Control of adrenocorticotropin secretion and adrenocortical sensitivity in neurohypophysectomized conscious dogs: effects of acute and chronic vasopressin replacement

We examined the effect of neurohypophysectomy with and without vasopressin replacement on the ACTH response to hypotension and ovine CRF infusion and on the adrenocortical response to ACTH and angiotensin II infusion in conscious dogs. Nitroprusside hypotension (decrease in mean arterial pressure of 25 mm Hg) in the intact state resulted in large increases in plasma arginine vasopressin (pAVP; from 2.6 +/- 0.3 to 296 +/- 63 pg/ml) and ACTH (from 35 +/- 6 to 395 +/- 92 pg/ml). Neurohypophysectomy resulted in greatly attenuated pAVP (8.4 +/- 1.6 pg/ml) and ACTH (80 +/- 10 pg/ml) responses to hypotension which were not normalized by physiological low dose vasopressin replacement (6-18 pg/kg.min continuously, iv, for 2 weeks). However, acute administration of vasopressin (4-6 ng/kg.min) simultaneously with hypotension in the neurohypophysectomized (neurohypox) dog, which produced pAVP levels equivalent to the hypotensive response to intact dogs, almost completely normalized the ACTH response to hypotension (to 248 +/- 74 pg/ml). The ACTH response to 20 ng/kg.min ovine CRF, iv (from 43 +/- 8 to 268 +/- 77 pg/ml), was not attenuated by neurohypophysectomy. The cortisol responses to infusion of 0.5 and 2 ng/kg.min ACTH-(1-24), iv, were essentially normal in neurohypox dogs. However, the ACTH and aldosterone responses to 5 ng/kg.min angiotensin II infusion iv were attenuated in neurohypox dogs off AVP replacement. Histological examination revealed normal adrenal glands and anterior pituitaries in neurohypox dogs. Immunocytochemical staining for vasopressin and neurophysin revealed normal cell bodies in the paraventricular and supraoptic nuclei of the hypothalami from neurohypox dogs. However, median eminence staining for AVP and neurophysin was greatly diminished in neurohypox dogs. In summary, neurohypophysectomy 1) attenuated the ACTH response to hypotension and angiotensin II, but not to CRF, and 2) attenuated the aldosterone response to high dose angiotensin II. Furthermore, the deficit in ACTH secretion was almost completely normalized by increasing plasma AVP levels to those observed in the intact dogs. We conclude that an action of circulating pAVP increases ACTH secretion by a direct effect at the pituitary and by activating afferent input to the hypothalamus.     

Inhibition of corticotropin release during hypothermia: the role of corticotropin-releasing factor, vasopressin, and oxytocin

To investigate the mechanism by which ACTH secretion is inhibited during hypothermia, hypophysial portal blood was collected from euthermic and hypothermic rats, and the concentrations of corticotropin-releasing factor (CRF), vasopressin (AVP), and oxytocin (OT) were measured by RIA. Whereas CRF levels in portal plasma were not different in the two groups, AVP and OT levels were significantly lower in hypothermic rats. The concentration of AVP and OT in peripheral plasma was also significantly lower in hypothermic rats compared with euthermic controls. The pituitary responsiveness to CRF during hypothermia was tested in vivo and in vitro. In pentobarbital-anesthetized male rats injected iv with 0.1 or 1.0 nmol CRF, the ACTH response was significantly smaller in hypothermic compared with euthermic animals. However, hemipituitaries superfused at 31 C released the same amount of ACTH in response to 1 nM CRF as hemipituitaries superfused at 37 C (31 C, 541 +/- 90 pg; 37 C, 563 +/- 29 pg) despite reduced baseline secretion (31 C, 77 +/- 10 pg/10 min; 37 C, 114 +/- 14 pg/10 min; P less than 0.05). The data suggest that the inhibition of ACTH secretion during hypothermia is mediated by decreased hypothalamic secretion of AVP and OT which in turn decreases the pituitary responsiveness to CRF.     

