Factors affecting ACTH release from perifused equine anterior pituitary cells

Assessment of the hypothalamic-pituitary-adrenocortical axis function using a vasopressin stimulation test in neonatal foals

BACKGROUND

Bacterial sepsis is the leading cause of death in foals and is associated with hypothalamic-pituitary-adrenocortical axis (HPAA) dysfunction. HPAA function can be evaluated by an arginine-vasopressin (AVP) stimulation test.

HYPOTHESES/OBJECTIVES

Administration of AVP will stimulate a dose-dependent rise in systemic adrenocorticotropin-releasing hormone (ACTH) and cortisol in neonatal foals. There will be no response seen in corticotropin-releasing hormone (CRH) and baseline AVP will be within reference interval.

ANIMALS

Twelve neonatal foals, <72 hours old.

METHODS

HPAA function was assessed in foals utilizing 3 doses of AVP (2.5, 5, and 7.5 IU), administered between 24 and 48 hours of age in this randomized cross-over study. Cortisol, ACTH, CRH and AVP were measured at 0 (baseline), 15, 30, 60 and 90 minutes after AVP administration with immunoassays. The fold increase in cortisol and ACTH was calculated at 15 and 30 minutes compared to baseline.

RESULTS

All doses of AVP resulted in a significant increase in cortisol concentration over time, and a dose-dependent increase in ACTH concentration over time. ACTH and cortisol were significantly increased at 15 and 30 minutes, respectively after all 3 doses of AVP compared to baseline (P < .01). There was no change in endogenous CRH after stimulation with AVP.

CONCLUSION AND CLINICAL IMPORTANCE

Administration of AVP is safe and results in a significant rise in ACTH and cortisol in neonatal foals. A stimulation test with AVP (5 IU) can be considered for HPAA assessment in septic foals.

Stress responsiveness of the hypothalamic-pituitary-adrenal axis: age-related features of the vasopressinergic regulation

The hypothalamic-pituitary-adrenal (HPA) axis plays a key role in adaptation to environmental stresses. Parvicellular neurons of the hypothalamic paraventricular nucleus secrete corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) into pituitary portal system; CRH and AVP stimulate adrenocorticotropic hormone (ACTH) release through specific G-protein-coupled membrane receptors on pituitary corticotrophs, CRHR1 for CRH and V1b for AVP; the adrenal gland cortex secretes glucocorticoids in response to ACTH. The glucocorticoids activate specific receptors in brain and peripheral tissues thereby triggering the necessary metabolic, immune, neuromodulatory, and behavioral changes to resist stress. While importance of CRH, as a key hypothalamic factor of HPA axis regulation in basal and stress conditions in most species, is generally recognized, role of AVP remains to be clarified. This review focuses on the role of AVP in the regulation of stress responsiveness of the HPA axis with emphasis on the effects of aging on vasopressinergic regulation of HPA axis stress responsiveness. Under most of the known stressors, AVP is necessary for acute ACTH secretion but in a context-specific manner. The current data on the AVP role in regulation of HPA responsiveness to chronic stress in adulthood are rather contradictory. The importance of the vasopressinergic regulation of the HPA stress responsiveness is greatest during fetal development, in neonatal period, and in the lactating adult. Aging associated with increased variability in several parameters of HPA function including basal state, responsiveness to stressors, and special testing. Reports on the possible role of the AVP/V1b receptor system in the increase of HPA axis hyperactivity with aging are contradictory and requires further research. Many contradictory results may be due to age and species differences in the HPA function of rodents and primates.

Hypothalamic-pituitary gland axis function and dysfunction in horses

Hypothalamic-pituitary (HP) dysfunction has been documented in a limited capacity in horses and foals associated with critical illness, stress, and pain. This article reviews species-specific details of anatomy, function, hormones, receptors, and testing of the HP axis in the horse. A discussion of critical care medicine relevant to HP dysfunction in the horse with some reference to current understanding in human medicine is made, focusing primarily on current and relevant literature. A brief mention of other conditions described in human and veterinary medicine is also provided for reference only, such as syndrome of inappropriate antidiuretic hormone secretion and other conditions.

