In vivo interactions between prejunctional alpha 2- and beta 2-adrenoceptors at the level of the adrenal medulla

The combined effect of a beta 2-antagonist and an alpha 2-agonist on the release of adrenal catecholamines was studied in the anaesthetized and vagotomized dog. The electrical stimulation of the splanchnic nerve (5-V pulses of 2 ms duration for 3 min at a frequency of 3 Hz) produced a significant rise in adrenal catecholamine release in the adrenal vein. Intravenous injection of a beta 2-antagonist significantly reduced this response and a subsequent injection of an alpha 2-agonist further reduced the release of catecholamines. However, if the alpha 2-agonist is injected first, the release is not different compared with the control stimulation, and the subsequent injection of the beta 2-antagonist also did not modify the release in response to electrical stimulation. These results suggest that the blockade of presynaptic beta 2-receptors reduces the release of adrenal catecholamines without interfering with the activation of the alpha 2-adrenoceptors. In contrast, the pretreatment with the alpha 2-agonist, which does not modify the release of catecholamine at 3 Hz, seems to interfere with the inhibitory effect of the beta 2-antagonist.