Corticosterone, but not glucose, treatment enables fasted adrenalectomized rats to survive moderate hemorrhage

Review article: Early steroid administration for traumatic haemorrhagic shock: A systematic review

Haemorrhagic shock after trauma is a leading cause of death worldwide, particularly in young individuals. Despite advances in trauma systems and resuscitation strategies, mortality from haemorrhagic shock has not declined over the previous two decades. A proportion of shocked trauma patients may experience a deficiency of cortisol relative to the severity of their injury. The benefit of exogenous steroid administration in patients suffering haemorrhagic shock as a result of injury is unclear. A systematic review of four databases (Ovid Medline, Ovid Embase, Cochrane, Scopus) was undertaken. Inclusion and exclusion criteria were pre-determined and two reviewers independently screened the articles with disagreements arbitrated by a third reviewer. The primary outcome variable was 28-day mortality. Quality of studies were assessed using the Cochrane-risk-of-bias (RoB 2) tool. Of the 2919 studies yielded by the search strategy, 1274 duplicates were removed and 1645 screened on title and abstract. After the full text of 33 studies were assessed, two articles were included. Both studies were over 30 years old with small numbers of participants and with primary outcomes not including mortality. Of the data available, no statistically significant difference in mortality was detected. Hospital length of stay, reversal of shock or adverse events were not reported. Both studies were at risk of bias. There are no high quality or recent studies in the English literature investigating the use of steroids for haemorrhagic shocked trauma patients. PROSPERO: CRD42021239656.

FKBP5 expression is related to HPA flexibility and the capacity to cope with stressors in female and male house sparrows

The hypothalamic-pituitary-adrenal (HPA) axis and its end products, the glucocorticoids, are critical to responding appropriately to stressors. Subsequently, many studies have sought relationships between glucocorticoids and measures of health or fitness, but such relationships are at best highly context dependent. Recently, some endocrinologists have started to suggest that a focus on HPA flexibility, the ability of an individual to mount appropriate responses to different stressors, could be useful. Here, we tested the hypothesis that expression of FKBP5, a cochaperone in the glucocorticoid receptor complex, is a simple and reliable proxy of HPA flexibility in a wild songbird, the house sparrow (Passer domesticus). We quantified HPA flexibility in a novel way, using guidance from research on heart rhythm regulation. As predicted, we found that adult sparrows with low stress-induced FKBP5 expression in the hypothalamus exhibited high HPA flexibility. Moreover, low FKBP5 expression was associated with greater exploratory disposition and were better at maintaining body mass under stressful conditions. Altogether, these results suggest that FKBP5 may be important in the regulation of HPA flexibility, potentially affecting how individuals cope with natural and anthropogenic adversity.

FKBP5: A Key Mediator of How Vertebrates Flexibly Cope with Adversity

Flexibility in the regulation of the hypothalamic–pituitary–adrenal (HPA) axis is an important mediator of stress resilience as it helps organisms adjust to, avoid, or compensate for acute and chronic challenges across changing environmental contexts. Glucocorticoids remain the favorite metric from medicine to conservation biology to attempt to quantify stress resilience despite the skepticism around their consistency in relation to individual health, welfare, and fitness. We suggest that a cochaperone molecule related to heat shock proteins and involved in glucocorticoid receptor activity, FKBP5, may mediate HPA flexibility and therefore stress resilience because it affects how individuals can regulate glucocorticoids and therefore capacitates their abilities to adjust phenotypes appropriately to prevailing, adverse conditions. Although the molecule is well studied in the biomedical literature, FKBP5 research in wild vertebrates is limited. In the present article, we highlight the potential major role of FKBP5 as mediator of HPA axis flexibility in response to adversity in humans and lab rodents.

Reproductive state and water deprivation increase plasma corticosterone in a capital breeder

Plasma corticosterone (CORT) concentrations fluctuate in response to homeostatic demands. CORT is widely recognized as an important hormone related to energy balance. However, far less attention has been given to the potential role of CORT in regulating salt and water balance or responding to osmotic imbalances. We examined the effects of reproductive and hydric states on CORT levels in breeding Children's pythons (Antaresia childreni), a species with substantial energetic and hydric costs associated with egg development. Using a 2 × 2 experimental design, we examined how reproduction and water deprivation, both separately and combined, impact CORT levels and how these changes correlate with hydration (plasma osmolality) and energy levels (blood glucose). We found that reproduction leads to increased CORT levels, as does dehydration induced by water deprivation. The combined impact of reproduction and water deprivation led to the largest increases in CORT levels. Additionally, we found significant positive relationships among CORT levels, plasma osmolality, and blood glucose. Our results provide evidence that both reproductive activity and increased plasma osmolality can lead to increased plasma CORT in an ectotherm, which could be explained by either CORT having a role as a mineralocorticoid or CORT being elevated as part of a stress response to resource imbalances.

Stress Resilience and the Dynamic Regulation of Glucocorticoids

Vertebrates respond to a diversity of stressors by rapidly elevating glucocorticoid (GC) levels. The changes in physiology and behavior triggered by this response can be crucial for surviving a variety of challenges. Yet the same process that is invaluable in coping with immediate threats can also impose substantial damage over time. In addition to the pathological effects of long-term exposure to stress hormones, even relatively brief elevations can impair the expression of a variety of behaviors and physiological processes central to fitness, including sexual behavior, parental behavior, and immune function. Therefore, the ability to rapidly and effectively terminate the short-term response to stress may be fundamental to surviving and reproducing in dynamic environments. Here we review the evidence that variation in the ability to terminate the stress response through negative feedback is an important component of stress coping capacity. We suggest that coping capacity may also be influenced by variation in the dynamic regulation of GCs-specifically, the ability to rapidly turn on and off the stress response. Most tests of the fitness effects of these traits to date have focused on organisms experiencing severe or prolonged stressors. Here we use data collected from a long-term study of tree swallows (Tachycineta bicolor) to test whether variation in negative feedback, or other measures of GC regulation, predict components of fitness in non-chronically stressed populations. We find relatively consistent, but generally weak relationships between different fitness components and the strength of negative feedback. Reproductive success was highest in individuals that both mounted a robust stress response and had strong negative feedback. We did not see consistent evidence of a relationship between negative feedback and adult or nestling survival: negative feedback was retained in the best supported models of nestling and adult survival, but in two of three survival-related analyses the intercept-only model received only slightly less support. Both negative feedback and stress-induced GC levels-but not baseline GCs-were individually repeatable. These measures of GC activity did not consistently covary across ages and life history stages, indicating that they are independently regulated. Overall, the patterns seen here are consistent with the predictions that negative feedback-and the dynamic regulation of GCs-are important components of stress coping capacity, but that the fitness benefits of having strong negative feedback during the reproductive period are likely to manifest primarily in individuals exposed to chronic or repeated stressors.