Postnatal development in vasopressin deficient Brattleboro rats with special attention to the hypothalamo-pituitary-adrenal axis function: the role of maternal genotype

Anomalies in hormonal and neurotransmitter status during perinatal period can lead to lifespan alterations in the central nervous system. Vasopressin is present early in the brain and has various mitogenic, metabolic and physiological actions, e.g. in water homeostasis or in the regulation of the hypothalamo-pituitary-adrenal (HPA) axis. Therefore we examine the possible role of vasopressin in perinatal development with special attention to the influence of maternal genotype and to the HPA axis regulation. We compared homozygous vasopressin deficient (di/di) Brattleboro rats to their heterozygous (di/+) littermates both from di/+ and di/di mother. Higher locomotion due to reduced adaptation was present at preweaning. During the first 10 days of life the di/di pups from di/di mother were the smallest, while in the later perinatal period the genotype of the pups became the more important determinant of the somatic development, namely the di/di pups from both mothers had reduced weight gain. Generally the lack of vasopressin in the pups fastened the somatic development (pinna detachment, eye and ear opening, incisor eruption) however the neurobehavioral development (palmar grasp reflex, righting reflex, negative geotaxis, etc.) was not influenced profoundly by either the mother's or the pup's genotype. The lack of vasopressin in pups abolished the 24 h maternal separation induced adrenocorticotrop hormone (ACTH) elevation while the accompanying corticosterone rises were even higher. The vasopressin deficiency of the mother reduced the resting ACTH and all corticosterone levels in all pups. So we can conclude that the lack of vasopressin speeds up the development, probably there is a greater drive for self-sufficiency in these animals. The mother's vasopressin deficiency reduced the HPA axis reactivity of the pups. The role of vasopressin in the HPA axis regulation is important during the perinatal period independently from the mother's genotype. The large discrepancy between ACTH and corticosterone regulation requires further studies.

Plasma vasopressin concentrations in healthy foals from birth to 3 months of age

BACKGROUND

Arginine vasopressin (AVP) has received increased attention in equine critical care but there is minimal information of AVP concentration in foals. The clinical usefulness of measuring AVP in ill foals depends on knowledge of age-related changes in AVP concentrations in healthy foals.

HYPOTHESIS

Plasma AVP concentrations will be significantly different when measured from birth to 3 months of age in healthy foals.

ANIMALS

Thirteen healthy university-owned foals.

METHODS

Prospective, observational study. Blood was collected from healthy foals at birth and 3, 5, 7, 10, 14, 21, 28, 42, 56, and 84 days of age. Plasma was harvested and plasma AVP concentrations were determined by radioimmunoassay.

RESULTS

No statistically significant differences were detected in plasma AVP concentrations over the study period. Plasma AVP concentrations over the entire study period was 6.2+/-2.5 pg/mL.

CONCLUSIONS AND CLINICAL IMPORTANCE

There was no age-related variation in plasma AVP concentrations detected in healthy foals from birth to 3 months of age suggesting that AVP concentrations are similar across foals of these ages.

Inter-relationships between the secretory dynamics of thyrotrophin-releasing hormone, thyrotrophin and prolactin in periovulatory mares: effect of hypothyroidism

