Adrenocortical responses to ACTH in neonatal rats: effect of hypoxia from birth on corticosterone, StAR, and PBR

Abstract

The adrenocortical response to hypoxia may be a critical component of the adaptation to this common neonatal stress. Little is known about adrenal function in vivo in hypoxic neonates. The purpose of this study was to evaluate adrenocortical responses to ACTH in suckling rat pups exposed to hypoxia from birth to 5-7 days of age compared with normoxic controls. We also evaluated potential cellular controllers of steroidogenic function in situ. In 7-day-old pups at 0800, hypoxia from birth resulted in increased basal (12.2 Ϯ 1.4 ng/ml; n ϭ 12) and ACTHstimulated (94.0 Ϯ 9.4 ng/ml; n ϭ 14) corticosterone levels compared with normoxic controls (basal ϭ 8.3 Ϯ 0.5 ng/ml; n ϭ 11; stimulated ϭ 51.3 Ϯ 3.8 ng/ml; n ϭ 8). This augmentation occurred despite no significant difference in plasma ACTH levels in normoxic vs. hypoxic pups before (85 Ϯ 4 vs. 78 Ϯ 8 pg/ml) or after (481 Ϯ 73 vs. 498 Ϯ 52 pg/ml) porcine ACTH injection (20 g/kg). This effect was similar in the afternoon at 6 days of age and even greater at 5 days of age at 0800. The aldosterone response to ACTH was not augmented by exposure to hypoxia from birth. Adrenocortical hypoxia-inducible factor (HIF)-1␣ mRNA was undetectable by RT-PCR. Steroidogenic acute regulatory (StAR) protein in adrenal subcapsules (zona fasciculata/reticularis) was augmented by exposure to hypoxia; this effect was greatest at 5 days of age. Peripheral-type benzodiazepine receptor (PBR) protein was also increased at 6 and 7 days of age in pups exposed to hypoxia from birth. We conclude that hypoxia from birth results in an augmentation of the corticosterone but not aldosterone response to ACTH. This effect appears to be mediated at least in part by an increase in controllers of mitochondrial cholesterol transport (StAR and PBR) and to occur independently of measurable changes in endogenous plasma ACTH. The augmentation of the corticosterone response to acute increases in ACTH in hypoxic pups is likely to be an important component of the overall physiological adaptation to hypoxia in the neonate. adrenocorticotropin; aldosterone; newborn; steroidogenic acute regulatory protein; peripheral-type benzodiazepine receptor; hypoxia-inducible factor HYPOXIA is one of the most common causes of neonatal morbidity and mortality (10, 17). Considerable attention has been paid to the short-and long-term neurological, cardiopulmonary, and renal consequences of neonatal hypoxia (3,8,14,34,36). In comparison, the adrenal adaptation in vivo to prolonged neonatal hypoxia has not been extensively studied. One pertinent study in human infants with hypoxemia due to bronchiolitis demonstrated an augmented cortisol, but not aldosterone, response to ACTH (12). This suggests a zone-specific adrenal adaptation to hypoxia.We have extensively examined dispersed adrenal cells in vitro removed from suckling rats exposed to hypoxia from birth (28). We have found that, as opposed to adult rats (29), steroidogenesis in vitro is augmented in adrenal cells from hypoxic neonatal rats despite no changes in steroidogenic enzy...

Metabolomic analysis of adrenal lipids during hypoxia in the neonatal rat: implications in steroidogenesis

“…An oxidized metabolite of 18:2 n-6 has been shown to stimulate basal aldosterone production in ZG cells (11,12), as well as augment basal and ACTH-stimulated corticosterone production in ZF/R cells (3,12). StAR and PBR proteins are significantly increased in the hypoxic adrenal (29), and the involvement of lipids in this process seems quite plausible given these previous findings.…”

“…These increases occur without an increase in plasma ACTH. Furthermore, the corticosterone response to exogenous ACTH is augmented in hypoxic pups (29). None of these changes could be explained by increased expression of genes encoding the steroidogenic enzymes, or by changes in the expression of the hypoxia-inducible transcription factor-1␣ (29,30).…”

“…Further studies on this phenomenon have measured small but significant increases in the expression of the steroidogenic acute regulatory (StAR) protein and the peripheral-type benzodiazepine receptor (PBR) protein in the hypoxic adrenal cortex (29). These changes occurred without a concomitant increase in plasma ACTH or plasma renin activity in the hypoxic pups (29,30).…”

Glucocorticoid feedback control of corticotropin in the hypoxic neonatal rat

“…Timed pregnant, Sprague-Dawley rats (Harlan, Indianapolis, IN, USA, nZ24) at 14 days gestation were obtained and maintained on a standard diet and water available ad libitum (0600-1800 h lights on). Immediately after parturition (days 21-22), dams and their pups were continuously exposed to either normoxia (21% O 2 ) or hypoxia (12% O 2 ) in an environmental chamber, as described in detail previously (Thomas & Marshall 1995, Raff et al 2000, 2003a. The experimental design is shown in Fig.…”

“…Understanding the mechanisms by which the resulting increase in glucocorticoid secretion occurs, as well as the physiological impact of this increase in glucocorticoids, will aid in devising strategies to mitigate the short-and long-term effects of neonatal hypoxia. We have previously demonstrated that the neonatal rat exposed to hypoxia from birth has increased plasma corticosterone that is driven by sympathetic input to the adrenal cortex rather than by corticotropin (ACTH; Raff et al 2003aRaff et al , 2004. It is possible that this ACTH-independent increase in corticosterone in the neonate exposed to chronic hypoxia from birth is a mechanism to increase circulating glucocorticoids by bypassing the stress-hyporesponsive hypothalamus and/or pituitary.…”