Effects of alpha adrenoceptor and dopamine receptor agonists and antagonists on ganglionic transmission

Effects of alpha adrenoceptor and dopamine (DA) receptor agonists and antagonists on ganglionic transmission were studied in pentobarbital-anesthetized, open-chest dogs. Changes in tachycardia produced by preganglionic cardiac nerve stimulation were monitored as a measure of ganglionic transmission. Several agonists, injected i.a. into the blood supply of the stellate ganglion, inhibited ganglion transmission. This effect was localized to the ganglion as none of the agonists, in the highest doses studied, inhibited tachycardia produced by postganglionic cardiac nerve stimulation. The potency order of the agonists was UK 14,304 (alpha-2 adrenoceptor agonist) greater than norepinephrine (NE) greater than dipropyl DA (DA2 dopamine receptor agonist) greater than DA greater than or equal to phenylephrine (alpha-1 adrenoceptor agonist) greater than fenoldopam (DA1 dopamine receptor agonist). UK 14,304 was over 200 times more potent than fenoldopam, being active in nanomole doses, whereas NE was more potent than DA. Rauwolscine, an alpha-2 adrenoceptor antagonist, antagonized the inhibitory effects of NE and DA and was the only antagonist, given i.a. or i.v., that facilitated ganglionic transmission; frequency-response curves of tachycardia induced by preganglionic nerve stimulation were dose-dependently augmented. SCH 23390, in double the full DA1 antagonist dose, had no effect on frequency-response curves or on NE- or DA-induced ganglionic inhibition. Domperidone antagonized the effect of DA but had no effect on ganglionic inhibition produced by NE or on frequency-response curves of tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)