Autonomic mechanisms in the ductus venosus of the lamb

Abstract

Morphological and pharmacological studies were performed in the ductus venosus from near-term fetal and neonatal (1 day of age) lambs. Light and electron microscopic analysis demonstrated a concentration of circularly oriented muscle fibers at the junction of the ductus with the portal sinus (the sphincter region). With the use of histochemical methods, adrenergic and cholinergic fibers were visualized in both the sphincter and extrasphincter regions of the ductus. At either site, nerve fibers were confined to the adventitial layer and never formed a plexus. Norepinephrine and transmural electrical stimulation contracted the ductus sphincter in vitro, and their action was abolished by dibenzyline. In contrast, atropine had no effect on the transmural response. Moreover, acetylcholine contracted the vessel inconsistently. In the fetus, both the norepinephrine-induced and the electrically induced contractions increased on raising the O2 tension of the bathing fluid from 17-31 to 520-705 mmHg. In addition, the transmural response increased with advancing gestation. Norepinephrine and transmural stimulation relaxed dibenzyline-treated preparations in which the tone had been raised with indomethacin. Norepinephrine relaxation was antagonized by propranolol. We conclude that the ductus venosus sphincter contains functional adrenergic nerves, causing contraction and relaxation via, respectively, alpha- and beta-adrenoceptors. The alpha-adrenoceptor-mediated contraction may have a role in postnatal closure of the vessel.