Hormonal and hemodynamic responses to 15 ml/kg hemorrhage in conscious dogs: responses correlate to body temperature

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

Repeated hypotension induced by nitroprusside and haemorrhage in sheep: effects on vasopressin release and recovery of arterial blood pressure

The arginine vasopressin (AVP) release in response to repeated hypotension caused by intravenous (i.v.) infusion of sodium nitroprusside (SNP) or haemorrhage was studied in conscious euhydrated sheep. Parallel determinations of renal excretion and plasma concentration of AVP were made in experiments involving two consecutive 10-min i.v. infusions of SNP (about 35 micrograms kg-1 min-1) with a 3-h interval between and repeated the next day. The AVP response to the second SNP administration was significantly reduced, but partial recovery was observed in response to the initial infusion the next day. Maximal fall in mean arterial blood pressure (MABP) and its recovery pattern did not differ in response to any of the four SNP infusions. In contrast, impaired recovery of the MABP together with markedly reduced AVP response was seen as a consequence of a hypotensive haemorrhage repeated after 3 h, but not when the interval between haemorrhages was extended to 24 h. The haemorrhage-induced increase in plasma renin activity was not affected by variations in the interval between experiments. It is concluded that the massive AVP liberation normally seen as an effect of acute isovolaemic hypotension becomes markedly reduced upon a renewed fall in the MABP occurring within 3 h. An iteration of hypotensive haemorrhage accentuates this fatigue of the hormonal response, which may contribute to the impaired recovery of the MABP.

Hypertonic saline attenuates the hormonal response to injury

We reported previously in a randomized double-blinded study in 20 postoperative coronary bypass patients that hypertonic saline (1.8% NaCl, HS) provides early hemodynamic benefits, increased osmolality and net negative fluid balance compared to 0.9% NaCl (NS). To investigate the effects of HS on the hormonal response to injury, we measured ACTH, cortisol, angiotensin II (AII), aldosterone, vasopressin (AVP), and atrial natriuretic factor (ANF) in these patients. ACTH and cortisol concentrations increased in the NS group but were suppressed in the HS group (p less than 0.05). Aldosterone increased in NS patients, but was suppressed in HS patients (HS: delta Aldosterone 13.0 +/- 3.0 vs. NS: delta Aldosterone 26.0 +/- 7.0 ng/dl, p less than 0.05). The AII response was suppressed at six and eight hours (p less than 0.05) in patients receiving HS but did not change in patients receiving NS. ANF did not change significantly for either group. The significant increases in AVP were similar in both groups (p less than 0.05), but correlated with increases in osmolality only in the NS group (r = 0.8, p less than 0.009). Other than AVP, HS suppressed the responses of some of the hormones that normally increase in response to injury, relative to NS. Attenuation of the neuroendocrine response and other previously reported effects of HS suggest that HS may be an efficacious solution for resuscitation in the postoperative and postinjury period.

Effects of catecholamines on secretion of adrenocorticotrophic hormone (ACTH) in man

The hypothalamus receives a rich supply of adrenergic and noradrenergic nerve fibres from the brain stem, terminating in many hypothalamic regions, including the paraventricular nucleus, which is the site of the cell bodies of corticotrophin releasing factor (CRF) neurones in man. Experimental evidence has shown that an alpha 1 adrenoceptor mechanism stimulates adrenocorticotrophic hormone (ACTH) secretion in man. The site of action of this mechanism seems to be within the blood brain barrier, presumably modulating the secretion of the CRF complex. This mechanism is important in the control of ACTH secretion in some physiological conditions in healthy subjects.

Cortisol-induced inhibition of ovine renin and aldosterone responses to hypotension

Previous studies from this laboratory have demonstrated that in preterm fetal sheep increases in plasma cortisol (F) concentration equal in amplitude to fetal F stress responses suppress plasma renin activity (PRA). The purpose of this study was to investigate the possibility that this negative interaction exists in adult sheep. Five conscious ewes with chronically prepared carotid arterial loops were infused intravenously with F (6 micrograms X kg-1 X min-1) or vehicle (5% ethanol in 0.9% saline) for 5 h. One hour after the end of F or vehicle infusion, renin secretion was stimulated by hypotension produced by infusion of sodium nitroprusside (20 micrograms X kg-1 X min-1, iv). F infusion increased plasma F from 26 +/- 3 to 46 +/- 8 ng/ml; during vehicle infusion plasma F did not change from 20 +/- 4 ng/ml. F infusion decreased hematocrit from 29 +/- 2 to 26 +/- 1%. Basal PRA in vehicle- and F-infused groups were 0.4 +/- 0 and 0.2 +/- 0.1 ng angiotensin I X ml-1 X h-1 and did not change. In vehicle-infused ewes, PRA increased from 0.4 +/- 0 and 0.2 +/- 0.1 ng angiotensin I X ml-1 X h-1 and did not change. In vehicle-infused ewes, PRA increased from 0.4 +/- 0 to 4.6 +/- 0.4 and plasma aldosterone from 26.0 +/- 1.0 to 173.1 +/- 21.8 pg/ml, while, in F-infused ewes, PRA increased from 0.2 +/- 1 to 3.3 +/- 0.4 ng angiotensin I X ml-1 X h-1 and aldosterone from 25.0 +/- 0 to 48.2 +/- 23.2 pg/ml, significantly smaller responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin release in response to acute hypotension induced at different time intervals in the conscious sheep