Effects of synthetic ovine corticotropin-releasing factor (CRF) on ACTH release in vitro

The effect of the synthetic ovine corticotropin-releasing factor (CRF) on adrenocorticotropin (ACTH) release was examined by the perifusion method using rat anterior pituitary tissue and rat monolayer cultured pituitary cells. Quartered anterior pituitaries were placed in a chamber and perifused at a rate of 400 microliters/min with Dulbecco's modified Eagle Medium (DMEM, pH 7.4) bubbled with a mixture of 95% O2 and 5% CO2. The perifused medium was fractionated, and the ACTH concentration was measured by radioimmunoassay. In the monolayer cultured pituitary cells, the amount of ACTH released in the culture medium during three hours incubation was assayed by radioimmunoassay. ACTH was released from the perifused anterior pituitary in a dose-related manner by the pulse administration of CRF or arginine vasopressin (AVP) at the concentration of 1 ng/ml, 10 ng/ml and 100 ng/ml. A significant difference was not found between CRF- and AVP-induced ACTH release. In the monolayer cultured pituitary cells, synthetic ovine CRF induced ACTH release in a dose-related manner between 30 pg/ml and 30 ng/ml, but AVP induced a slight ACTH release. ACTH release was pulsatile during the continuous administration of 2.5 ng/ml of CRF for 150 min, although if gradually increased during the continuous administration of 10 ng/ml or 20 ng/ml of CRF. The continuous administration of AVP also caused pulsatile ACTH release at 10 ng/ml, but the ACTH release gradually decreased during the continuous administration of AVP. The interaction between CRF and AVP on ACTH release was examined by two methods. When CRF and AVP were given simultaneously, a mainly additive effect on ACTH release was observed. However, a low concentration of CRF seemed to potentiate AVP-induced ACTH release. These results show that both CRF and AVP have a significant CRF activity on the perifusion system, that AVP induced a slight ACTH release in monolayer cultured pituitary cells, and that CRF acts additively or potently with AVP to control the ACTH release from the anterior pituitary gland.     

Calmodulin inhibitors decrease the CRF-and AVP-induced ACTH release in vitro: interaction of calcium-calmodulin and the cyclic AMP system

The effect of N-(6-aminohexyl)-5-chloro-naphthalene-1-sulfomide (W-7) and trifluoperazine (TFP) was examined on ACTH release from cultured rat anterior pituitary cells and pituitary halves. These drugs significantly inhibited the ACTH release induced by synthetic ovine corticotropin-releasing factor (CRF) in a dose-related manner. In pituitary halves, arginine vasopressin (AVP) at 10 and 100 ng/ml showed almost the same ACTH-releasing activity as CRF at the same concentrations. W-7 and TFP inhibited the CRF-and AVP-induced ACTH release from pituitary halves. The cyclic AMP levels in the pituitary halves were significantly increased by CRF, but not AVP. Although W-7 inhibited CRF-induced ACTH release, it did not have an effect in cyclic AMP accumulation. These results suggest that CRF exerts ACTH-releasing activity through both the calcium-calmodulin system and cyclic AMP system and that AVP stimulates ACTH release mainly through the calcium-calmodulin system.     

In-vitro effects of corticosterone, synthetic ovine corticotrophin releasing factor and arginine vasopressin on the release of adrenocorticotrophin by fetal rat pituitary glands

