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SUMMARY
Experiments were performed on male albino rats in urethane anaesthesia. The pituitary gland was exposed, the pituitary stalk was cut and the whole gland was removed. Isotonic sodium chloride solution used for washing the cut pituitary stalk was collected and its antidiuretic activity assayed. Antidiuretic substance was liberated from the infundibular axons into the washing fluid during continuous intravenous infusion of a hypotonic saline. The antidiuretic activity of the washing fluid was assayed in ethanol-anaesthetized rats. A 5% sodium chloride solution, infused intravenously to the limit of 1% of the body weight, caused a significant release of antidiuretic substance from the cut infundibular axons. A similar effect was observed after the i.v. infusion of 1 ml. of a solution containing 0·06 mm-CaCl2. The i.v. infusion of 0·02 mm-MgCl2 decreased the amount of antidiuretic substance liberated from the pituitary stalk significantly.
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ABSTRACT
Patterns of neurohypophysial hormone secretion and changes in the hormone content of the hypothalamus and posterior pituitary lobe were monitored in the male rat for cycles of 24 h in association with changes in food and water intake and fluid excretion. Plasma oxytocin and vasopressin concentrations were seen to rise significantly over the hours of daylight, decreasing during the night. Parallel changes were seen in the immunoreactive material in the hypothalamus, whilst the content of the neurohypophysis was inversely related to plasma concentrations. The ratio of plasma oxytocin:vasopressin reached a significant peak at about 02.00 h which might be related to the feeding activity of the rats, food and water intake being largely confined to the night, as was fluid excretion. On exposure to constant light, despite initial disruption hormonal rhythms were still seen but showed a phase shift. The relationships between plasma and tissue levels were maintained. Patterns of food and water intake and urinary excretion were little affected by exposure to constant light, remaining largely confined to the former night phase. The hormonal rhythms appeared to be more closely related to the activity of the rats, which also showed a phase shift during constant light.
Journal of Endocrinology (1992) 133, 283–290