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ABSTRACT

The purpose of this study was to determine the effect of water restriction on the vasopressin response to hypoxia in conscious Long–Evans rats. Rats were prepared with chronic indwelling femoral artery and vein catheters 1 week before experimentation. At 24 h before the first blood sample, the supply of drinking water was maintained ad libitum (water replete) or removed (water deplete). At 24 h, a control blood sample was taken and then normoxia (21% O₂) was maintained or hypoxia (10% O₂) induced. Additional blood samples were taken at 1, 18 and 24 h. All blood samples (2·5 ml) were simultaneously replaced with donor blood to maintain isovolaemia. Hypoxia led to a very small and transient increase in vasopressin in the water-replete rats. The combination of hypoxia and water restriction led to a greatly augmented vasopressin response at 1 h (60 ± 16 pmol/l); this response was also not sustained. Additional non-cannulated rats were exposed to 24 h of normoxia or hypoxia with or without water available ad libitum and posterior pituitaries were collected after decapitation for measurement of vasopressin content. Water restriction, hypoxia and water restriction plus hypoxia all led to decreased pituitary vasopressin content. We conclude that the vasopressin response to hypoxia in conscious rats is small and transient, and that concomitant water restriction augments the vasopressin response to acute but not chronic hypoxia.

Journal of Endocrinology (1990) 125, 61–66

ABSTRACT

Angiotensin II (AII) and extracellular K⁺, acting through different intracellular mechanisms, stimulate aldosterone release in a synergistic fashion. We have previously shown that decreases in oxygen (O₂) within the physiological range inhibit AII, cyclic AMP (cAMP) and ACTH-stimulated aldosterone release. The present experiment evaluated the effect of various concentrations of O₂ on K⁺-stimulated aldosterone release in the presence and absence of AII. Dispersed bovine adrenal glomerulosa cells were incubated with different concentrations of K⁺ (0·9–5·4 mmol/l) without and with AII (10 nmol/l) under different concentrations of O₂ (0, 5 or 50%); 21% O₂ (pO₂ = 19·9±0·5 kPa, n = 9) was used as reference control for comparison.

In all cases, increases in K⁺ stimulated aldosterone release, an effect augmented by AII. Under 0% O₂ (pO₂ = 8·1±0·3 kPa, n = 3) and 5% O₂ (pO₂ = 12·8±0·5 kPa, n= 3), aldosterone release stimulated by K⁺ or K⁺/AII was significantly inhibited compared with that under 21% O₂. Conversely, under 50% O₂ (pO₂ = 36·3±2·5 kPa, n= 3), aldosterone release stimulated by K⁺ or K⁺/AII was significantly augmented. Cortisol secretion was not significantly affected by 5% or 50% O₂ but was significantly decreased under 0% O₂.

The effect of O₂ on K⁺/AII stimulation of aldosterone release, as well as previous experiments with cAMP, progesterone and ACTH, suggest a final common post-receptor oxygen-sensitive component of the aldosterone synthetic pathway. It is suggested that one or more enzymes in the aldosterone synthetic pathway is/are exquisitely sensitive to small changes in O₂ within the physiological range.

Journal of Endocrinology (1991) 129, 43–48

ABSTRACT

Glucocorticoids are known to inhibit the ACTH response to a variety of stimuli. It has been suggested that vasopressin secretion is also inhibited by glucocorticoid negative feedback. The purpose of this study was to (1) determine the ACTH response to hypertonic saline and its sensitivity to glucocorticoid negative feedback and (2) to determine whether physiological elevations of plasma cortisol inhibit subsequent vasopressin responses to hypertonic saline. Five mongrel dogs (15–18 kg) were prepared with chronic arterial and venous catheters and studied while conscious. Ten experiments were performed on each dog in a randomized design separated by at least 5 days. Each experiment consisted of a pretreatment period (from −60 to −30 min except for dexamethasone administration) during which a glucocorticoid feedback signal was applied and a stimulus period (from 0 to 30 min) during which hypertonic saline was infused. The pretreatment and stimulus periods were separated by 30 min. Pretreatments were as follows: isotonic saline (control), half-maximal and maximal cortisol infusion (5·5 or 11 nmol/kg per min), ACTH(1–24) infusion (6·8 pmol/kg per min) which produces increases in endogenous cortisol, and dexamethasone (1·5 mg i.m.) given at 17.00 h the day before experimentation. Stimuli were as follows: hypertonic saline was infused at 0·2 or 0·4 mmol/kg per min which increased plasma sodium by about 6 or 12 mmol/l respectively. NaCl infusion at 0·2 mmol/kg per min had no effect on plasma ACTH or cortisol except when subsequent to ACTH(1–24) pretreatment when plasma ACTH actually increased to 41·4 ± 2·9 pmol/l in response to hypertonic saline. NaCl infusion at 0·4 mmol/kg per min resulted in a significant increase in plasma ACTH from 5·9 ± 0·9 to 11·7 ± 2·0 pmol/l in the control group. This ACTH response was blocked by pretreatment with either dose of cortisol and dexamethasone. ACTH pretreatment, however, did not completely block the ACTH subsequent response to infusion of 0·4 mmol NaCl/kg per min. The two doses of NaCl led to significant and dose-related increases in plasma vasopressin. None of the pretreatments significantly affected the vasopressin response to hypertonic saline except for significant inhibition after overnight dexamethasone. We conclude that (1) hypertonic saline can stimulate ACTH release if plasma sodium is increased sufficiently, (2) the ACTH response to hypertonic saline is potentiated by pretreatment with ACTH making it different from other stimuli studied previously, and (3) the vasopressin response to hypertonic saline is not inhibited by short-term elevations of plasma cortisol within the physiological range.