The in-vitro release of ACTH by fetal rat pituitary glands on days 17, 19 and 21 of pregnancy was measured using radioimmunoassay. The spontaneous release of ACTH, expressed in pg ACTH/gland per h, increased with fetal age, in correlation with the sharp rise in pituitary ACTH content. However, since pituitary ACTH content was nearly sevenfold higher at term than on day 17, while basal release of ACTH was only threefold higher, one can speculate that the spontaneous release of ACTH was proportionally greater on day 17 than on day 21 of gestation. As corticosterone, at a physiological concentration (865 nmol/l), reduced ACTH release, it was concluded that the pituitary gland was one site of the negative feedback action of the corticosteroids during fetal life. Quantities of synthetic ovine corticotrophin releasing factor (CRF) which gave concentrations of 0.3-30 nmol/l in the incubation medium induced a sharp rise in ACTH release which was log-dose dependent between 0.3 and 3 nmolCRF /1 on day 17 and between 0.3 and 30 nmolCRF /1 on days 19 and 21. The response to CRF increased with fetal age. Quantities of arginine vasopressin (AVP) which gave concentrations of 2-200 nmol/l stimulated ACTH release at all stages of gestation investigated. However, the response to AVP was much lower than that to CRF. Potentiation of CRF-induced ACTH release was not observed when whole pituitary glands from 21-day-old fetuses were incubated with AVP (20 nmol/1) + CRF ( 3nmol /1). Such results were correlated with the ontogenesis of immunoreactive vasopressin- and CRF-containing fibres in the median eminence of the rat fetus, as well as with the CRF-like immunoreactivity present in adult rat pituitary portal plasma and the AVP content of the fetal rat hypophysis.     

Role of vasopressin in cardiovascular and neurohormonal responses to intracerebroventricular hypertonic NaCl

To determine the significance of vasopressin in cardiovascular and neurohormonal responses caused by centrally administered hypertonic NaCl, we examined the effects of a vasopressin antagonist on blood pressure, heart rate, plasma levels of catecholamines, cortisol and renin activity in anesthetized dogs. Intracerebroventricular (ICV) injections of 0.2 ml of 1.5 M NaCl increased mean arterial blood pressure (+29.7 +/- 3.0 mmHg, mean +/- SE), heart rate (+27.9 +/- 7.0 beats/min), plasma concentrations of vasopressin (+48.9 +/- 8.2 pg/ml), norepinephrine (+40.0 +/- 6.2 pg/ml), epinephrine (+231.4 +/- 21.4 pg/ml) and cortisol (+5.3 +/- 1.1 micrograms/dl) and decreased plasma renin activity (-2.0 +/- 0.4 ng/ml/hr). An intravenous vasopressin antagonist, d(CH2)5Tyr(Me)AVP, at a dose of 10 micrograms/kg, attenuated the pressor response and augmented the heart rate response to ICV 1.5 M NaCl. The vasopressin antagonist also augmented the change in plasma norepinephrine and significantly attenuated the responses of cortisol and renin. Baseline levels of these variables were not altered by the vasopressin antagonist except for an increase in renin activity. Two injections of hypertonic NaCl without any pretreatment produced similar cardiovascular and hormonal responses. These results suggest that vasopressin contributes not only to an increase in blood pressure, but also to changes in the sympathetic nervous system, the hypothalamo-adrenocortical axis and the peripheral renin-angiotensin system in response to a central sodium stimulus.     

Brain content and plasma concentrations of arginine vasopressin in an Australian marsupial, the brushtail possum Trichosurus vulpecula.

Arginine vasopressin (AVP) has been identified and quantified in the brain and plasma of the possum using a highly specific radioimmunoassay and high-performance liquid chromatography. Large amounts of AVP were found in the pituitary (16.3 +/- 0.56 micrograms/pituitary, n = 5) and hypothalamus (398 +/- 82.5 ng/hypothalamus), and significant amounts of AVP were also present in the cerebral cortex (26.8 +/- 11.5 ng/cortex). Plasma AVP concentrations were significantly lower (2.2 +/- 0.45 pg/ml, n = 10) during anesthesia compared to concentrations while conscious (4.5 +/- 1.19 pg/ml). Severe hemorrhage markedly increased plasma concentrations to 1091 +/- 225 pg/ml (n = 8). It was concluded that AVP is present in the possum brain, pituitary, and plasma, and that its secretion is stimulated by hypovolemia and inhibited by surgical stress.     

