regulation of HR at rest vs exercise. HR was used as the covariate because exercise significantly increased baseline HR. There was no significant treatment effect (rest vs exercise) for the reflex inhibition of HR during mechanical stimulation. However, the two-way ANCOVA revealed a significant treatment effect (rest vs exercise) for the reflex inhibition of HR during chemical stimulation. The reflex decreases in HR were enhanced (-delta 23 +/- 8 vs -delta 133 +/- 47 and -delta 208 +/- 40 vs -delta 374 +/- 10 bpm at 2.5 and 5 micrograms.kg-1, respectively). These data suggest that factors associated with exercise enhanced the cardiopulmonary reflex inhibition of heart rate during chemical stimulation.