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, PEG, 2.5 and 5 mu g . 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) stimula tion of cardiopulmonary receptors was examined at rest and during exer cise (6 m . min(-1) 10% grade). A two-way analysis of covariance (ANCO VA) with repeated measures was used to test for differences in the ref lex regulation of HR at rest vs exercise. HR was used as the covariate because exercise significantly increased baseline HR. There was no si gnificant treatment effect (rest vs exercise) for the reflex inhibitio n of HR during mechanical stimulation. However, the two-way ANCOVA rev ealed 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 mu g . kg(-1), respectiv ely). These data suggest that factors associated with exercise enhance d the cardiopulmonary reflex inhibition of heart rate during chemical stimulation.