Glucocorticoids and "Stress" Are Not Synonymous

Reference to glucocorticoids as “stress hormones” has been growing in prevalence in the literature, including in comparative and environmental endocrinology. Although glucocorticoids are elevated in response to a variety of stressors in vertebrate animals, the primary functions of glucocorticoids are not responding to stressors and they are only one component of complex suite of physiological and behavioral responses to stressors. Thus, the use of the short-hand phrase “stress hormone” can be misleading. Further, simply measuring glucocorticoids is not equivalent to measuring a stress response, nor is manipulating glucocorticoids equivalent to exposing an animal to a stressor. In this commentary we highlight the problems with using functional names for hormones, and of treating cortisol or corticosterone as synonymous with stress. We provide recommendations to add clarity to the presentation of research on this topic, and to avoid conflation of glucocorticoids with stressors and the stress response in the design of experiments.

Cell signaling dependence of rapid glucocorticoid-induced endocannabinoid synthesis in hypothalamic neuroendocrine cells

Glucocorticoids induce a rapid synthesis of endocannabinoid in hypothalamic neuroendocrine cells by activation of a putative membrane receptor. Somato-dendritically released endocannabinoid acts as a retrograde messenger to suppress excitatory synaptic inputs to corticotropin-releasing hormone-, oxytocin-, and vasopressin-secreting cells. The non-genomic signaling mechanism responsible for rapid endocannabinoid synthesis by glucocorticoids has yet to be fully characterized. Here we manipulated cell signaling molecules pharmacologically using an intracellular approach to elucidate the signaling pathway activated by the membrane glucocorticoid receptor in hypothalamic neuroendocrine cells. We found that rapid glucocorticoid-induced endocannabinoid synthesis in magnocellular neuroendocrine cells requires the sequential activation of multiple kinases, phospholipase C, and intracellular calcium mobilization. While there remain gaps in our understanding, our findings reveal many of the critical players in the rapid glucocorticoid signaling that culminates in the retrograde endocannabinoid modulation of excitatory synaptic transmission.

The role of glucocorticoids in the vertebrate response to weather

Changes in the environment related to inclement weather can threaten survival and reproductive success both through direct adverse exposure and indirectly by decreasing food availability. Glucocorticoids, released during activation of the hypothalamic-pituitary-adrenal axis as part of the stress response, are an important candidate for linking vertebrate coping mechanisms to weather. This review attempts to determine if there is a consensus response of glucocorticoids to exposure to weather-related stimuli, including food availability, precipitation, temperature and barometric pressure. The included studies cover field and laboratory studies for all vertebrate taxa, and are separated into four exposure periods, e.g., hours, days, weeks and months. Each reported result was assigned a score based on the glucocorticoid response, e.g., increased, no change, or decreased. Short-term exposure to weather-related stimuli, of up to 24 h, is generally associated with increased glucocorticoids (79% of studies), suggesting that these stimuli are perceived as stressors by most animals. In contrast, the pattern for exposures longer than 24 h shows more variation, even though a majority of studies still report an increase (64%). Lack of glucocorticoid increases appeared to result from instances where: (1) prolonged exposure was a predictable part of the life history of an animal; (2) environmental context was important for the ultimate effect of a stimulus (e.g., precipitation limited food availability in one environment, but increased food in another); (3) prolonged exposure induced chronic stress; and (4) long-term responses appeared to reflect adaptations to seasonal shifts, instead of to short-term weather. However, there is a strong bias towards studies in domesticated laboratory species and wild animals held in captivity, indicating a need for field studies, especially in reptiles and amphibians. In conclusion, the accumulated literature supports the hypothesis that glucocorticoids can serve as the physiological mechanism promoting fitness during inclement weather.

The next step for stress research in primates: To identify relationships between glucocorticoid secretion and fitness

Glucocorticoids are hormones that mediate the energetic demands that accompany environmental challenges. It is therefore not surprising that these metabolic hormones have come to dominate endocrine research on the health and fitness of wild populations. Yet, several problems have been identified in the vertebrate research that also apply to the non-human primate research. First, glucocorticoids should not be used as a proxy for fitness (unless a link has previously been established between glucocorticoids and fitness for a particular population). Second, stress research in behavioral ecology has been overly focused on "chronic stress" despite little evidence that chronic stress hampers fitness in wild animals. Third, research effort has been disproportionately focused on the causes of glucocorticoid variation rather than the fitness consequences. With these problems in mind, we have three objectives for this review. We describe the conceptual framework behind the "stress concept", emphasizing that high glucocorticoids do not necessarily indicate a stress response, and that a stress response does not necessarily indicate an animal is in poor health. Then, we conduct a comprehensive review of all studies on "stress" in wild primates, including any study that examined environmental factors, the stress response, and/or fitness (or proxies for fitness). Remarkably, not a single primate study establishes a connection between all three. Finally, we provide several recommendations for future research in the field of primate behavioral endocrinology, primarily the need to move beyond identifying the factors that cause glucocorticoid secretion to additionally focus on the relationship between glucocorticoids and fitness. We believe that this is an important next step for research on stress physiology in primates.

