As authorized by the World Health Organization 29th Expert Committee on Biological Standardization, the preparation of human prolactin in ampoules coded 75/504 has been established as the International Reference Preparation (IRP) of human prolactin for immunoassay. From the results of a collaborative study, to which 15 laboratories in nine countries contributed, with the agreement of the participants, the content of each ampoule is defined as 0·650 International Units (i.u.; 650 mi.u.) immunoassay. The results of this collaborative study show that the IRP is adequately stable and suitable for use as a standard for the determination of prolactin in human plasma and serum. Estimates of the prolactin content of human plasma and serum made in the various laboratories have been compared and show good agreement in ranking order, but only fair agreement in the numerical value of the estimates. Numerical agreement is poor between estimates of the human prolactin content of two samples identical except for coding; this shows the difficulty in achieving continuity of estimates when any laboratory calibrates a replacement standard.
P. MARY COTES and B. A. CROSS
Changes in body weight and food intake were studied for 14 days post partum in primiparous rats.
The growth increments in suckled rats with galactophores cut to prevent milk withdrawal and in normally lactating rats were greater than in unsuckled controls. The growth increments in rats with galactophores cut were accompanied by parallel increases in food intake.
Injection of 3 i.u. prolactin daily did not reproduce these changes in body weight and food intake in non-suckled rats.
The results suggest that the main factor in the extra growth of lactating rats is an increased food intake in excess of the metabolic needs for milk secretion induced by the stimulus of suckling.
P. MARY COTES, W. A. BARTLETT, ROSE E. GAINES DAS, P. FLECKNELL and R. TERMEER
Different methods for administration of human growth hormone (hGH) have been examined with a view to efficient use of the limited amounts of hGH at present available for clinical use.
We found that in hypophysectomized rats (1) hGH administered by continuous subcutaneous infusion induced a greater increase in body weight (referred to throughout as growth) than hGH administered by intermittent (daily) injection and (2) intermittent injections of hGH dissolved in 16% gelatin induced more growth than hGH dissolved in a glycine buffer.
It was further found that (1) hGH dissolved in 16% gelatin compared with hGH dissolved in a glycine buffer induced lower maximal levels of immunoreactive plasma hGH and between 7 and 9 h after treatment higher plasma levels when injected subcutaneously in rabbits, (2) 125I-labelled hGH added as a tracer to hGH in gelatin was removed more slowly from subcutaneous injection sites in rabbits than 125I-labelled hGH given with hGH in glycine buffer and (3) changes in the ratio of hGH to gelatin had little effect on the time-course of plasma levels of hGH in the rabbit. Addition of the protease inhibitors aprotinin or 6-aminohexanoic acid, to injection of hGH in gelatin or glycine did not induce any consistent increase in plasma levels of hGH.
P. MARY COTES, MARJORIE V. MUSSETT, I. BERRYMAN, R. EKINS, S. GLOVER, N. HALES, W. M. HUNTER, CLARA LOWY, R. W. J. NEVILLE, E. SAMOLS and PATRICIA M. WOODWARD
Results of a collaborative study in which the insulin concentrations of a number of plasma samples were estimated in several laboratories are reported. They indicate that: (1) laboratories agreed on the ranking of plasma samples according to insulin content; (2) the reproducibility of replicate estimates obtained in the same assay usually overestimated the reproducibility of estimates made on different occasions; (3) values reported for the insulin concentration of the same plasma examined in each of a number of laboratories were likely to range from about one half to twice the inter-laboratory mean estimate.
Possible reasons for the variability of estimates are discussed.