The oxytocic substance of posterior pituitary extracts is normally present in equal amounts in both sexes (van Dyke, Adamsons & Engel, 1955). So far two physiological functions have been ascribed to oxytocin, namely, as one of the factors causing contraction of the uterus in parturition, and as the factor responsible for contraction of the mammary myo-epithelial cells in lactation. Several workers have tried to find some other function for this substance. Positive results were obtained by Dicker & Heller (1946) who reported that when Pitocin (oxytocin; Parke, Davis) was given subcutaneously to rats in doses of 3 m-u/100 g rat, glomerular filtration rate (G.F.R.) and renal plasma flOW (R.P.F.) were increased and Cl reabsorption diminished. Demunbrun, Keller, Levkoff & Purser (1954) observed that Pitocin and du Vigneaud's highly purified oxytocin when infused intravenously in large amounts, or given subcutaneously in doses of 2-7-5 i.u., raised renal clearances in dogs with diabetes insipidus, and they suggested that one of the normal functions of oxytocin was to maintain renal blood flow. Brunner, Kuschinsky & Peters (1956) gave doses of 1-50 m-u. oxytocic extract subcutaneously to rats and observed that following hydration with saline solutions, or when the animals were dehydrated, the extract acted as a diuretic and increased Na and K excretion in parallel with the increase in rate of urine flow.The experiments described below were made in another attempt to find out whether oxytocin has some other function than its acknowledged and intermittent ones and whether there is a physiological interdependence of vasopressin and oxytocin. The latter question arises since it has been observed that whatever stimulus is applied to the posterior pituitary-electric, suckling, osmotic etc.-the gland liberates simultaneously both the vasopressor and oxytocic factors, and that the oxytocic one is liberated in greater amounts than the vasopressor by about 4-20 times (Harris, 1955). It was therefore

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In two recent papers (Lloyd, 1959a, b) it was shown that the vascular responses of the rat to oxytocin and vasopressin varied with the concentration of ovarian hormones in the body. For example, oxytocin was dilator in the dioestrous rat, though without effect on the blood pressure, while in the oestrous animal, during late pregnancy, or after ovarian hormone administration, oxytocin was pressor and constrictor. It was not known how far this reversal of effect was due to an altered state of the peripheral vasculature, or how far central mechanisms were involved. Since peripheral vasodilatation is one of the consequences of the administration of oestrogen, it was of interest to test the effect of other dilator substances on the response to oxytocin and vasopressin in the rat, and for this purpose infusions of histamine, acetylcholine, isoprenaline and 5-hydroxytryptamine (5-HT) were used. In addition, procedures such as pithing, decerebration, and the administration of autonomic blocking agents were used to determine whether reduction of vasomotor tone would affect the responses to posterior pituitary hormones, and to discover if any part was played by the central and peripheral nervous systems. METHODSAll experiments were made on rats of approximately 200 g body weight. Anaesthesia, injection of drugs, and recording of blood pressure were effected by the methods previously described (Lloyd, 1959a, b). The stage of the reproductive cycle was checked in all females by vaginal smears stained with Leishmann stain. All drugs were dissolved in NaCl solution 0-9 g/100 ml. Single intravenous injections were made up to a volume of 0-3 ml.; intravenous infusions were made into a cannulated femoral vein, at a rate of 0-05 ml./min. Autonomic blocking agents were either given intravenously during the experiments, or subcutaneously 2-3 hr before observations were begun. In experiments in which rats were pre-treated with an oestrogen, stilboestrol dipropionate was used, given in a dose of 3-5 pg/100 g 24 hr before observations were made.

In an earlier paper (Pickford, 1939) (Theobald, 1934) that this substance interferes but little with the excretion of water.Dogs and bitches were used. At a preliminary aseptic operation the kidneys were denervated by a thorough stripping of their pedicles, and the urethra made easily accessible by the dorsal slitting of the perineum in the female, and by exposing, slitting and stitching open the membranous urethra in the male. One to two weeks after operation the pituitary gland and optic chiasma were exposed through the mouth under soluble pentobarbitone anaesthesia. The soft palate was left unsutured. Early the next morning when the animal w&s well and lively, it was allowed a half pint of milk. Two hours later a hydrating dose of 250-300 c.c. water was given by mouth. Two to three hours later again 250-350 c.c. water containing 0-14 g. chloralose per kg. body weight were given by mouth, and urine collection begun by means of a soft rubber catheter passed into the bladder. When renal clearances were to be measured, -hr. before the dose of water and chloralose 10 g. inulin in 50 c.c. 0-9 % NaCl and 6 c.c. diodone were injected subcutaneously in the flanks. This subcutaneous administration was found to maintain, for some hours, adequate and fairly steady plasma concentrations of inulin and diodone. If creatinine was used, this was given by mouth in a 4 g. dose in 100 c.c. water at the time of the diodone administration.

RECENT work has made it appear possible that acetylcholine is the effective transmitter of impulses in the central as well as in certain distributions of the peripheral nervous system. Dikshit's work [1934, 1935] first brought this idea into the realm of probability. He injected acetylcholine into the hypothalamus and ventricles of cats and found that the effects produced were similar to those of electrical stimulation of those parts. Later, Chang, Chia, Hsi! & Lim [1937] showed that a reflex secretion of a pressor substance of posterior pituitary origin could be elicited by the stimulation of the central end of the divided vagus nerve. It seemed reasonable, therefore, to try whether acetylcholine' played any part in the liberation of the posterior pituitary hormones, using the inhibition of water diuresis as a test forthe presence of one of the hormones. METHODSBitches were used, in which at a preliminary aseptic operation the fundus of the bladder had been removed to lessen the dead space, and the perineum slit dorsally in order to expose the urethral opening and make catheterization easy. 250 c.c. of water were given by stomach tube every morning as a hydrating dose, and 34 hr. later water diuresis was induced by giving 250-350 c.c. of water according to the size of the dog. The urine was collected and its rate of flow measured by means of a selfretaining catheter leading to a series of graduated tubes. When atropine sulphate was used it was injected subcutaneously in 1-2 mg. doses at the time of the second administration of water. Injections of acetylcholine in a solution of 5 % sodium dihydrogen phosphate were made into the ankle vein with no apparent disturbance of the animal, the volume 1 Molitor & Pick [1924], using dogs, found that choline inhibited water diuresis. They ascribed this effect to a change in water distribution.