Corticosterone and aldosterone dose-dependent responses to ACTH and angiotensin II in the duck (Anas platyrhynchos)

Expanding the actions of cortisol and corticosterone in wild vertebrates: A necessary step to overcome the emerging challenges

We conducted a review of scientific articles published between 2000 and 2014 and evaluated how frequently various aspects of cortisol and corticosterone (CORT) actions have been considered in studies on wild vertebrates. Results show that (1) the notion that CORT are stress-responsive hormones is central in our theoretical frameworks and it is reflected by the fact that several articles refer to CORT as "stress hormones". (2) The large majority of studies do not contemplate the possibility of decrease and no change in CORT levels in response to chronic stressors. (3) Our ideas about CORT actions on energy balance are slanted towards the mobilization of energy, though there are several studies considering -and empirically addressing- CORT's orexigenic actions, particularly in birds. (4) The roles of CORT in mineral-water balance, though widely documented in the biomedical area, are virtually ignored in the literature about wild vertebrates, with the exception of studies in fish. (5) Adrenocorticotropic hormone (ACTH) independent regulation of CORT secretion is also very scarcely considered. (6) The preparative, permissive, suppressive and stimulatory actions of CORT, as described by Sapolsky et al. (2000), are not currently considered by the large majority of authors. We include an extension of the Preparative Hypothesis, proposing that the priming effects of baseline and stress-induced CORT levels increase the threshold of severity necessary for subsequent stimuli to become stressors. Studies on animal ecology and conservation require integration with novel aspects of CORT actions and perspectives developed in other research areas.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Effects of ACTH and cAMP on steroidogenic acute regulatory protein and P450 11beta-hydroxylase messenger RNAs in rainbow trout interrenal cells: relationship with in vitro cortisol production

Steroidogenic acute regulatory protein (StAR) transfers cholesterol over the inner mitochondrial membrane, thereby making the molecule available for cholesterol side-chain cleavage enzyme, which carries out the first conversion in the steroidogenic pathway. In mammals, StAR controls this rate limiting step in steroidogenesis, and both StAR protein and StAR mRNA levels become rapidly elevated in response to tropic hormone stimulation. The relationship between StAR gene expression and steroid production in fish has not yet been well explored. We investigated the relationship between adrenocorticotropic hormone (ACTH)- and cAMP-stimulated cortisol production in vitro and levels of StAR transcripts in interrenal cells of rainbow trout. To assess the effect of ACTH on mRNA levels of a downstream steroidogenic enzyme, we also investigated the effects of ACTH on transcripts encoding 11beta hydroxylase (P450 11beta). In a series of experiments, juvenile rainbow trout head kidney tissue containing interrenal cells was incubated with either ACTH or dibutyryl cyclic AMP (dbcAMP). Cortisol in incubation media were measured by radioimmunoassay and total RNA was isolated from the tissue for Northern analysis or for quantitative real-time PCR. Incubation of tissue with 150 ng/mL ACTH for 1-18 h induced a progressive increase in cortisol accumulation in media, but StAR mRNA levels increased modestly and mostly insignificantly over 18 h, irrespective of treatment. Exposure of tissue for 18 h to 5, 150, 500 or 1,500 ng ACTH/mL resulted in a strong increase in cortisol production, with a peak response (15-fold increase over controls) achieved with 150 ng/mL ACTH. Although there was a trend towards a dose-response effect, mean StAR mRNA levels were only significantly affected by the highest concentration of ACTH used (1,500 ng/mL), which induced a less than 2-fold increase in StAR transcripts. However, there was a significant linear relationship between StAR mRNA levels and ACTH-induced cortisol accumulation in media (p<0.001, r(2)=0.55). Incubation of tissue with 5mM dbcAMP for 6 or 18 h induced large increases in cortisol accumulation in media over controls, but had no significant effect on StAR mRNA levels. By contrast, ACTH induced a clear dose-dependent increase in P450 11beta transcripts, with 150 ng/mL ACTH inducing an 8-fold increase in levels compared to control; nonetheless, only a weak correlation existed between transcript levels and ACTH-induced cortisol secretion (p<0.003, r(2)=0.26). Thus, despite the relatively high degree of conservation of StAR proteins in vertebrates, we have been unable to demonstrate that a rapid, acute increase in transcription of the StAR gene is the dominant mechanism supporting flow of cholesterol to the mitochondria during acute increases in cortisol production in rainbow trout. The strong stimulation of P450 11beta gene transcription by ACTH probably enhances biosynthetic capacity during longer term chronic ACTH stimulation.