Effects of etomidate in the adrenal and cytokine responses to hemorrhagic shock in rats

Hemorrhagic shock (HS) induces a compensatory endocrine and cytokine response which aims to restore homeostasis. This response can be modulated by general anesthetics. To our knowledge, no studies have evaluated if etomidate modulates this response in experimental HS. After being premedicated with buprenorphine (0.05 mg/kg subcutaneously), male Wistar rats were anaesthetized with 5% isoflurane and divided into three groups: G1 (control, n = 16), G2 (n = 13), and G3 (n = 14). G2 and G3 were subjected to HS by collecting 30% of their blood volume and resuscitated 90 min later with the collected blood and normal saline, in a 1:3 ratio, respectively. G3 received etomidate (1 mg/kg IV) before HS. Blood gas analysis, adrenocorticotropic hormone (ACTH), corticosterone, and plasma levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-10 and of TNF-α, IL-6, and IL-10 mRNA obtained through real-time polymerase chain reaction (RT-PCR) were measured at 0, 90, 150, and 240 min after HS induction. Compared with G2, etomidate-treated animals had significantly lower corticosterone, PO2, PO2/FiO2, base excess and HCO3, and higher TNF-α, IL-6, IL-10, and TNF-α mRNA levels ( P <0.05). Etomidate-treated rats showed impaired adrenal and increased cytokine response to HS and evidence of worse tissue oxygenation and lung dysfunction. Based on these results, and until further studies are performed to confirm if these findings occur in clinical patients, we suggest that etomidate should be used cautiously in HS.

Endocannabinoid Regulation of Neuroendocrine Systems

The hypothalamus is a part of the brain that is critical for sustaining life through its homeostatic control and integrative regulation of the autonomic nervous system and neuroendocrine systems. Neuroendocrine function in mammals is mediated mainly through the control of pituitary hormone secretion by diverse neuroendocrine cell groups in the hypothalamus. Cannabinoid receptors are expressed throughout the hypothalamus, and endocannabinoids have been found to exert pronounced regulatory effects on neuroendocrine function via modulation of the outputs of several neuroendocrine systems. Here, we review the physiological regulation of neuroendocrine function by endocannabinoids, focusing on the role of endocannabinoids in the neuroendocrine regulation of the stress response, food intake, fluid homeostasis, and reproductive function. Cannabis sativa (marijuana) has a long history of recreational and/or medicinal use dating back to ancient times. It was used as an analgesic, anesthetic, and antianxiety herb as early as 2600 B.C. The hedonic, anxiolytic, and mood-elevating properties of cannabis have also been cited in ancient records from different cultures. However, it was not until 1964 that the psychoactive constituent of cannabis, Δ(9)-tetrahydrocannabinol, was isolated and its chemical structure determined (Gaoni & Mechoulam, 1964).

Septic serum induces glucocorticoid resistance and modifies the expression of glucocorticoid isoforms receptors: a prospective cohort study and in vitro experimental assay

Background

A protective role for glucocorticoid therapy in animal models of sepsis was shown many decades ago. In human sepsis, there is new interest in glucocorticoid therapy at a physiological dose after reports of improved response to vasopressor drugs and decreased mortality in a selected group of patients. However, other reports have not confirmed these results. Cellular glucocorticoid resistance could explain a possible cause of that. To evaluate this hypothesis, we evaluated the expression of glucocorticoid receptor beta, the dominant negative isoform of glucocorticoid receptor, in peripheral mononuclear cells of septic patients and the effect of serum septic patients over glucocorticoid receptor expression and glucocorticoid sensitivity in immune cells culture.

Methods

A prospective cohort study and an in vitro experimental study with matched controls were developed. Nine patients with septic shock and nine healthy controls were prospectively enrolled. Mononuclear cells and serum samples were obtained from the patients with sepsis on admission to the Intensive Care Unit and on the day of discharge from hospital, and from healthy volunteers matched by age and sex with the patients. Glucocorticoid receptor alpha and beta expression from patients and from immune cell lines cultured in the presence of serum from septic patients were studied by western blot. Glucocorticoid sensitivity was studied in control mononuclear cells cultured in the presence of serum from normal or septic patients. A statistical analysis was performed using a Mann-Whitney test for non-parametric data and analysis of variance for multiple comparison; P < 0.05 was considered significant.

Results

The patients' glucocorticoid receptor beta expression was significantly higher on admission than on discharge, whereas the alpha receptor was not significantly different. In vitro, septic serum induced increased expression of both receptors in T and B cells in culture, with a greater effect on receptor beta than the control serum. Septic serum induced glucocorticoid resistance in control mononuclear cells.

Conclusion

There is a transient increased expression of glucocorticoid receptor beta in mononuclear cells from septic patients. Serum from septic patients induces cell glucocorticoid resistance in vitro. Our findings support a possible cell glucocorticoid resistance in sepsis.

Fecal cortisol metabolite levels in free-ranging North American red squirrels: Assay validation and the effects of reproductive condition

Patterns in stress hormone (glucocorticoid: GC) levels and their relationship to reproductive condition in natural populations are rarely investigated. In this study, we (1) validate an enzyme-immunoassay to measure fecal cortisol metabolite (FCM) levels in North American red squirrels (Tamiasciurus hudsonicus), and (2) examine relationships between FCM levels and reproductive condition in a free-ranging red squirrel population. Injected radiolabeled cortisol was entirely metabolized and excreted in both the urine (mean+/-SE; 70.3+/-0.02%) and feces (29.7+/-0.02%), with a lag time to peak excretion in the feces of 10.9+/-2.3h. Our antibody reacted with several cortisol metabolites, and an adrenocorticotropic injection significantly increased FCM levels above baseline levels at 8h post-injection. Relative to baseline levels, manipulation by handling also tended to increase FCM levels at 8h post-manipulation, but this difference was not significant. FCM levels did not differ significantly between samples frozen immediately and 5h after collection. Reproductive condition significantly affected FCM levels in free-ranging females (pregnant>lactating>post-lactating>non-breeding) but not males (scrotal testes vs. abdominal testes). Among females with known parturition dates, FCM levels increased during gestation, peaked at parturition, and declined during lactation. The difference between pregnant and lactating females was therefore dependent upon when the fecal samples were obtained during these periods, suggesting caution in categorizing reproductive stages. This study demonstrates the utility of fecal hormone metabolite assays to document patterns of glucocorticoid levels in free-ranging animals.

