BIOLOGICAL ASSAY AND STANDARDISATION OF MELANOPHORE EXPANDING PITUITARY HORMONE

Abstract

1. Detailed instructions are given for assay of melanophore‐expanding hormone using Xenopus laevis as a test animal. 10 per cent. accuracy is quickly and easily obtained. 2. The criterion of potency is the maximum melanophore index attained after injection of extracts into the dorsal lymph sacs of fully pale intact or hypophysectomised animals. 3. Intact Xenopus kept on a white “background” for long periods sustain an actual loss of pigment and their sensitivity to pituitary extracts is increased. 4. The first international standard posterior lobe powder contains 97 per cent. of the melanophore‐expanding activity of the new standard. 5. We suggest that an international unit of melanophore activity be defined as that amount in 0·5 mgm. of the international standard powder. 6. The ratio of melanophore‐expanding, pressor, and oxytocic activities is not the same in all posterior lobe powders. 7. The maximum melanophore index evoked by injection of a posterior lobe extract is not influenced by the amounts of pressor and oxytocin normally present. So one substandard powder may be used for assay of all activities, provided a separate figure is assigned for the potency of each activity in terms of the corresponding activity of the international standard powder. Our best thanks are due to Dr. F. G. Young of the Medical Research Council for a generous supply of posterior lobe powder. We gratefully acknowledge an expenses grant to one of us (H. W.) from the Medical Research Council.