We used our nonsurgical technique for collecting pituitary venous blood to relate the dynamics of thyrotrophin-releasing hormone (TRH) secretion to the secretion patterns of both prolactin and thyrotrophin in periovulatory mares, either euthyroid (n = 5) or made hypothyroid by treatment with propyl-thiouracil (n = 5). Pituitary venous blood was collected continuously and divided into 1-min aliquots for 4 h. To test the effect of dopamine on the relationship between secretion patterns, sulpiride, a selective D2 receptor antagonist, was given i.m. after 2 h of sampling. Thorough testing of the model and blood collection procedure revealed no sites of TRH loss. Hypothyroidism increased the mean secretion rates of TRH (P = 0.04) and thyrotrophin (P < 0.0001) but not prolactin. Sulpiride increased prolactin secretion rates in hypothyroid (P < 0.0001) and control (P = 0.007) mares, but did not alter TRH or thyrotrophin secretion rates. In both groups of mares, all three hormones were secreted episodically but not rhythmically. In both groups, the secretion pattern of TRH was almost always significantly related to that of thyrotrophin, as assessed by cross correlation and cross approximate entropy (ApEn) analysis. However, the degree of linear correlation was weak, with only 14% (hypothyroid) or 8% (controls) of the variation in thyrotrophin secretion rates attributable to TRH. Prolactin and TRH secretion patterns before sulpiride were coupled on cross ApEn analysis in both groups, and the minute-to-minute secretion rates of the two hormones were correlated in four hypothyroid and three euthyroid mares. Overall, the small, but significant, degree of association between TRH and prolactin was similar to that between TRH and thyrotrophin. In hypothyroid mares, sulpiride increased (P = 0.02) the synchrony between TRH and prolactin patterns. We conclude that in horses: (i) little TRH degradation occurs during passage through the pituitary or in blood after 1 h at 37 degrees C; (ii) TRH is not the major factor controlling minute-to-minute fluctuations in either thyrotrophin or prolactin; and (iii) reducing two strongly inhibitory inputs (i.e. dopamine and thyroid hormones) may magnify the stimulatory effect of TRH on prolactin secretion.

The effect of endotoxin administration on the secretory dynamics of oxytocin in follicular phase mares: relationship to stress axis hormones

The primary aim of this study was to define the secretory dynamics of oxytocin and vasopressin in pituitary venous effluent from ambulatory horses during acute endotoxaemia, a stimulus that may release both hormones. Our secondary aim was to investigate the role of oxytocin in regulating adrenocorticotropic hormone (ACTH) secretion by comparing oxytocin, vasopressin, corticotropin-releasing hormone (CRH) and ACTH secretory profiles during endotoxaemia and by monitoring the ACTH response to oxytocin administration. Pituitary venous blood was collected nonsurgically continuously and divided into 1-min segments from eight follicular phase mares. Four mares were sampled for 30 min before and 3.5 h after receiving an i.v. infusion of bacterial endotoxin (TOX). Four control mares were sampled for 2.5 h without infusion of TOX. Another three follicular phase mares were given 5 U of oxytocin to replicate the peak response to TOX and pituitary blood collected every 1 min for 10 min before and 15 min after injection. Endotoxin raised the secretion rates of all hormones measured. All hormones were released episodically throughout the experiment, with TOX increasing the amplitude of peaks in each hormone. Peaks in oxytocin and vasopressin were coincident in each treated mare. Similarly, ACTH peaks were coincident with peaks of oxytocin and vasopressin in each treated mare, and with peaks of CRH in three mares. However, oxytocin administration did not affect ACTH secretion. We conclude that during endotoxaemia in horses: (i) oxytocin and vasopressin are secreted synchronously; (ii) oxytocin is unlikely to be acting as an ACTH secretagogue since inducing peak oxytocin concentrations observed during TOX does not raise ACTH; and therefore (iii) the close relationship between oxytocin and ACTH secretion is circumstantial and due to the fact that oxytocin secretion is concurrent with that of vasopressin, a proven ACTH secretagogue in horses.

Acute and chronic regulation of pituitary receptors for vasopressin and corticotropin releasing hormone