Glucocorticoids regulate glutamate and GABA synapse-specific retrograde transmission via divergent nongenomic signaling pathways

Glucocorticoids exert an opposing rapid regulation of glutamate and GABA synaptic inputs to hypothalamic magnocellular neurons via the activation of postsynaptic membrane-associated receptors and the release of retrograde messengers. Glucocorticoids suppress synaptic glutamate release via the retrograde release of endocannabinoids and facilitate synaptic GABA release via an unknown retrograde messenger. Here, we show that the glucocorticoid facilitation of GABA inputs is due to the retrograde release of neuronal nitric oxide and that glucocorticoid-induced endocannabinoid synthesis and nitric oxide synthesis are mediated by divergent G-protein signaling mechanisms. While the glucocorticoid-induced, endocannabinoid-mediated suppression of glutamate release is dependent on activation of the G(alpha)s G-protein subunit and cAMP-cAMP-dependent protein kinase activation, the nitric oxide facilitation of GABA release is mediated by G(beta)gamma signaling that leads to activation of neuronal nitric oxide synthase. Our findings indicate, therefore, that glucocorticoids exert opposing rapid actions on glutamate and GABA release by activating divergent G-protein signaling pathways that trigger the synthesis of, and glutamate and GABA synapse-specific retrograde actions of, endocannabinoids and nitric oxide, respectively. The simultaneous rapid stimulation of nitric oxide and endocannabinoid synthesis by glucocorticoids has important implications for the impact of stress on the brain as well as on neural-immune interactions in the hypothalamus.

Preoperative glucocorticoid administration attenuates the systemic stress response and hyperglycemia after surgical trauma in the rat

The stress response to surgery is characterized by activation of the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system, and by an inflammatory response and hyperglycemia. The aim of the present study was to investigate if preoperative corticosterone could reduce the postoperative systemic stress response, without aggravating hyperglycemia or interfering with activation of the hypothalamic-pituitary-adrenal axis, in a standardized rat model of surgical trauma. We used a standardized experimental model of intestinal resection in the rat. Exogenous corticosterone (8 mg/kg body weight) or vehicle was administered 2 hours before surgery; and postoperative plasma concentrations of interleukin-6, interleukin-10, adrenaline, noradrenaline, glucose, and insulin were determined. Exogenous corticosterone decreased preoperative plasma adrenaline but did not change plasma glucose or insulin levels. Moreover, corticosterone reduced postoperative plasma interleukin-6, catecholamines, and glucose (all P < .001-.05) without any effect on the plasma corticosterone concentration compared with vehicle-treated controls. A preoperative 2-hour exposure of physiologic poststress corticosterone concentrations not only suppressed plasma IL-6 levels but also inhibited surgery-induced adrenaline release and suppressed plasma glucose levels. We hypothesize that glucocorticoids attenuated the inflammatory response in injured tissues that reduced afferent input into brain areas regulating the neuroendocrine response.

An experimental model of intestinal resection and compensated non-hypotensive blood loss

BACKGROUND

Massive hemorrhage results in hypovolemia and hypotension that activate the sympathetic-adrenal-system, the hypothalamic-pituitary-adrenal axis, and induce metabolic changes such as hyperglycemia. Blood loss during surgery, however, is rapidly compensated by intravenous fluids to minimize negative impact on circulation. The aim of this study was to develop an experimental model for studies of the stress response and metabolic alterations seen after major abdominal surgery by combining intestinal resection with compensated non-hypotensive blood loss.

MATERIAL AND METHODS

Intestinal resection in the rat was combined blood loss (7% and 16% of total blood volume) with subsequent fluid replacement (2 x volume of saline + hydroxyethyl starch, 50:50) to investigate the effects on blood pressure, plasma adrenaline, noradrenaline (0-2 h postoperatively), glucose, insulin, corticosterone, and hematocrit (0-24 h postoperatively).

RESULTS

Surgical trauma alone or in combination with blood loss and fluid replacement did not decrease mean arterial blood pressure (MAP) compared with anesthetized controls. Plasma levels of catecholamines were unchanged during the period of blood loss. After surgery and blood loss, plasma glucose showed a blood loss volume-dependent increase up to 12 h postoperatively compared to surgery alone (P < 0.001-0.05). Plasma corticosterone increased after surgery alone and in combination with blood loss but the changes were not dependent on the magnitude of blood loss.

CONCLUSIONS

Surgical trauma combined with compensated non-hypotensive blood loss induced persistent volume-dependent hyperglycemia that did not depend on changes in plasma corticosterone and, in the early postoperative phase, MAP and catecholamines. Our data indicate that blood loss per se, without causing hypotension or acute elevations in catecholamines, can induce marked postoperative hyperglycemia. The experimental model presented in this report will be a useful tool to further elucidate mechanisms underlying the changes in glucose metabolism seen after surgical injury.

Recommendations for the diagnosis and management of corticosteroid insufficiency in critically ill adult patients: consensus statements from an international task force by the American College of Critical Care Medicine

OBJECTIVE

To develop consensus statements for the diagnosis and management of corticosteroid insufficiency in critically ill adult patients.

PARTICIPANTS

A multidisciplinary, multispecialty task force of experts in critical care medicine was convened from the membership of the Society of Critical Care Medicine and the European Society of Intensive Care Medicine. In addition, international experts in endocrinology were invited to participate.

DESIGN/METHODS

The task force members reviewed published literature and provided expert opinion from which the consensus was derived. The consensus statements were developed using a modified Delphi methodology. The strength of each recommendation was quantified using the Modified GRADE system, which classifies recommendations as strong (grade 1) or weak (grade 2) and the quality of evidence as high (grade A), moderate (grade B), or low (grade C) based on factors that include the study design, the consistency of the results, and the directness of the evidence.

RESULTS

The task force coined the term critical illness-related corticosteroid insufficiency to describe the dysfunction of the hypothalamic-pituitary-adrenal axis that occurs during critical illness. Critical illness-related corticosteroid insufficiency is caused by adrenal insufficiency together with tissue corticosteroid resistance and is characterized by an exaggerated and protracted proinflammatory response. Critical illness-related corticosteroid insufficiency should be suspected in hypotensive patients who have responded poorly to fluids and vasopressor agents, particularly in the setting of sepsis. At this time, the diagnosis of tissue corticosteroid resistance remains problematic. Adrenal insufficiency in critically ill patients is best made by a delta total serum cortisol of < 9 microg/dL after adrenocorticotrophic hormone (250 microg) administration or a random total cortisol of < 10 microg/dL. The benefit of treatment with glucocorticoids at this time seems to be limited to patients with vasopressor-dependent septic shock and patients with early severe acute respiratory distress syndrome (PaO2/FiO2 of < 200 and within 14 days of onset). The adrenocorticotrophic hormone stimulation test should not be used to identify those patients with septic shock or acute respiratory distress syndrome who should receive glucocorticoids. Hydrocortisone in a dose of 200 mg/day in four divided doses or as a continuous infusion in a dose of 240 mg/day (10 mg/hr) for > or = 7 days is recommended for septic shock. Methylprednisolone in a dose of 1 mg x kg(-1) x day(-1) for > or = 14 days is recommended in patients with severe early acute respiratory distress syndrome. Glucocorticoids should be weaned and not stopped abruptly. Reinstitution of treatment should be considered with recurrence of signs of sepsis, hypotension, or worsening oxygenation. Dexamethasone is not recommended to treat critical illness-related corticosteroid insufficiency. The role of glucocorticoids in the management of patients with community-acquired pneumonia, liver failure, pancreatitis, those undergoing cardiac surgery, and other groups of critically ill patients requires further investigation.