Maturation-dependent changes of angiotensin receptor expression in fowl

An angiotensin (ANG) receptor homologous to the type 1 receptor (AT1) has been cloned in chickens (cAT1). We investigated whether cAT1 expression in various tissues shows maturation/age-dependent changes. cAT1 mRNA levels detected in renal glomeruli [in situ hybridization (ISH)] and kidney extract (RT-PCR) are significantly (P < 0.01) higher in 19-day embryos (EB) than in chicks (CH, 2-3 wk) and pullets/cockerels (PL/CK, 14-16 wk). The levels in adrenal glands (concentrated in subcapsular regions) are high in EB and further increased in CH and PL/CK. cAT1 mRNA is also detectable in smooth muscle (SM)/adventitia of EB and CH aorta and in the adventitia, but not SM, from PL/CK aortas. The endothelia from small arteries and arterioles, but not from aorta, express cAT1 mRNA (ISH). In all age groups, ANG II induces profound endothelium-dependent relaxation of abdominal aorta, partly (37-47%) inhibitable (P < 0.01) by Nomega-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), suggesting the presence of ANG receptor in endothelium. l-NAME-resistant ANG II relaxation, examined in a limited number of EB or CH aortas, was reduced by 125 mM K+ or apamin plus charybdotoxin. The results suggest that 1) cAT1 is present in kidney, adrenal gland, and vascular endothelium (heterogeneity exists among arteries) of EB, CH, and PL/CK, and in aortic SM/adventitia of EB/CH but only in adventitia of PL/CK; 2) levels of cAT1 gene expression change during maturation in a tissue-specific manner; and 3) ANG II-induced relaxation may be partly attributable to nitric oxide and potassium channel activation.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

Urinary clearance of angiotensin II in Pekin ducks

To quantify the renal excretion of angiotensin II (AII) in birds, synthetic AII was intravenously infused into conscious Pekin ducks at rates of 5, 15 and 45 ng/kg/min for 30 min and the relationships between plasma and urine AII concentrations were monitored by radioimmunoassay. The infusions of AII produced dose-dependent elevations in the plasma concentrations of the hormone, however, urinary concentrations and excretion rates did not change significantly from the basal values of 11.6 +/- 1.2 pg/ml and 4.3 +/- 1.0 pg/min, respectively. The metabolic clearance rate of AII remained constant at each infusion dose (approximately 120 ml/min/kg). The urinary clearance rates (< 0.3 ml/min/kg) indicated that the fraction of AII cleared from the blood via the urine was less than 0.2%. Clearly, excretion of intact AII via this route is not a quantitatively important means of elimination for this hormone in birds.

Thyrotropic activity of the ovine corticotropin-releasing factor in the chick embryo

An injection of several doses of ovine corticotropin-releasing factor (oCRF) was made into an allantoic blood vessel of 18-day-old chick embryos. All doses used (0.5, 1, 2, and 5 micrograms) induce a quick increase of plasma corticosterone concentrations after 15 and 30 min and 1 and 2 hr and this in a dose-dependent manner with the high doses having a prolonged effect lasting up to 2 hr. An increase in plasma levels of thyroxine (T4) and triiodothyronine (T3) was observed after 1 and 2 hr. After the oCRF injection, no stimulation of the hepatic 5'-monodeiodination activity was observed and there was no increase, but even a small decrease in the T3 to T4 ratio could be calculated. It was therefore concluded that oCRF in the chick embryo has a thyrotropic effect presumably by stimulating the release of thyrotropin from the pituitary.