Effects of O 2 -CO 2 changes on hypothalamoypophyseal-adrenocortical activation

An etiologic model proposing that non-insulin-dependent diabetes mellitus is chronic hypoxic stress hyperglycemia

This etiologic model equates non-insulin-dependent diabetes mellitus (NIDDM) to chronic hypoxic stress hyperglycemia produced by increased stimulation of the sympathetic nervous system and hypothalamic-pituitary-adrenal axis. In the initial stages of the disease, hypoxia is believed to result from hemodilutional anemia precipitated by a reduction in vascular smooth muscle tone. The reduction increases lumen diameter necessitating an increased blood volume to maintain pressure. Increased lumen diameter may also trigger atherosclerotic changes that characterize the later stages of NIDDM. The increased diameter decreases the shear stress experienced by endothelial cells and they respond by releasing endothelin, a smooth muscle constrictor and mitogen. The constricting action is hypothesized to be relatively ineffective in NIDDM leading to long-term endothelin release and activation of its mitogenic properties. The resulting increase in the number of smooth muscle cells may explain the intimal thickening of atherosclerosis. Restoration of vascular muscle tone is proposed as a treatment strategy for mild NIDDM.

Pituitary-adrenal response to bacterial endotoxin in developing rats

The neonatal rat is very sensitive to the lethal effects of bacterial endotoxin. Because of the adaptive importance of pituitary-adrenal secretions to stress, this study examined the ontogeny of the plasma corticosterone and adrenocorticotropic hormone (ACTH) responses to endotoxin. The lethal sensitivity of young rats to endotoxin ranged from 0.5 to 30 mg/kg (ip) in the 1- to 21-day-old rat. After endotoxin treatment, the 1- and 2-day-old rat showed marked elevations of corticosterone similar in magnitude to that seen in 21-day-old and adult rats; however, significantly depressed corticosterone increments were observed in the 5-, 10-, and 14-day-old rats. This age-related pattern of adrenocortical secretion was correlated with the developing rat's corticosterone response to exogenous ACTH. In contrast, endotoxin administered to 5-, 10-, and 14-day-old rats resulted in increments of plasma ACTH similar to those observed in the 21-day-old and adult rats. Although plasma ACTH levels increased by 84-127% in the 1- and 2-day-old rats, these increases were significantly less than those of rats at all other ages tested. Thus the newborn rat mounts an effective corticosterone response to endotoxin, loses this ability between ages 5-14 days, and regains this response at 21 days of age. Because the hyporesponsive ages exhibit a marked increase in ACTH secretion, the loss of the adrenocortical response to endotoxin appears to be a result of a depressed responsiveness of the adrenal cortex to ACTH.

Vasopressin, ACTH, and corticosteroids during hypercapnia and graded hypoxia in dogs

We examined the interaction of graded hypoxia and hypercapnia on stimulation of vasopressin (AVP), ACTH, and corticosteroids in nonsurgically stressed, pentobarbital-anesthetized, gallamine-paralyzed ventilated dogs. Partial pressure of O2 in arterial blood (PaO2) levels of approximately 26-29, 38-41, 54-57, and 83-88 Torr were achieved by altering the fractional concentration of O2 in dry inspired gas with a normocapnic (PaCO2, 35 Torr) and hypercapnic (PaCO2, 59 Torr) background. Normocapnic hypoxia produced a PaO2-dependent increase in AVP, ACTH, and corticosteroids. The threshold PaO2 for AVP was lower (approximately 35 Torr) than for ACTH (approximately 45 Torr). AVP, ACTH, and corticosteroids at all PaO2 levels were higher during hypercapnia. In addition, an ACTH and corticosteroid dose-response correlation estimated the threshhold ACTH to be 20-30 pg/ml. The PaO2-dependent hormone increases and the augmentation of these relationships by hypercapnia are consistent with a peripheral chemoreceptor-mediated reflex. In addition, hypoxia and hypercapnia did not seem to alter the high sensitivity of the adrenal cortex for ACTH.

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.

Adrenal and pancreatic endocrine responses to hypoxia in the conscious calf

1. Pancreatic and adrenal responses to intense hypoxia have been examined in conscious unrestrained calves 3-5 weeks after birth. 2. The outputs of both cortisol and corticosterone from the right adrenal gland rose steadily in response to hypoxia and this cortical secretory response was accompanied by a pronounced increase in blood flow through the gland. The changes in both steroid output and adrenal blood flow corresponded with those which occur in response to supramaximal doses of corticotrophin in calves of the same age. 3. Neither adrenaline nor noradrenaline were released in significant amounts from the adrenal medulla until the arterial PO2 had fallen below 15 mmHg. Such severe hypoxia caused secretion of catecholamines at rates comparable with those recorded during maximal stimulation of the sympathetic innervation to the gland in anaesthetized calves. The response to intense hypoxia in these conscious calves differed from that which occurs under anaesthesia in that the amount of adrenaline released was invariably greater than that of noradrenaline. 4. Severe hypoxia produced a rapid but transient increase in plasma glucagon concentration, followed by a pronounced rise in plasma glucose concentration in animals with abundant liver glycogen. No change in plasma insulin concentration was observed during hypoxia although it rose subsequently in response to hyperglycaemia. 5. Bilateral section of the splanchnic nerves virtually abolished the release of catecholamines in response to hypoxia but the adrenal cortical and pancreatic responses did not appear to be affected.