CONCLUSION

Evidence-linked consensus statements with regard to the diagnosis and management of corticosteroid deficiency in critically ill patients have been developed by a multidisciplinary, multispecialty task force.

Seasonal glucocorticoid responses to capture in wild free-living mammals

We determined baseline and capture-induced glucocorticoid concentrations during two different seasons in three species of wild free-living rodents: brown lemmings ( Lemmus trimucronatus), golden-mantled ground squirrels ( Spermophilus saturatus), and yellow-pine chipmunks ( Tamias amoenus). Initial blood samples were obtained within 3 min of capture, so that initial glucocorticoid levels reflect baseline titers of undisturbed animals. Animals were held for an additional 30 min, when a second blood sample was taken to measure stress-induced glucocorticoid titers. The primary glucocorticoid differed in each species. Lemmings secreted extremely large amounts of corticosterone (as high as 8,000 ng/ml). These high concentrations were accompanied by high corticosterone-binding globulin capacity and resistance to negative feedback. Squirrels and chipmunks secreted a mixture of cortisol and corticosterone (10–400 ng/ml). In males of all three species and female squirrels and chipmunks, glucocorticoid levels were significantly elevated 30 min after capture. Baseline and 30-min glucocorticoid levels differed seasonally in each species. Levels were higher during summer (with no snow cover) than in spring (with ∼60% snow cover) in female lemmings, higher during breeding than before hibernation in squirrels, and higher postreproductively than during breeding in chipmunks. Together, these data indicate that glucocorticoid responses to stress in these free-living species are similar to those in laboratory species, but the magnitude of the response appears to depend on life-history features specific to each species.

Rapid glucocorticoid actions in the hypothalamus as a mechanism of homeostatic integration

The hypothalamic paraventricular nucleus (PVN) is a major integrative site for the control of homeostasis, including energy balance, through coordinated regulation of neuroendocrine and autonomic outputs. However, cross-talk regulation of PVN neuroendocrine and preautonomic systems is poorly understood. The stress response invokes the coordinated control of motor, hormonal, and vegetative systems to establish homeostasis after an environmental perturbation. Elevated stress levels of circulating glucocorticoids give rise to multiple, complex physiological effects. The complexity of the glucocorticoid actions is caused by the wide range of glucocorticoid target tissues and to the broad time scale over which the actions occur. Recent studies have revealed rapid glucocorticoid actions in the hypothalamus that may provide an integrative signal linking stress with the regulation of energy and fluid homeostasis. Glucocorticoids inhibit PVN and supraoptic nucleus neurons by stimulating a rapid synthesis and retrograde release of endocannabinoids, which suppress synaptic excitation through presynaptic CB1 receptor activation. The glucocorticoid-induced endocannabinoid synthesis is mediated apparently by a novel membrane-associated glucocorticoid receptor found in multiple subpopulations of hypothalamic neuroendocrine cells. It may, therefore, represent a mechanism for rapid glucocorticoid control of activity among different neuroendocrine systems to coordinate a global response to stress. In support of this, leptin, a circulating adipose signal that regulates food intake and energy expenditure through central actions, blocks the glucocorticoid-mediated endocannabinoid release in the PVN. This represents a means by which the regulation of stress and feeding may interface in the PVN, thus providing a possible mechanism for the integration of multiple homeostatic functions.

Inosine infusion prevents mortality in endotoxic shock

BACKGROUND

Substances that inhibit Na/K ATPase activity appear in plasma during severe septic shock causing Na and fluid to move into cells and K to move out, resulting in cell swelling and an elevation of plasma K. These changes contribute to the morbidity of sepsis. Recently, we reported that inosine and other purine nucleosides stimulate Na/K ATPase activity, prolong survival in hemorrhagic shock, and lower the plasma potassium in that condition. Here, we determine whether inosine prolongs survival in lipopolysaccharide-induced sepsis shock.

METHODS

Pentobarbital-anesthetized rats underwent cannulation of a femoral artery and vein, and lipopolysaccharide was injected by intravenous bolus (10 mg/kg). Rats were than resuscitated (5 mL/hr) with inosine (5 mmol/L) in saline, saline alone, inosine with S-4-nitrobenzyl-6-thioinosine (NBTI, 10 micromol/L, an equilibrative nucleoside transporter blocker), NBTI alone, or no resuscitation.

RESULTS

Inosine significantly and dramatically prolongs survival of rats in endotoxic shock as compared with saline resuscitation or to no resuscitation. Furthermore, resuscitation with NBTI (10 micromol/L) prevented prolonged survival with inosine.

CONCLUSION

Inosine prevents mortality in lipopolysaccharide-induced septic shock in rats. The mechanism of action must be intracellular, as blockers of the equilibrative nucleoside transporter prevented prolonged survival with inosine.

Corticosterone inhibits feather growth: Potential mechanism explaining seasonal down regulation of corticosterone during molt

Corticosterone (CORT) is seasonally modulated in many passerines, with plasma CORT concentrations lowest during the prebasic molt when all feathers are replaced. To explain why, we proposed that the birds downregulate natural CORT release during molt in order to avoid CORT's degradative effects on proteins and its inhibition of protein synthesis. If CORT exerted these effects during molt, it could slow protein deposition during feather production and potentially result in a longer period of degraded flight performance. To test this hypothesis, either empty or CORT-filled silastic implants were inserted into captive European starlings (Sturnus vulgaris) and white-crowned sparrows (Zonotrichia leucophrys) undergoing induced (feather replacement after plucking) and natural molts. We then measured the rate of feather re-growth by regularly measuring the length of primary, secondary, and tail feathers. CORT implanted birds showed a significantly decreased rate of feather growth compared to control animals. Basal CORT concentrations of induced molt and non-molting birds were also compared but no difference was noted. The results suggest a tradeoff; a complete set of new feathers may be more important to the survival of a bird than the ability of CORT to respond maximally to a stressor.

