Effects of clonidine on canine cardiac neuroeffector structures controlling heart rate

1 In intact dogs anaesthetized with pentobarbitone, clonidine (10 mug/kg, i.v.) produced a sustained decrease in heart rate. This effect was significantly smaller in vagotomized dogs in which the sympathetic drive to the heart was either left intact or experimentally created by continuous electrical stimulation of the decentralized cardioaccelerator nerve. In the latter preparation, the negative chronotropic action of clonidine was reversed by an intravenous injection of phentolamine, whereas in the former experimental situation it was antagonized only by an intravenous plus an intravertebral artery injection of phentolamine.2 In dogs with denervated hearts the tachycardia produced by electrical stimulation of the cardioaccelerator nerve was accompanied by a rise in noradrenaline overflowing into the coronary sinus plasma. Clonidine inhibited both these effects and phentolamine restored them to pre-clonidine levels.3 Clonidine decreased heart rate in dogs with an intact parasympathetically innervated heart and decentralized stellate ganglia. When the low basal heart rate of this preparation was elevated by electrical stimulation of the cardioaccelerator nerve, clonidine had a negative chronotropic effect, the degree of which was similar to that observed in intact dogs.4 Clonidine neither modified baseline heart rates of dogs with denervated hearts nor the levels of heart rate which in this preparation were reduced by a sustained electrical stimulation of the right vagus or increased by intravenous infusions of either isoprenaline or noradrenaline.5 These findings indicate that in the intact dog, bradycardia induced by clonidine resulted both from a reduction of sympathetic drive and from a concomitant increase in parasympathetic tone. The latter action did not occur at the level of cardiac neuroeffector structures since it was observed only in the presence of centrally connected vagal pathways. The inhibition of cardiac sympathetic tone was of both peripheral and central origin. Clonidine, in fact, diminished the quantity of noradrenaline overflowing into the coronary sinus plasma in cardiac denervated dogs with a tachycardia elicited by electrical stimulation of the decentralized cardioaccelerator nerve. This peripheral effect was probably due to an activation of alpha-adrenoceptors located on sympathetic nerve terminals since it was antagonized by phentolamine. However, in vagotomized dogs (intact sympathetic pathways) intravenous phentolamine failed to antagonize the heart rate effects of clonidine which were abolished by a subsequent injection of phentolamine into the vertebral artery. Thus, the clonidine-induced inhibition of both the peripheral and central sympathetic drive to the heart would appear to be mediated via alpha-adrenoceptors.