INVOLVEMENT OF INHIBITORY DOPAMINE-2 RECEPTORS IN RESTING BRADYCARDIAIN EXERCISE-CONDITIONED RATS

The purpose of this investigation was to examine the underlying cause for the resting bradycardia and lower resting blood pressure demonstra ted in conscious rats that performed 12 wk of treadmill exercise condi tioning. The influence of inhibitory dopamine (DA2) receptors and alph a2-adrenoceptors, which are known to mediate bradycardia and hypotensi on, was assessed in exercise-conditioned (EC) and nonexercised conditi oned (NC) rats. To accomplish this, preferential DA2 and alpha2-agonis ts and antagonists were administered at rest to conscious rats after t hey participated in an exercise conditioning program. The results obta ined with the DA2 antagonist metoclopramide (15 mg/kg ip) alone sugges t that there is physiological activation of cardiovascular DA2 recepto rs in EC rats but not in NC rats. Furthermore, the results obtained wi th the DA2 agonist bromocriptine (1.5 mg/kg ip) suggest that the DA2 r eceptor-mediated bradycardia and hypotension are greater in EC rats th an in NC rats. In addition, heart rate and blood pressure responses to the alpha2-agonist clonidine (0.1 mg/kg ip) and antagonist yohimbine (1 mg/kg ip) were not different between EC and NC rats. These data sug gest that enhanced DA2 receptor influence accounts, in part, for the r esting bradycardia and lower resting blood pressure demonstrated in EC rats after 12 wk of exercise conditioning.