The hepatoadrenal syndrome: a common yet unrecognized clinical condition

OBJECTIVE

Adrenal failure is common in critically ill patients, particularly those with sepsis. As liver failure and sepsis are both associated with increased circulating levels of endotoxin and proinflammatory mediators and reduced levels of apoprotein-1/high-density lipoprotein, we postulated that adrenal failure may be common in patients with liver disease.

DESIGN

Clinical study.

SETTING

Liver transplant intensive care unit.

PATIENTS

The study cohort included 340 patients with liver disease.

INTERVENTIONS

Based on preliminary observational data, all patients admitted to our 28-bed liver transplant intensive care unit (LTICU) undergo adrenal function testing. An honest broker system was used to extract clinical, hemodynamic, medication, and laboratory data on patients admitted to the LTICU from March 2002 to March 2004. A random (stress) cortisol level <20 microg/dL in a highly stressed patient (respiratory failure, hypotension) was used to diagnose adrenal insufficiency. In all other patients, a random cortisol level <15 microg/dL or a 30-min level <20 microg/dL post-low-dose (1 microg) cosyntropin was considered diagnostic of adrenal insufficiency. Patients were grouped as follows: a) chronic liver failure; b) fulminant hepatic failure; c) patients immediately status post-orthotopic liver transplantation receiving a steroid-free protocol of immunosuppression; and d) patients status post-remote liver transplant (>/=6 months). The decision to treat patients with stress doses of hydrocortisone was at the discretion of the treating intensivist and transplant surgeon.

MEASUREMENTS AND MAIN RESULTS

Two-hundred and forty-five (72%) patients met our criteria for adrenal insufficiency (the hepatoadrenal syndrome). Eight (33%) patients with fulminant hepatic failure, 97 (66%) patients with chronic liver disease, 31(61%) patients with a remote history of liver transplantation, and 109 (92%) patients who had undergone liver transplantation under steroid-free immunosuppression were diagnosed with adrenal insufficiency. The high-density lipoprotein level at the time of adrenal testing was the only variable predictive of adrenal insufficiency (p < .0001). In vasopressor-dependent patients with adrenal insufficiency, treatment with hydrocortisone was associated with a significant reduction (p = .02) in the dose of norepinephrine at 24 hrs, whereas the dose of norepinephrine was significantly higher (p = .04) in those patients with adrenal failure not treated with hydrocortisone. In vasopressor-dependent patients without adrenal insufficiency, treatment with hydrocortisone did not affect vasopressor dose at 24 hrs. One hundred and forty-one patients (26.4%) died during their hospitalization. The baseline serum cortisol was 18.8 +/- 16.2 microg/dL in the nonsurvivors compared with 13.0 +/- 11.8 microg/dL in the survivors (p < .001). Of those patients with adrenal failure who were treated with glucocorticoids, the mortality rate was 26% compared with 46% (p = .002) in those who were not treated. In those patients receiving vasopressor agents at the time of adrenal testing, the baseline cortisol was 10.0 +/- 4.8 microg/dL in those with adrenal insufficiency compared with 35.6 +/- 21.2 microg/dL in those with normal adrenal function. Vasopressor-dependent patients who did not have adrenal failure had a mortality rate of 75%.

CONCLUSIONS

Patients with liver failure and patients post-liver transplantation have an exceedingly high incidence of adrenal failure, which may be pathophysiologically related to low levels of high-density lipoprotein. Treatment of patients with adrenal failure may improve outcome. High baseline serum cortisol levels may be a maker of disease severity and portend a poor prognosis.

Purine nucleosides stimulate Na/K ATPase, and prolong survival in hemorrhagic shock

BACKGROUND

Hemorrhagic shock leads to the appearance of substances in plasma that depress Na/K ATPase activity leading to a rise in plasma potassium. Recently, we reported that adenosine can stimulate Na/K ATPase activity, lower the plasma potassium back to control and prolong survival in shocked rats. However, adenosine also caused bradycardia. We therefore searched for adenosine analogs that stimulate Na/K ATPase without the side effects of bradycardia.

METHODS

Na/K ATPase activity was assessed using Rb uptake in erythrocytes. Pentobarbital anesthetized rats had their femoral artery and vein cannulated, bled to 35 mm Hg for 1 hour and resuscitated.

RESULTS

We found that the purine nucleosides, inosine, guanosine, adenosine, deoxyadenosine and deoxyguanosine, stimulated Na/K ATPase in a dose-dependent manner and overcame partial inhibition by ouabain. However, the de-ribosylated bases, the nucleotides and the pyrimidines had little or no effect on Na/K ATPase activity. Purine nucleosides did not stimulate Na/K ATPase activity through adenosine receptors, as caffeine (1 mmol/L) or aminophylline (1 mmol/L) did not block stimulation. However, stimulation was blocked by inhibitors of the equilibrative nucleoside transporter (dipyridamole, 1 mmol/L, or S-(4-nitrobenzyl)-6-thioinosine, 10 micromol/L), suggesting that the mechanism of action is intracellular. Inosine, guanosine and adenosine (2.5 mmol/L) significantly increased survival of rats in hemorrhagic shock as compared with saline and cytidine controls, and lowered the shock-elevated plasma K.

CONCLUSIONS

Purine nucleosides stimulate Na/K ATPase and prolong survival in hemorrhagic shock in rats, probably through an intracellular mechanism.

