EFFECT OF HYPOXIA ON SECRETION OF ACTH IN THE RAT

Upregulation of adrenocorticotrophic hormone in the corticotrophs and downregulation of surface receptors and antigens on the macrophages in the adenohypophysis following an exposure to high altitude

Altitude exposures lead to the development of hypobaric hypoxia because of low oxygen tension in the ambient air. This study has shown the vigorous upregulation of adrenocorticotrophic hormone (ACTH) expression in corticotrophs of the pars distalis (adenohypophysis) of rats 1-7 days after an altitude exposure. Concomitant to this was the increase in number and hypertrophy of the immunoreactive corticotrophs. It was suggested that this had resulted in an upsurge of ACTH production which may have suppressed the immuno-expression of complement type 3 receptors and major histocompatibility complex class II antigens constitutively expressed by the parenchymal macrophages through paracrine action. Along with ACTH, altered levels of other hormones following such exposures may also contribute to suppression of antigen presenting function and phagocytic activity of macrophages. The effects of altitude (hypobaric hypoxia) exposure, however, were reversible as the above immunohistochemical changes returned to normal 21-28 days after the hypobaric hypoxic insult.

Prolonged hypoxic stress increases adrenal cholesterol reserve in rats without causing adrenal hypertrophy

1. It is known that, in rats, hypoxia stimulates adrenal steroidogenesis, but our understanding of the hypoxic effect on the glandular parameters remains incomplete. 2. Adrenals were collected and analysed from rats that had been exposed to hypoxic conditions for 3 weeks. 3. The results reveal increased adrenal concentrations of corticosterone, free cholesterol and total cholesterol without a change in glandular weight and protein concentration. The increased total cholesterol is primarily associated with enriched cholesteryl adrenate (CE22: 4), cholesteryl arachidonate (CE20: 4) and cholesteryl oleate (CE18: 1).

Diminished adrenal steroidogenic activity in aging rats: new evidence from adrenal cells cultured from young and aged normal and hypoxic animals

Adrenal cells from 2-6-month-old young rats (Y cells) and from 19-25-month-old aged male rats (O cells) were adapted to primary monolayer culture. The cultures of Y and O cells appeared to be primarily epithelial and rounded up in response to stimulation with adrenocorticotropic hormone (ACTH). The general morphology of O cells was comparable to that observed in Y cells except for the presence of lipofuscin-like granules, a cellular marker of aging, in O cells, but not in Y cells. ACTH-stimulated steroid production by O cells was 52% lower than that by Y cells. Exposure of intact young rats to hypoxia (0.5 atmosphere) for 21 days prior to sacrifice and culture resulted in a 122% increase of ACTH-stimulated adrenal steroidogenic activity in the cultured cells, but this effect was not observed in adrenal cells cultured from hypoxic aged rats. The results suggest that there is an age-related diminution in rat adrenal steroidogenic capacity in response to ACTH stimulation in culture derived from Y and O animals; hypoxic stress magnifies this difference.

Adrenal corticomedullary hyperplasia in hypobaric hypoxia

Continuous exposure of adult male Wistar albino rats to a barometric pressure of 380 mm Hg (equivalent to an altitude of 5500 m) for 28 days causes a marked increase in adrenal gland weight due to hyperplasia of both adrenal cortex and medulla. The size of the cortical and medullary components of the gland relative to each other is unaffected; in both hypoxic and control animals the cortex occupies approximately 85 per cent of the overall volume of the gland. Cortical hyperplasia is consistent with the elevated levels of ACTH which have previously been shown to occur during hypoxia. Medullary hyperplasia under such conditions presumably reflects a continuing increased requirement for catecholamines in an hypoxic environment. The morphology of these adrenal medullae bears a striking structural resemblance to some phaeochromocytomata. Adrenal medullary hyperplasia does not appear to have been described previously under such conditions and may be an important effect of prolonged severe hypoxia.

The effects of hypobaric hypoxia on the corticotroph population of the adenohypophysis of the male rat

The effect of hypobaric hypoxia on the corticotroph population of the adenohypophysis has been studied in male rats. After 28 days exposure to a barometric pressure of 380 mm Hg, equivalent to an altitude of 5500 m, there was an increase in their number relative to the total cell population. This may reflect a requirement for a sustained increase in the level of adrenocortical secretion in an hypoxic environment. Alternatively, higher levels of ACTH may be required to maintain normal levels of adrenocortical hormone secretion under these conditions.

Adrenal and pancreatic endocrine responses to hypoxia and hypercapnia in the calf

1. Adrenal and pancreatic endocrine responses to hypoxia and hypercapnia, of differing degrees of intensity, have been examined in conscious, unrestrained calves 3-5 weeks after birth. 2. The outputs of cortisol and corticosterone from the right adrenal gland were found to vary inversely with arterial Po2 between 17 and 55 mmHg. Significant increase in mean adrenal blood flow was not observed at arterial oxygen tensions above about 30 mmHg. 3. Release of physiologically effective amounts of catecholamines from the adrenal medulla occurred only in response to intense hypoxia (arterial Po2 17-1 +/- 2-8 mmHg) and was effectively abolished by section of both splanchnic nerves. Release of pancreatic glucagon in response to such intense hypoxia was unaffected by section of both splanchnic nerves and administration of atropine. In contrast, the rise in plasma pancreatic glucagon concentration during less intense hypoxia was abolished by autonomic blockade. 4. Hypercapnia produced by inhalation of either 5% or 10% CO2 for 30 min stimulated maximal release of adrenal glucocorticoids and caused a substantial rise in plasma glucagon concentration. In contrast, the adrenal medulla was found to be extremely resistant to hypercapnia. Significant release of catecholamines was only observed during intense hypercapnia (inhalation of 10% CO2) and noradrenaline was invariably found to be the predominant amine. 5. The results of these experiments show how endocrine responses to hypoxia and hypercapnia are graded in the conscious calf. Of the mechanisms we have examined the pituitary-adrenal cortical axis is the most sensitive and the adrenal medulla the most resistant, while the pancreatic alpha cell occupies an intermediate position.

Studies on hypoxia. II. Autoradiographic quantitation of proline-3H incorporation by connective tissue cells of the neonatal hamster

Quantitative evaluation of autoradiographic grains from anoxia-treated and control neonatal hamster indicated that anoxia impairs collagen synthesis and significantly suppresses the incorporation of proline-sH in the odontoblast, fibroblast, and osteoblast by 24 hours after the insult. However, the rapid and apparently total recovery of the dental and paradental connective tissue cells suggests that anomalies of dentition, heretofore attributed to brief spells of anoxia at birth, may not be due to such a hypoxic insult during the perinatal period.