The relationship of saline-induced changes in vasopressin secretion to basal and corticotropin-releasing hormone-stimulated adrenocorticotropin and cortisol secretion in man

To investigate the effect of endogenous arginine vasopressin (AVP) on ACTH secretion, normal subjects were given infusions of either hypertonic saline (HS) or isotonic saline (NS) combined with human corticotropin-releasing hormone (CRH) or placebo. Basal plasma AVP was 2.3 +/- 0.3 (+/- SE) pg/ml, did not change with NS treatment, and rose to 5.4 +/- 0.6 pg/ml during HS infusion (P less than 0.01). Both basal and CRH-stimulated plasma ACTH and cortisol concentrations increased during HS infusion. Peak plasma ACTH and cortisol levels were 11.4 +/- 1.5 pg/ml and 8.6 +/- 0.8 micrograms/dl, respectively, during the HS (plus placebo) infusion. During the NS (plus placebo) infusion, plasma ACTH and cortisol gradually declined to 6.8 +/- 0.5 pg/ml and 2.6 +/- 0.4 micrograms/dl. The timing of the rise in ACTH during the HS infusion paralleled the rise in AVP. When an iv dose of 1 microgram/kg CRH was administered during the saline infusions, peak plasma ACTH and cortisol levels were 27.7 +/- 6.3 pg/ml and 17.5 +/- 1.0 micrograms/dl, respectively, during the HS infusion and 15.6 +/- 1.7 pg/ml and 13.4 +/- 1.2 micrograms/dl during the NS infusion. When the areas under the hormone response curves were compared, CRH stimulated ACTH and cortisol secretion to a greater extent than did HS (P less than 0.05). The hormonal stimulation due to combined CRH and hypertonic saline was greater than that attributable to either factor alone (P less than 0.025), but was not different than the sum of the effects of the individual factors. These results indicate that increases in endogenous AVP produced by HS are associated with increases in both basal and CRH-stimulated ACTH and cortisol release. The effect of HS appears to be additive to but not consistently synergistic with the effect of CRH.     

The concentration of arginine vasopressin in pituitary stalk plasma of the rat after adrenalectomy or morphine

We have investigated the role of adrenal steroids and the opiates in regulating arginine vasopressin (AVP) secretion into the pituitary stalk blood of the rat. The portal plasma concentration of AVP in urethane-anesthetized male rats was 532 +/- 68 pg/ml (mean +/- SEM), while the peripheral plasma AVP concentration in intact urethane-anesthetized rats was 20.7 +/- 5.7 pg/ml. Column chromatography on Sephadex G-25 of an extract of a pool of portal plasma revealed that the material being assayed comigrated with synthetic AVP. Bilateral adrenalectomy (ADX) 5 days before the collection of portal blood elevated portal plasma AVP concentrations approximately 6-fold (655 +/- 124 pg/ml in controls vs. 4090 +/- 504 pg/ml in adrenalectomized animals). Dexamethasone administration (15 micrograms/kg X day) for 5 days prevented the ADX-induced increase in portal plasma AVP concentrations without significantly changing portal plasma AVP concentrations in intact rats. Portal plasma concentrations of beta-endorphin were not changed by ADX or dexamethasone treatment. The iv infusion of morphine sulfate (3 mg/kg) dramatically decreased the concentration of AVP in the portal plasma of the rat (501 +/- 101 pg/ml before morphine vs. 185 +/- 50 pg/ml after morphine). The inhibitory effect of morphine was reversed by naltrexone (1.0 mg/kg), whereas naltrexone alone did not alter AVP secretion. Morphine administration also decreased systemic plasma AVP concentrations in urethane-anesthetized rats (27.1 +/- 6.6 pg/ml in controls vs. 3.3 +/- 1.3 pg/ml in morphine-treated rats). Naltrexone treatment reversed this effect. These results suggest that AVP secretion into pituitary stalk blood is under the inhibitory influence of the adrenal steroids, and the increased concentration of AVP found in portal blood may be partially responsible for the elevated levels of ACTH after ADX. Furthermore, morphine-induced activation of the pituitary-adrenal axis is apparently independent of hypothalamic AVP secretion.     