The renal arginine vasopressin (AVP) excretion in response to acute systemic hypotension induced by intravenous infusion of sodium nitroprusside (SNP) (30-40 micrograms/kg min-1) at different experiment intervals (0, 2, 4, 7 and greater than or equal to 12 days) was studied in the conscious hyperhydrated sheep. During the first post-infusion hour, 2.5 times more AVP was excreted in response to hypotension induced at greater than or equal to 12 day intervals than that observed at intervals of 0-7 days. No interexperimental time dependence of the AVP response to SNP infusion was seen with intervals of 0-7 days. The attenuated AVP release obtained with reduced experiment intervals (0-7 days) was accompanied by shorter antidiuresis and a less accentuated natriuresis during the post-hypotensive period in comparison to what was observed with greater than or equal to 12 day experiment intervals. There were no interval-dependent differences in maximal fall of mean arterial pressure, or onset and recovery of the hypotension induced by SNP administration. It is suggested that acute systemic hypotension causes such a massive AVP release that more than one week is needed for complete restoration of a releasable neurohypophyseal pool of the hormone.

Vasopressin, ACTH, and blood pressure during hypoxia induced at different rates

We decreased arterial PO2 at three different rates and measured blood pressure (BP), vasopressin (AVP), ACTH, and corticosteroid levels in nonsurgically stressed, anesthetized, paralyzed dogs. PaO2 was lowered to 28 Torr in 2 (fast), 10 (moderate), and 20 min (slow). The fast dPO2/dt produced a large spike in BP. Increases in AVP, ACTH, and corticosteroids were similar regardless of the dPO2/dt. When the spike in BP during the fast dPO2/dt was prevented with nitroprusside, hormone levels increased more quickly and were higher during the first 20-30 min of hypoxia. By 60 min, hormone levels were not different between experiments. The data suggest that 1) faster decreases in PO2 produce larger increases in BP, 2) increases in AVP, ACTH, and corticosteroids are primarily sensitive to the level of steady-state PaO2, and 3) increases in BP inhibit stress-induced increases in AVP and ACTH.

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

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

Epochal trace elements and evolution

The use of some trace elements by plants and animals during the evolutionary process has resulted in epochal changes. Noteworthy is the fact that plants (but not animals) needed boron in order to grow stems and roots as they left the seas and became anchored on land. Iodine is plentiful in sea water but rare on land. Therefore, the iodination of tyrosine provided an iodine transport mechanism which allowed for the metamorphosis and the development of warm bloodedness--a great evolutionary advantage. Zinc from clay was needed for the formation of the first primitive nucleic acids and, later, the presence of zinc in the retina provide the enhanced night vision of the nocturnal predators--a natural advantage. Hence, boron, iodine and zinc can be termed epochal trace elements. Inquiry should be directed towards the possible roles of other trace elements, which may have been epochal in evolution.

Comparison of canine corticosteroid responses to mean and phasic increases in ACTH

To determine the dynamics and magnitudes of adrenal corticosteroid responses to ACTH, we measured arterial plasma ACTH and corticosteroid concentrations in conscious dogs during infusions of ACTH or saline. Synthetic alpha 1-24-ACTH was infused at rates of 300,900, or 4,500 ng/30 min either as constant infusions or as three equal short infusions at 10-min intervals. In dogs infused with saline, plasma ACTH fluctuated, whereas corticosteroids did not, suggesting that ACTH is secreted episodically in dogs as in man. The magnitudes of the plasma corticosteroid responses to ACTH infusions were linearly related to the logarithm of the total amount of ACTH infused in 30 min and not to the pattern of administration. In all ACTH infusion experiments, the lag between an increase in arterial ACTH and corticosteroids was not less than 3 min. Mean ACTH half-disappearance time, metabolic clearance rate, and volume of distribution estimated from the different experiments ranged between 1.8 and 2.1 min, 24 and 38 ml . kg-1 . min-1, and 95 and 114 ml/kg, respectively. Collectively, these results explain the apparent paradox that corticosteroid responses to ACTH-releasing stimuli can be initiated before a detectable increase in ACTH above the highest control value (Wood et al. Apparent dissociation of ACTH and corticosteroid responses to ml/kg hemorrhage in conscious dogs. Endocrinology In press).