THE ASSAY OF MILK‐EJECTING ACTIVITY IN THE LACTATING RAT

Abstract

One of the most sensitive methods for assaying oxytocin is to measure the increase in milk-ejection pressure which results from a close arterial injection of the hormone into the lactating mammary gland.This method originates from the work of Cross & Harris (1952) on the role of the neurohypophysis in the milk-ejection reflex.In this work, a teat duct was cannulated in the lactating rabbit and the cannula was partly filled with citrate solution.In response to electrical stimulation of the neurohypophysis or to the intravenous injection of posterior pituitary extract, milk was ejected into the cannula and a volume change was registered by means of a piston recorder and kymograph.Van Dyke, Adamsons & Engels (1955) developed a quantitative assay method in which volume recorders were replaced by highly sensitive pressure transducers and milk ejection pressure was recorded from the cannulated duct with minimal displacement of fluid.Fitzpatrick (1961) achieved an almost 40-fold increase in the sensitivity of the assay in the lactating rabbit by giving close arterial, instead of intravenous, injections of oxytocin.Using a similar technique in the guinea-pig, Tindal & Yokoyama (1962) obtained a further increase in sensitivity and their method of assay has been used to estimate small amounts of oxytocin in blood during suckling and parturition (Folley & Knaggs, 1965a; Fitzpatrick & Walmsley, 1965).Many of our investigations have been concerned with the problem of demonstrating independent release of the neurohypophysial hormones (Haldar, 1966; Bisset, Hilton & Poisner, 1967; Beleslin, Bisset, Haldar & Polak, 1967).In this work an even greater sensitivity than we have been able to obtain with the lactating guinea-pig preparation is often required to exclude the presence of oxytocin in blood extracts.For this purpose the use of the lactating rat has been investigated.Since the work began, Grosvenor (1965) and DeNuccio & Grosvenor (1967) have recorded increases in milk-ejection pressure in the rat in response to oxytocin given by the intravenous route, but the limit of sensitivity Permanent addresses:

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