A Sensitive Method for the Assay of Oxytocin in the Circulating Blood

Abstract

The sensitivity of a wide range of isolated tissues to oxytocin was investigated. The longitudinal muscle strip of duck pulmonary vein proved the most suitable tissue for use with the superfusion technique, contracting to concentrations of oxytocin as low as 10 μu/ml. The duck pulmonary vein superfused with Krebs solution was contracted by oxytocin, the vasopressins, adrenaline, noradrenaline, 5‐hydroxytryptamine, histamine and angiotensin II. Pre‐treatment of the preparation with phenoxybenzamine (1–2 μg/ml) abolished the contractions to catecholamines and reduced the effects of histamine and 5‐hydroxytryptamine without affecting the sensitivity to oxytocin. The pulmonary vein contracted when superfused with blood from an anaesthetized dog. This contraction was accompanied by a non‐specific loss of responsiveness. When the pulmonary vein was superfused with Krebs solution that had been dialysed against blood the initial contraction was greatly reduced or abolished as was the loss in responsiveness. Oxytocin was stable in circulating dog's blood but approximately 50% was bound to plasma proteins. Oxytocin was not destroyed in the pulmonary circulation or the hind limbs. In the doses used oxytocin had a half‐life of 60–90 s in the circulation of the dog under steady‐state conditions. Disappearance occurred chiefly in the visceral vascular beds.

Oxytocin-induced changes in single cell K+ currents and smooth muscle contraction of guinea-pig gastric antrum

“…Summarizing the data in the literature, Nessen (41) has assumed that the circulating hormone may exert a prolonged in vivo action on gastrointestinal motility due to the absence of oxytocinase activity in gastrointestinal smooth muscles. However, oxytocinase activity has recently been detec¬ ted in the visceral vascular bed (42), which means that if oxytocin modulates the gastric smooth muscle locally, it has to be released by the nerve endings at the place of its action. Indeed, the existence of oxytocinergic neurones is well documented for the spinal cord and thoracic ganglia of some mammals (43,44), which implies that oxytocin may serve as a putative neuro¬ transmitter in the periphery.…”

The contribution of prostaglandin production to contractions of the isolated uterus of the rat

“…This tissue, which is contracted by oxytocin, was prepared according to Vane & Williams (1970 Residues were taken up in 2 ml of Krebs solution and assayed for prostaglandinlike activity on a rat stomach strip, rat colon and chick rectum superfused with Krebs solution (6-8 ml/min) containing a mixture of antagonists (Gilmore, Vane & Wyllie, 1968 (Green & Samuelsson, 1964). The positions of the authentic prostaglandins were visualized and the corresponding parts of the bath fluid sample regions, as well as the intermediate zones, were scraped off the plate and taken up with Krebs solution.…”