Journal of Applied Physiology, Vol 74, Issue 5 2086-2091, Copyright © 1993 by American Physiological Society
Involvement of inhibitory dopamine-2 receptors in resting bradycardia in exercise-conditioned rats
K. J. Slavik and J. LaPointe
Department of Biology, New Mexico State University, Las Cruces 88003.
The purpose of this investigation was to examine the underlying cause for
the resting bradycardia and lower resting blood pressure demonstrated in
conscious rats that performed 12 wk of treadmill exercise conditioning. The
influence of inhibitory dopamine (DA2) receptors and alpha 2-adrenoceptors,
which are known to mediate bradycardia and hypotension, was assessed in
exercise-conditioned (EC) and nonexercised conditioned (NC) rats. To
accomplish this, preferential DA2 and alpha 2-agonists and antagonists were
administered at rest to conscious rats after they participated in an
exercise conditioning program. The results obtained with the DA2 antagonist
metoclopramide (15 mg/kg ip) alone suggest that there is physiological
activation of cardiovascular DA2 receptors in EC rats but not in NC rats.
Furthermore, the results obtained with the DA2 agonist bromocriptine (1.5
mg/kg ip) suggest that the DA2 receptor-mediated bradycardia and
hypotension are greater in EC rats than in NC rats. In addition, heart rate
and blood pressure responses to the alpha 2-agonist clonidine (0.1 mg/kg
ip) and antagonist yohimbine (1 mg/kg ip) were not different between EC and
NC rats. These data suggest that enhanced DA2 receptor influence accounts,
in part, for the resting bradycardia and lower resting blood pressure
demonstrated in EC rats after 12 wk of exercise conditioning.
