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The specificities of two radioimmunoassays (RIA) for relaxin, based upon crude porcine relaxin (NIH-R-P1; RIA I) and a highly purified porcine relaxin (RIA II) have been studied concurrently using purified hormones and plasma samples. A labelled fraction, selected from radio-iodinated NIH-R-P1 and used in that RIA, was also bound to antiserum raised to the highly purified relaxin. Hence a third RIA was possible in which both the crude and the purified relaxins inhibited in the ng/ml range. Porcine insulin and the connecting peptide of porcine proinsulin did not inhibit any of the assay systems whereas porcine proinsulin did inhibit in each assay at the μg/ml range. Concurrent measurements by assays I and II have been made in sheep plasma obtained during both delivery of the lamb and suckling. The peak values obtained by assays I and II are 3 and 6 min out of phase during suckling and delivery respectively; the NIH-R-P1 relaxin immunoactivity appearing first. The plasma inhibition curves of both appear to be the sum of individual contributions from relaxin and relaxin-like peptides, such as prorelaxin and its fragments, as seen by different antisera. Both assays, however, give qualitatively similar indices of relaxin immunoactivity. The RIA developed for the more purified peptide would be expected to yield a better quantitative estimate of relaxin secretion but this, like specificity, cannot be shown absolutely.
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Considerable indirect evidence exists that certain drugs, including phenothiazine derivatives, stimulate the release of prolactin from the anterior pituitary (see review by Meites, Nicoll & Talwalker, 1963). As a direct test of this hypothesis a radioimmunoassay for sheep prolactin (Bryant & Greenwood, 1968) was used to measure plasma prolactin levels after the i.m. or i.v. injection of acepromazine in eight sheep of the Clun Forest breed (Table 1). Evidence for the specificity of the plasma prolactin measurements has been adduced (Bryant & Greenwood, 1968). Nevertheless prolactin concentration in plasma has been expressed in terms of the weight of the reference standard preparation (NIH-P-S 6). If biological and immunological measurements were identical, the levels of plasma prolactin of 50–500 ng./ml. obtained here would be equivalent to 1·25–12·5 m-u./ml.
The four female sheep investigated formed part of a study relating the degree of parasitic infestation with the reproductive cycle (Connan, 1968), and
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Relaxin reference preparations NIH-R-P1 and Warner Lambert W1164-3, and purified relaxin peptides, CM-a′, CM-a and CM-B, were assayed in the mouse interpubic ligament and rat uterine inhibition bioassays. There was evidence that CM-a and CM-B would bind to glass and significant apparent increases in potency in the case of these two peptides alone resulted from the use of silicone-coated glassware. Using treated glassware, CM-a′, CM-a and CM-B were equipotent in the mouse interpubic ligament bioassay with potencies relative to NIH-R-P1 of 3·97-, 4·85- and 3·64-fold respectively. In the rat uterine inhibition bioassay only W1164-3 and CM-a′ gave dose-response curves which were parallel to NIH-R-P1. In this bioassay, potencies relative to NIH-R-P1 for W1164-3 and CM-a' were 0·35-and 19·6-fold respectively.
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SUMMARY
Studies of the effect of mating on the release of prolactin over a 47-h period from the onset of oestrus in the ewe are presented. Surges of prolactin occurred around the time of pre-ovulatory luteinizing hormone release, and in some instances copulation appeared to result in an additional surge of prolactin secretion.