Glucocorticoid regulation of the inflammatory response to injury

During the first half of the 20th century, physiologists were interested in the adrenal glands primarily because adrenalectomized animals failed to survive even mild degrees of systemic stress. It eventually became clear that hormones secreted by the adrenal cortex were critical for survival and, in this context, adrenal cortical hormones were widely considered to support or stimulate important responses to stress or injury. With the purification and manufacture of adrenal cortical hormones in the 1930s and 1940s, clinicians suddenly discovered the potent anti-inflammatory actions of glucocorticoids (GCs). This dramatic, and unexpected, discovery has dominated clinical and laboratory research into GC actions throughout the second half of the 20th century. More recent research is again reporting GC-induced stimulatory effects on a variety of inflammatory response components. These effects are usually observed at low GC concentrations, close to concentrations that are observed in vivo during basal, unstimulated states. For example, GC-mediated stimulation has been reported for the hepatic acute-phase response, for cytokine secretion, expression of cytokine/chemokine receptors, and for the pro-inflammatory mediator, macrophage migration inhibition factor. It seems clear that the long-held clinical view that GCs act solely as anti-inflammatory agents needs to be re-assessed. Varying doses of GCs do not lead simply to varying degrees of inflammation suppression, but rather GCs can exert a full range of effects from permissive to stimulatory to suppressive.

Reduced blood-to-tissue albumin movement after plasmapheresis

We tested the hypothesis that a decrease in the blood-to-tissue movement of albumin contributes to the recovery of plasma albumin and plasma volume after acute plasma protein depletion (plasmapheresis). Awake and unrestrained male Sprague-Dawley rats (220-320 g) fitted with jugular catheters were plasmapheresed, and plasma volume, plasma albumin, and total plasma protein content were measured at 1, 5, 24, and 48 h postplasmapheresis. Plasma volume recovered to baseline within 1 h (4.6 +/- 0.42 vs. 4.7 +/- 0.46 mL/100 g body weight (bw), remained at baseline from 5 h to 24 h but increased to 5.5 + 0.57 mL/100 g bw at 48 h (P < 0.05). Plasma albumin and total protein content recovered rapidly but remained below baseline levels at 1 h (10.05 +/- 0.98 vs. 12.33 +/- 1.29 and 19.75 +/- 1.75 vs. 24.73 +/- 2.56 mg/100 g bw, respectively). Plasma protein content retumed to baseline by 5 h of recovery. Tissue uptake of I125-labeled albumin decreased in the heart, skin, skeletal muscle, and small Intestines of plasmapheresed rats (P < 0.05). These data support the hypothesis that a reduction in albumin efflux from the vascular space contrlbutes to the recovery of plasma albumin and total protein content during plasma volume recovery and eventual expansion after plasmapheresis.

Adrenal insufficiency in the critically ill: a new look at an old problem

Stress from many sources, including pain, fever, and hypotension, activates the hypothalamic-pituitary-adrenal (HPA) axis with the sustained secretion of corticotropin and cortisol. Increased glucocorticoid action is an essential component of the stress response, and even minor degrees of adrenal insufficiency can be fatal in the stressed host. HPA dysfunction is a common and underdiagnosed disorder in the critically ill. We review the risk factors, pathophysiology, diagnostic approach, and management of HPA dysfunction in the critically ill.

Detrimental effects of rapid fluid resuscitation on hepatocellular function and survival after hemorrhagic shock

Because end-organ injury can occur with reperfusion following hemorrhage or ischemia, we hypothesized that aggressive intravenous fluid resuscitation would aggravate tissue injury in a fixed-volume model of hemorrhagic shock. Unanesthetized chronically prepared male rats were hemorrhaged 33-36 mL/kg for 2.5 h. Then Lactated Ringers Solution (3x hemorrhage volume) was infused over 5 min (FAST), 20 min (MEDIUM), 180 min (SLOW), or not at all (NO RESUS). Plasma ornithine carbamoyltransferase (OCT), lactate, and creatinine were measured as indices of hepatocellular injury, anaerobic metabolism, and renal function, respectively. At 1 h post-resuscitation (PR), MAP was greater after SLOW and MEDIUM treatment (tx) than after other txs (P < 0.05). OCT increased earliest after FAST tx to values greater than those after other txs from 30 min to 24 h PR (P < 0.01). Plasma lactate was elevated immediately before resuscitation in all groups (P < 0.01) and returned to baseline at 3 h PR after SLOW tx compared to 5 h PR after FAST tx (P < 0.05). Creatinine at 5 h PR was less in the groups treated with intravenous fluid compared to the NO RESUS group, P < 0.05. Survival at 72 h was reduced in the FAST (57%) and NO RESUS (58%) groups compared to the SLOW (87%) and MEDIUM (85%) groups (P < 0.05). Thus, overly aggressive fluid tx accelerates hepatocellular injury, is no better than lesser rates of resuscitation at correcting plasma lactate and preserving renal function, and provides no overall survival benefit.

Seasonal changes in plasma glucocorticoid concentrations in free-living vertebrates

The vertebrate stress response helps animals respond to environmental dangers such as predators or storms. An important component of the stress response is glucocorticoid (GC) release, resulting from activation of the hypothalamic-pituitary-adrenal axis. After release, GCs induce a variety of behavioral and physiological changes that presumably help the animal respond appropriately to the situation. Consequently, GC secretion is often considered an obligatory response to stressful situations. Evidence now indicates, however, that free-living species from many taxa can seasonally modulate GC release. In other words, the magnitudes of both unstressed and stressed GC concentrations change depending upon the time of year. This review examines the growing evidence that GC concentrations in free-living reptiles, amphibians, and birds, but not mammals, are commonly elevated during the breeding season. This evidence is then used to test three hypotheses with different focuses on GC's energetic or behavioral effects, as well as on GC's role in preparing the animal for subsequent stressors. These hypotheses attempt to place annual GC rhythms into a physiological or behavioral context. Integrating seasonal differences in GC concentrations with either different physiological states or different life history stages provides clues to a new understanding of how GCs actually help in survival during stress. Consequently, understanding seasonal modulation of GC release has far-reaching importance for both the physiology of the stress response and the short-term survival of individual animals.