Journal of Endocrinology (1989) 122, 41–48

Abstract

Although corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) have been extensively characterized as stimulators, and glucocorticoids as inhibitors of ACTH secretion, far less is known about the control of the secretion of ACTH precursors from the anterior pituitary or about the types of corticotrophs involved. The present study was designed to systematically evaluate the actions of stimulatory and inhibitory factors on the secretion of ACTH and ACTH precursors (pro-opiomelanocortin, M _r 31 000; pro-ACTH, M _r 22 000) from dissociated ovine anterior pituitary cells. The cells were stimulated for 3 h with CRH (10 nmol/l) and AVP (100 nmol/l), alone or in combination with the synthetic glucocorticoid dexamethasone. In designated wells, cells were treated with dexamethasone, (100 nmol/l), beginning 16–18 h before and continuing through the 3-h secretion experiments in the presence of CRH and AVP. Secretion of ACTH-like peptides from intact cultures was compared with that from cultures which had been pretreated with a cytotoxic CRH conjugate (cytotoxin) to eliminate CRH-target cells specifically. Immunoreactive (ir)-ACTH was measured by radioimmunoassay (RIA); ACTH(1–39) and ACTH precursors were specifically measured by two-site immunoradiometric assays that discriminate between the two. In intact populations of cells, dexamethasone had no effect on basal ACTH(1–39) secretion, but decreased the secretion of ACTH(1–39) in response to CRH or AVP. Pretreatment of cells in the same experiments with cytotoxin (for 18 h, beginning 3·5 days before secretion studies) also had no significant effect on basal ACTH(1–39) secretion, but eliminated the response to CRH and decreased the response to AVP. In contrast to the situation in intact populations, dexamethasone had no effect on the residual secretion of ACTH(1–39) in response to AVP. These results mirrored those for secretion of ir-ACTH, measured by RIA.

Secretion of ACTH precursors followed a different pattern from that for ir-ACTH and ACTH(1–39). In intact populations, dexamethasone decreased the secretion of ACTH precursors in response to CRH, but had no effect on basal secretion or the precursor response to AVP. Elimination of CRH-target cells also had no effect on basal precursor secretion and eliminated the secretion of precursors in response to CRH. Loss of CRH-target cells was accompanied by a smaller decrease in the secretion of ACTH precursors than ir-ACTH and ACTH(1–39) in response to AVP. Interestingly, dexamethasone significantly increased the secretion of ACTH precursors in response to AVP after cytotoxin.

These results suggest either that the inhibition by glucocorticoids of the ACTH(1–39) secretory response to AVP is confined to those AVP-responsive cells that are sensitive to the CRH-target-specific cytotoxin, or that glucocorticoid-induced inhibition of the response to AVP depends on the functional presence of CRH-responsive cells. The results further suggest that the secretion of ACTH precursors in response to AVP is resistant to inhibition by glucocorticoids, regardless of the presence of CRH-target cells and is, generally, much less influenced by, or dependent upon, CRH-target cells. Taken together, the data suggest that those corticotrophs which are resistant to cytotoxin are the source of ACTH precursors secreted in response to AVP, and resist inhibition by glucocorticoids.

Journal of Endocrinology (1994) 140, 189–195