At least two hypothalamic peptides, corticotropin releasing hormone (CRH) and vasopressin (VP), are important in regulating adrenocorticotropin (ACTH) release from the anterior pituitary. Both are secreted in a pulsatile manner and stimulate ACTH secretion by interacting with G protein-coupled receptors (GPCRs), namely the type 1 CRH receptor and V1b receptor, respectively. Repeated or prolonged stimulation with either peptide can cause reduced ACTH responsiveness or desensitisation, both in vivo and in vitro. Desensitisation of perifused sheep anterior pituitary cells to VP was found to be rapid and occurred following treatment with 5 nM VP for 5 min. This is within the range of concentrations and durations of VP pulses seen in sheep portal blood during acute stress. In contrast, significant desensitisation of the ACTH response to CRH required pre-treatment for longer than 25 min with a CRH concentration of 1 nM, suggesting that endogenous pulses may not elicit desensitisation. Although rapid GPCR desensitisation involves uncoupling of receptors from their G proteins, commonly mediated by receptor phosphorylation, and internalisation of receptors, desensitisation of neither the CRH nor VP receptor was mediated by PKA or PKC, respectively. Desensitisation of the response to VP was found to be dependent upon receptor internalisation, and resensitisation could be delayed by treatment with a protein phosphatase 2B inhibitor. The rapid kinetics of desensitisation of the ACTH response to VP suggest that this process is important in regulating the response to acute rather than chronic stress. If, as has been suggested, CRH acts in a permissive way to set corticotrope gain, desensitisation to CRH could also be important in long term regulation of ACTH secretion.

The effect of the alpha-2-adrenergic agonist, clonidine, on secretion patterns and rates of adrenocorticotropic hormone and its secretagogues in the horse

Alpha-2-adrenoceptor activation may lower adrenocorticotropic hormone (ACTH) by reducing secretagogue input and/or increasing the release of an inhibitory factor (CIF). To investigate this, we gave clonidine, an alpha-2-agonist, to seven horses, and collected pituitary venous blood every minute for 20 min before treatment and 40 min after treatment. Six horses were given saline vehicle. Mean secretion rates of corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP) and ACTH were calculated before and during four 5-min then two 10-min periods after clonidine or saline. Reduction in ACTH secretion without corresponding changes in CRH and/or AVP would imply the presence of CIF. Secretion rates of ACTH (P = 0.008) and AVP (P = 0.0005) fell after clonidine and remained lower than baseline values for 20 min and 10 min, respectively. The CRH secretion rate decreased slightly but not significantly after clonidine. In controls, hormone secretion rates did not alter during the experiment. Multiple linear regression showed that CRH and AVP secretion accounted for 69% (treated) or 45% (controls) of the variation in ACTH secretion (P < 0.0001 for each). CRH alone contributed 80% (treated) or 76% (controls) of the fit to this model, which is consistent with the concept that CRH 'sets the gain' of the response of corticotrophs to fluctuations in AVP. Accordingly, minute-to-minute changes in pituitary concentrations of AVP and ACTH were synchronous when all data were considered (% concordant changes: controls, 68%, P < 0.0001; treated, 76%, P < 0.0001) and the percentage of concordant movement was unaffected by clonidine (before 72%; after 73%; P = 0.80). In treated horses but not controls, the ratio between the secretion rates of ACTH and AVP fell (P = 0.009), while the ACTH : CRH ratio tended to fall after clonidine, implying reduced responsiveness to stimulation. Moreover, one horse showed a drop in ACTH and a rise in CRH and AVP secretion after clonidine. We conclude that in horses alpha-2-adrenoceptor activation lowers ACTH secretion primarily by reducing the secretion of AVP and possibly CRH. While there was some evidence that a CIF may participate in the clonidine-induced suppression of ACTH, the subtlety of the discordance between ACTH and its secretagogues in most horses and the rarity of complete dissociation indicate that it does not play a major role.

Interactions of CRH, AVP and cortisol in the secretion of ACTH from perifused equine anterior pituitary cells: "permissive" roles for cortisol and CRH

