Adrenoceptors and cardiovascular regulation in conscious, unrestrained, Long Evans and Brattleboro rats.

In conscious rats, in the presence of prazosin and idazoxan (alpha 1- and alpha 2-adrenoceptor antagonists, respectively), blood pressure (BP) and heart rate (HR) become unstable, due to the occurrence of 'spontaneous' depressor and tachycardic episodes. These events could be triggered also by auditory stimuli, and were blocked by antagonism of beta 2-adrenoceptors. In adrenal-demedullated animals treated with prazosin and idazoxan, BP was less variable than in sham-operated animals, but bolus injections of adrenaline or salbutamol could simulate the 'spontaneous' depressor events seen in the latter. The marked swings in HR were not explained by sensitization of the cardiac baroreflex in the presence of idazoxan, but were probably due to blockade of prejunctional autoinhibitory alpha 2-adrenoceptors, amplifying the effects of intermittent sympatho-adrenal activation.