Testosterone receptor blockade after trauma and hemorrhage attenuates depressed adrenal function

Although the testosterone receptor antagonist flutamide restores the depressed immune function in males after trauma and hemorrhage, it remains unknown whether this agent has any salutary effects on adrenal function. To study this, male rats underwent laparotomy and were bled to and maintained at a blood pressure of 40 mmHg until 40% of the shed blood volume was returned in the form of Ringer lactate. Animals were then resuscitated and flutamide (25 mg/kg body wt) was administered subcutaneously. Plasma adrenocorticotropic hormone (ACTH) and corticosterone, as well as adrenal corticosterone and cAMP were measured 20 h after resuscitation. In additional animals, ACTH was administered and ACTH-induced corticosterone release and adrenal cAMP were determined. The results indicate that adrenal contents of corticosterone and cAMP were significantly decreased and morphology was altered after hemorrhage. Administration of flutamide improved corticosterone content, restored cAMP content, and attenuated adrenal morphological alterations. Flutamide also improved the diminished ACTH-induced corticosterone release and adrenal cAMP response at 20 h after hemorrhage and resuscitation. Furthermore, the diminished corticosterone response to ACTH stimulation in the isolated adrenal preparation was improved with flutamide. These results suggest that flutamide is a useful adjunct for improving adrenal function in males following trauma and hemorrhage.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Blood flow, vascular resistance, and blood volume after hemorrhage in conscious adrenalectomized rat

Hemorrhage leads to cardiovascular collapse and death in adrenal-insufficient animals. To determine whether the cardiovascular collapse is due to vasodilation and/or failure to restore blood volume, we used radiolabeled microspheres and 125I-labeled albumin to measure blood flow and blood volume in conscious adrenalectomized (ADX) rats after 15 ml.kg-1.3 min-1 hemorrhage. In ADX rats, hemorrhage led to a greater fall than in sham rats in blood flow in the stomach, small intestines, cecum, colon, spleen, hepatic portal vein, kidney, testis, lung, thymus, bone, fat, forebrain, cerebellum, and brainstem. The greater fall in blood flow was caused by an increase in vascular resistance in these organs except brain and hepatic artery. Sham rats maintained or increased brain and hepatic artery blood flow after hemorrhage whereas flow decreased and remained depressed in ADX rats. ADX rats failed to restore blood volume, whereas sham rats completely restored blood flow by 2 h. We conclude that cardiovascular collapse in ADX rats does not result from vasodilatation but may result from a failure to restore blood volume. The failure to restore blood volume and the low blood flow to organs, especially brain and liver, may contribute to mortality in ADX rats after hemorrhage.

Differential glucocorticoid effects on catecholamine responses to stress

There have been relatively few studies of the relationships between glucocorticoid and catecholamine responses to stress. We have therefore determined plasma levels of adrenocorticotropic hormone (ACTH), cortisol (F), norepinephrine (NE), and epinephrine (Epi) in four intact sheep treated with cortisol, dexamethasone (DM), or saline (S) and subjected to both audiovisual (barking dog) and insulin-induced hypoglycemic stress. In control sheep, exposure to both stressors resulted in acute rises in ACTH, F, Epi, and NE, with the rises after insulin being greater than those after dog stress. Pretreatment with DM blocked the ACTH and F responses to stress. Both DM and F markedly attenuated the Epi response to hypoglycemia, whereas the rises in NE after each stress and those of Epi after dog stress were unaffected by steroid treatment. In a second experiment with six sheep treated with S or DM only and subjected to the same stressors in reverse order, the same results were obtained, excluding a confounding effect of prior stress. We conclude that 1) the poststress release of NE, presumably primarily from extra-adrenal sources, is largely steroid independent; 2) the mechanism of release of Epi from the adrenal medulla in response to audiovisual stress is different from that after hypoglycemia; and 3) this may reflect different inputs from central glucocorticoid receptors to splanchnic outflow in the two situations.

Stress update Adaptation of the hypothalamic-pituitary-adrenal axis to chronic stress

The hypothalamic-pituitary-adrenal (HPA) axis exhibits a circadian rhythm, activation by stress, and inhibition by corticosteroids. Activity in the HPA axis is very sensitive to inhibition by corticosteroids when they are administered exogenously. When stress-induced corticosteroid secretion occurs, however, normal activity in the HPA is not inhibited and may even be augmented. Experiments in rats have shown that stress also induces facilitation of subsequent activity in the HPA axis that appears to balance the inhibitory effects of corticosterone and thus maintains responsiveness to new, acute stresses in chronically stressed rats. Stress-induced facilitation of HPA axis activity may be mediated by a parallel stress-induced (CRH-dependent) increase in the capacity of brain noradrenergic cell groups to respond to acute stress. A continually responsive HPA axis, even under conditions of chronic stress, appears to be important for survival. Stress-induced increases in glucocorticoid secretion to levels sufficient to occupy glucocorticoid receptors enable appropriate thermoregulatory and cardiovascular responses to acute stress. There is, however, an overall metabolic cost to the animal of maintaining continued activity in the HPA axis during chronic stress.

The interaction between atrial natriuretic peptide and cardiac parasympathetic function

We have demonstrated previously that atrial natriuretic peptide (ANP) inhibits hypotension-induced reflex tachycardia via a parasympathetic mechanism. The present study further defines that parasympathetic mechanism. We tested the hypothesis that ANP, during vagus nerve stimulation, acts as a physiological antagonist to interfere with alpha 1-adrenoceptor modulation of efferent cardiac vagal action. Sprague Dawley rats were divided into five groups, each group receiving a different infusion. Infusates included one of vehicle (Ringer's solution; RS), an alpha 1-adrenoceptor agonist (phenylephrine; PE), a combination of agonist and either a known alpha 1-adrenoceptor antagonist (prazosin; PE+PRZ) or the putative physiologic antagonist, ANP (PE+ANP). The fifth group received all three drugs, PE+PRZ+ANP. Under Inactin anesthesia (100 mg/kg i.p.), efferent autonomic input to the heart was surgically interrupted. Animals were also adrenalectomized to limit the effects of circulating catecholamines. We then monitored each group for the change in heart rate (delta HR) in response to efferent vagus nerve stimulation at various frequencies (2 Hz, 5 Hz, 10 Hz). Infusion of PE significantly (P < 0.01 by ANOVA) attenuated the magnitude of delta HR when compared to the RS group. This attenuation of vagally-induced bradycardia was eliminated by the addition of the alpha 1-adrenoceptor antagonist, prazosin (PE+PRZ group). The PE+ANP group responded with results similar to those of the PE+PRZ group. There was no difference between delta HR responses of the PE+PRZ+ANP group and the PE+PRZ group.(ABSTRACT TRUNCATED AT 250 WORDS)