To further elucidate the interaction of CRH, AVP and cortisol in the control of ACTH secretion, we used an in vitro perifusion model with dispersed equine anterior pituitary cells. To approximate the in vivo milieu in the horse, CRH was perifused continuously (at 0, 2 and 20 pmol/L) and 5-min pulses of AVP (0, 1, 3 and 10 nmol/L) were given every 30 min in the presence of 0 or 100 nmol/L cortisol. Total (baseline + incremental) ACTH secretion increased as both the CRH (p<0.001) and the AVP (p<0.001) concentration increased and interaction between CRH and AVP was significant (p=0.042). Cortisol reduced total ACTH secretion in the presence of 2 pmol CRH/L (p=0.001) but not 0 or 20 pmol CRH/L. For incremental ACTH there was interaction between CRH and AVP (p<0.0001), with increased secretion at higher concentrations, and no significant main effect of cortisol. There was significant (p=0.001) interaction between cortisol and CRH, with cortisol attenuating ACTH release at 0 pmol CRH/L (p=0.008), having no effect at 2 pmol CRH/L and potentiating it at 20 pmol CRH/L (p=0.026). We conclude that (1) CRH at high physiological levels has a "permissive" role in preventing the cortisol inhibition of the ACTH response to AVP, and (2) basal cortisol levels have a "permissive" action in priming the HPA axis for maximal responsiveness to stimulated levels of CRH and AVP.

In the trout, CRH and AVT synergize to stimulate ACTH release

Anterior pituitaries of the rainbow trout (Oncorhynchus mykiss) were incubated with graded concentrations of arginine vasotocin (AVT) or synthetic rat corticotrophin-releasing hormone (rCRH-41), alone or in combination, and the ACTH secreted into the medium was measured by a sensitive cytochemical bioassay. The aim was to determine the relative potencies of the two secretogogues and whether, in this fish species, they act synergistically. Rat CRF-41 and AVT both produced concentration-dependent increases in ACTH release. The minimum effective concentration for both peptides was approximately 1 nM but, at higher concentrations, the efficacy of CRF-41 was greater than that of AVT. Clear evidence of synergy between the two peptides was obtained. The response of the trout thus falls in line with observations in mammals and contrasts with findings for the goldfish.

Effects of corticotropin-releasing hormone, lysine vasopressin, oxytocin, and angiotensin II on adrenocorticotropin secretion from porcine anterior pituitary cells

The aim of this study was to determine the ability of corticotropin-releasing hormone (CRH), lysine vasopressin (LVP), oxytocin (OT), and angiotensin II (AII) to stimulate adrenocorticotropin (ACTH) secretion from porcine anterior pituitary (AP) cells in vitro and to evaluate the role of protein kinase C (PKC) in the interaction between CRH and LVP. In this study, porcine AP cells were enzymatically and mechanically dispersed, cultured (150,000 cells/well) for 4 d, and then challenged with doses of various neuropeptides for 3 hr. CRH (10(-7)-10(-10) M) was the most potent of the peptides tested in stimulating ACTH release from porcine AP cells. In fact, none of the other peptides consistently affected ACTH concentrations relative to basal levels. However, LVP potentiated CRH action, even though by itself, it failed to stimulate ACTH production. Neither OT or AII potentiated CRH-stimulated ACTH release from porcine AP cells. To determine whether the inter-action between CRH and LVP was regulated partially by the protein Kinase C (PKC) pathway, we challenged AP cells in a 30-min incubation with 10(-7) M staurosporine (ST), a treatment predicted to decrease PKC activity. Then, cells were washed and challenged with 10(-9) M LVP, 10(-9) M CRH, and 10(-9) M CRH + LVP. Treatment with ST decreased (P < 0.05) CRH + LVP-stimulated ACTH release. To further demonstrate an interaction between protein kinase A (PKA) and PKC transduction pathways in the observed synergism between CRH and LVP to enhance ACTH secretion, we also challenged AP cells with 10(-7) M phorbol 12, 13-myristate acetate (PMA) and 5 microM forskolin (FOR) for 3 hr. This treatment was predicted to enhance PKA and PKC activities, respectively, and thereby enhance ACTH concentrations. Challenging cells with FOR + PMA enhanced (P < 0.001) ACTH release above basal concentrations, but more important, it increased (P < 0.001) ACTH concentration above that elicited by either drug given alone. Taken together, our in vitro studies support the conclusion that CRH is the principal regulator of ACTH secretion in the pig. In contrast to the results in most other species evaluated, vasopressin alone did not affect ACTH release. However, LVP can enhance the effectiveness of CRH in releasing ACTH, and this enhancement appears to rely, at least in part, on the activation of the PKC signal transduction pathway.