Synergism between systemic corticotropin-releasing factor and arginine vasopressin on adrenocorticotropin release in vivo varies as a function of gestational age in the ovine fetus

In sheep, parturition is associated with maturation of fetal pituitary-adrenal function, and with rises in the concentrations of ACTH and cortisol (F) in fetal plasma. We examined the hypothesis that pituitary ACTH output in response to arginine vasopressin (AVP) and CRF separately and together might change during late pregnancy as a function of fetal age. Fetal sheep were chronically catheterized, and bolus iv injections of equimolar AVP, CRF, AVP plus CRF, or saline (controls) were given on days 110-115, 125-130, and 135-140 of gestation. AVP evoked significant rises in plasma ACTH on days 110-115 and 125-130, but not on days 135-140. After AVP, the peak plasma concentrations of ACTH were attained at 5-10 min, and basal (preinjection) values were reestablished by 30-60 min. After CRF treatment, plasma ACTH rose progressively throughout the 240 min of the study. Evidence was obtained in support of an increase in pituitary responsiveness to CRF between days 110-115 and 125-130 and a decrease in response on days 135-140, when basal F concentrations were higher. The ACTH response to AVP, relative to that to CRF, was greatest in the youngest fetuses. On days 110-115 only, CRF and AVP showed a synergistic response in ACTH output, especially during the first 30 min after agonist injection. Plasma F rose in response to the changes in endogenously released ACTH in a manner consistent with progressive fetal adrenal maturation between days 110-140 of pregnancy. We conclude that in vivo the ovine fetal pituitary responds separately and synergistically to AVP and CRF on days 110-115 of gestation, but the relative role of AVP in stimulating ACTH release decreases with progressive gestational age.     

Effect of passive immunization against corticotropin-releasing factor (CRF) on the postadrenalectomy changes of CRF binding sites in the rat anterior pituitary gland

The concentration of corticotropin-releasing factor (CRF)-binding sites decreases in the rat anterior pituitary after adrenalectomy; this change may be related either to a direct effect of the circulating glucocorticoids at the pituitary level or to a desensitization of CRF receptors through an increased CRG release in hypophysial portal blood. In order to examine the latter possibility we have measured plasma adrenocorticotropin hormone (ACTH) levels and the number of anterior pituitary CRF binding sites in sham-operated and 24-hour adrenalectomized rats after blockade of endogenous CRF by passive immunization with an antiserum anti-rat CRF (CRF-AS), or after injection of normal rabbit serum (NRS). In NRS-injected rats, after sham operation, plasma ACTH concentration increased (227 +/- 34 vs. 118 +/- 19 pg/ml in controls) without change in CRF-binding sites capacity (20.7 +/- 2.6 vs. 24.6 +/- 3.5 fmol/mg protein in controls). Adrenalectomy induced a large rise in plasma ACTH (785 +/- 89 pg/ml) and a decrease in the number of CRF-binding sites (12.2 +/- 1.7 fmol/mg protein). After CRF-AS injection, plasma ACTH was normalized in sham-operated animals (149 +/- 24 pg/ml) and significantly reduced in adrenalectomized rats (472 +/- 76 pg/ml); the adrenalectomy-induced decrease in the number of CRF-binding sites was unaffected by the CRF-AS administration (12.2 +/- 1.7 fmol/mg protein). The administration of dexamethasone to adrenalectomized rats significantly reduced plasma ACTH concentrations (23.3 +/- 10.6 pg/ml) and prevented the loss in CRF-binding sites capacity (20.7 +/- 1.3 fmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of acute exercise on the secretion of corticotropin-releasing factor, arginine vasopressin, and adrenocorticotropin as measured in pituitary venous blood from the horse.

