An Examination of Factors Affecting the Precision of the Assay of the Oxytocic Hormone in Posterior Pituitary Lobe Preparations

Abstract

Summary Work has been described which shows the errors which may be encountered under different experimental conditions of the uterine assay of oxytocin, or commercial posterior pituitary extracts where the ratio of oxytocin to vasopressin is not unity. By increasing the concentration of magnesium chloride in the British Pharmacopoeia 1948 Ringer’s solution, the response of the guinea-pig uterus to pitocin is increased. The response of the guinea-pig uterus to pitressin is also potentiated by increasing the concentration of magnesium chloride, but to a different extent than is pitocin. Different concentrations of calcium chloride in the Ringer’s solution produce potentiation to both pitocin and pitressin, but to different extents. The pressor principle does not produce any oxytocic effect on the uterus suspended in Ringer’s solution from which calcium chloride is absent. The absence of glucose from the Ringer’s solution produces rapid fatiguing of the uterine strip. Temperature regulation of the Ringer’s solution is very important. The sensitivity of uteri from guinea-pigs treated with stilbæstrol is greater than the uteri of progesterone-treated guinea-pigs to both pitocin and pitressin. The ratio of the sensitivity of uteri from progesterone-treated guinea-pigs to pitressin and pitocin is almost unity, whilst the ratio of sensitivity for the uteri of guinea-pigs treated with stilbæstrol indicates that the uteri are more sensitive to pitocin than to pitressin. Seasonal variation in the number of virgin albino guinea-pigs used per assay was not found to be significant. The ratio of oxytocin to vasopressin in the test samples must be the same as the ratio which is stated for the standard. The type of uterus and the physiological salt solution should be stated. It is recommended that the modification of Ringer’s solution proposed by the British Pharmacopoeia 1948 should be used for the guinea-pig assay, and that the uterus of the guinea-pig should be that of a virgin in diæstrus. The chicken blood depressor method offers many advantages over the isolated guinea-pig uterine method. It is cheaper and quicker to perform, and is more accurate in that narrow fiducial limits can be obtained. Until the chicken blood depressor method has been studied exhaustively in many laboratories engaged on the assay of the oxytocic principle, the guinea-pig uterine assay should be run in parallel with the former.