Dihydroergotoxine-induced bradycardia in rats

Effect of co-dergocrine in the autoperfused superior mesenteric vascular bed of the rat

1. The influence of local administration of co-dergocrine in the in situ, constant flow autoperfused, superior mesenteric vascular bed of the rat was investigated. 2. Local infusion of co-dergocrine (0.1 micrograms kg-1 min-1) reduced the pressor response to electrical stimulation of the periarterial sympathetic nerves for the whole frequency range studied during cumulative frequency-response curves (1-8 Hz). 3. Co-dergocrine (0.01, 0.03, 0.1 micrograms kg-1 min-1) dose-dependently inhibited the pressor response to electrical stimulation at 4 Hz; increases in perfusion pressure produced by locally administered noradrenaline were inhibited to the same degree. 4. Pressor responses induced by locally administered phenylephrine were also inhibited by co-dergocrine (0.1 micrograms kg-1 min-1) while those produced by local injection of angiotensin II were not. 5. Local administration of the DA1-/DA2-receptor antagonist haloperidol (1 microgram kg-1), the DA2-receptor antagonist domperidone (10 micrograms kg-1) and the alpha 2-adrenoceptor antagonist rauwolscine (100 micrograms kg-1) partially prevented the influence of co-dergocrine on the electrically induced response. 6. The results show that the effect of co-dergocrine in the rat superior mesenteric vascular bed cannot simply be ascribed to interaction with the presynaptic inhibitory DA2-receptors on the noradrenergic nerve endings; it also has an antagonistic action at the postsynaptic alpha 1-adrenoceptors and an agonist action at the presynaptic alpha 2-adrenoceptors.