Plasma corticosterone clearance in anaesthetized rats was measured from the disappearance of radioactivity after a bolus injection of [3~corticosterone. Mean fractional clearance rates were significantly (P < 0.05) reduced after a 48 h fast, by 32 and 22% for males and females respectively.Plasma corticosterone concentrations were increased by fasting in both sexes. Corticosterone secretion rates, calculated as the product of fractional clearance and plasma corticosterone concentration, did not differ between fed and fasted groups in either sex. The mean activity (U/liver) of the rate-limiting enzyme for corticosterone degradation, hepatic 4,5-dihydrocorticosterone :NADP+ A4-oxidoreductase, was significantly reduced by 51 and 78 % after fasting in males and females respectively. This was due to changes in both the soluble and microsomal forms of the enzyme. The binding capacity of corticosterone-binding globulin in plasma was significantly reduced by fasting in females (P < 0.001), but was not altered in males. The results suggest that reduced hormone clearance is the dominant cause of fasting hypercorticosteronaemia in the rat. Fasting: Corticosterone metabolism: RatCorticosteroids play a major role in the metabolic response to undernutrition, by causing increased release of amino acids from skeletal muscle, and their subsequent conversion to glucose in the liver (Waterlow et al. 1978;Munck et al. 1984;Tischler et al. 1988). Plasma corticosteroid concentration is generally assumed to be controlled by pituitary release of adrenocorticotrophin, which increases adrenal steroid secretion. Thus, increased levels of plasma adrenocorticotrophin and corticosteroids have been found in fasted humans (Beer et a/. 1989) and in cockerels given a diet deficient in protein (Carsia et al. 1988).The capacity of the liver to catabolize corticosterone is, however, highly dependent on the level of food consumption (Herbst et al. 1960). Since this organ is the major site of corticosteroid metabolism (Norman & Litwack, 1987), a decreased ability to catabolize corticosterone might increase plasma steroid levels independently of changes in adrenal secretion. The aim of the present investigation was to measure the turnover rate of plasma corticosterone, in order to assess the relative contributions of reduced clearance and increased secretion to the raised corticosterone levels found during fasting. In addition, measurements have been made of the binding capacity of plasma corticosterone-binding globulin (CBG), and of the activity of hepatic 4,5-dihydrocorticosterone : NADP+ A4-oxidoreductase (corticosterone reductase), the rate-limiting enzyme for corticosterone degradation. A preliminary report of this work has already been given (Woodward et al. 1990). M A T E R I A L S A N D M E T H O D S AnimalsThe rats were F1 hybrids obtained from our breeding colonies by crossing PVG/C Lister Hooded males with WAG/C Wistar Albino females. Three experiments were carried out.https://www.cambridge.org/core/terms. https://doi

SUMMARY1. Stimulation of left atrial receptors by distending balloons located at the junction between the pulmonary veins and the left atrium and in the body of the left atrium resulted in a diuresis in anaesthetized dogs.2. In the same dogs the diuresis was not consistently associated with a reduction in the antidiuretic activity of plasma as compared with that during the initial control period. The antidiuretic activity of dog plasma was assayed in water-loaded rats anaesthetized with ethanol.3. Intravenous injections of vasopressin suppressed a water diuresis in water-loaded dogs anaesthetized with chloralose. The changes in antidiuretic activity of dog plasma which resulted from these injections were reliably detected by the bio-assay in the rat.4. It is concluded that the diuretic response to the stimulation of left atrial receptors is not accompanied by a reduction in the plasma antidiuretic hormone level.

SUMMARY1. Stimulation of left atrial receptors by distension of a balloon in the lumen of the left atrium of anaesthetized dogs was shown to result in an increase in urine flow. Plasma samples obtained from these dogs during control periods and during periods of stimulation were applied to the Malpighian tubules of Rhodnius prolixus.2. It was found that the tubules suspended in test plasma secreted at a significantly lower rate than those suspended in control plasma.3. These differences were also evident in extracts of plasma prepared using the solvent n-butanol.4. Cutting or cooling the cervical vagi abolished these differences along with the increase in urine flow. It is argued that this preparation of the Malpighian tubule of Rhodnius prolixus could be used as a means of detecting the diuretic agent responsible for the increase in urine flow.

The plasma oestradiol-17 beta concentrations of obese and non-obese female Zucker rats have been measured in three phases of the oestrous cycle. The oestradiol concentrations of both phenotypes were similar, and changed normally with the oestrous cycle. The weights of the uteri also changed normally with the cycle. Plasma androgen concentrations in male Zucker rats have also been measured: the mean concentration was slightly but significantly lower in obese rats, and androgen-sensitive tissues were slightly reduced in weight. The oestradiol-17 beta concentrations in males of both phenotypes were similar. It seems unlikely that deficient plasma concentrations of gonadal hormones cause the infertility of obese rats of either sex.

SUMMARY1. In anaesthetized dogs, stimulation of atrial receptors after destruction of the pituitary gland results in a diuresis. This response was not abolished by the administration ofbretylium tosylate and was also observed in a surgically denervated kidney.2. The diuresis is qualitatively similar to that observed in anaesthetized dogs with intact pituitary glands.3. It is concluded that the diuresis which results from stimulation of the left atrial receptors is mediated by a blood-borne agent which is not the antidiuretic hormone.