We have used the technique which we have developed for collecting pituitary venous blood from conscious, undisturbed horses to study the effect of acute vigorous exercise on the secretion of CRF, arginine vasopressin (AVP) and ACTH. Pituitary venous (pit) blood was collected every 1-5 min from nine trained racehorses at rest in the stable. The horses then trotted quietly for 10 min, after which they galloped as fast as possible for 4-6 min, before returning to the stable where sampling continued. In Exp 1 (n = 5) no blood samples were taken during exercise, whereas in Exp 2 (n = 4), pit blood was collected every 30 sec during exercise. Immediately after exercise, significant elevations in heart rate (P less than 0.001), body temperature (P less than 0.01) and hematocrit (P less than 0.001) were observed as compared with preexercise values. Jugular cortisol levels were higher after exercise (301.9 +/- 35.2 nmol/liter; mean +/- SEM) than before (187.3 +/- 34.8; P less than 0.01; n = 9). Likewise, jugular AVP levels increased with exercise (before, 0.65 +/- 0.11 pmol/liter; after 3.2 +/- 0.6; P less than 0.01; n = 6), whereas jugular CRF was not altered by exercise (before, 0.38 +/- 0.08 pmol/liter; after, 0.93 +/- 0.31; n = 6; NS). In Exp 1, no significant changes in pit ACTH, AVP, or CRF were observed after exercise. However in Exp 2 when pit blood was sampled during exercise all horses showed an immediate and dramatic rise in ACTH (P less than 0.01) and AVP (P less than 0.005) secretion which peaked during galloping with mean fractional changes above resting levels of 23.6 +/- 9.9 for ACTH and 51.7 +/- 24.0 for AVP. After exercise pit AVP levels were not different from resting, whereas ACTH remained elevated (11.4 +/- 6.9-fold above resting levels). By contrast, pit CRF levels were not altered by exercise. In both experiments together, pit AVP and ACTH concentrations were correlated in eight of the nine horses, whereas pit CRF and ACTH concentrations were positively correlated in only one of seven horses. We conclude that acute exercise causes a transient increase in ACTH secretion which occurs synchronously with an increase in AVP secretion. CRF does not appear to play a major role in mediating the initial ACTH response to exercise.     

Arginine vasopressin is a much more potent stimulus to ACTH release from ovine anterior pituitary cells than ovine corticotropin-releasing factor. 1. In vitro studies

Cultured rat and ovine anterior pituitary cells were treated with a range of doses (0.01-1,000 nM) of arginine vasopressin (AVP) and ovine corticotropin-releasing factor (CRF), alone or in combination, and medium and cell content of immunoreactive (ir-)ACTH determined. In rat cells, a dose-response curve to CRF was obtained, with a threshold dose of 0.1 nM; AVP was much less effective alone, but augmented CRF responses when administered with CRF. In ovine pituitary cells AVP markedly stimulated ACTH release in a dose-dependent fashion, and with a threshold of 0.1 nM; in contrast, CRF increased ACTH release over basal only at doses greater than 100 nM. In combination, subthreshold doses of AVP potentiated rat pituitary cell responses to CRF; addition of 1 nM of AVP to varying doses of CRF was more effective in terms of ACTH release than addition of 1 nM of CRF to increasing doses of AVP. In contrast, in ovine cells the addition of 1 nM CRF to increasing doses of AVP elicited a larger ACTH response than the addition of 1 nM AVP to increasing doses of CRF. Dexamethasone pretreatment (5 nM) for 48 h significantly decreased CRF potentiation of AVP-stimulated ACTH release in ovine cells. These studies confirm that CRF is a more potent stimulus of ACTH release than AVP in the rat, and establish that in contrast AVP is a much more potent stimulus of ACTH secretion than CRF in isolated ovine pituitary cells.     

Increased pituitary sensitivity to glucocorticoid feedback during the stress nonresponsive period in the neonatal rat

Neonatal rats show a diminished response to stress [the stress-nonresponsive period (SNRP)] from day 2-3 until day 14 of age; the physiological bases for the SNRP are unknown. We examined whether enhanced sensitivity of the brain or pituitary to the inhibitory feedback effects of circulating glucocorticoids (GC) contributes to the SNRP. Age-related changes in the ability of corticosterone (CORT) and dexamethasone (DEX) to inhibit the ACTH secretion induced by urethane or CRF were studied. We also examined the ACTH response to ether stress or CRF in intact or 24 h-adrenalectomized 5-day-old rats. Plasma ACTH did not increase in intact rats after ether stress (basal: 64.6 +/- 9.1 pg/ml vs. stressed: 66.8 +/- 8.9 pg/ml; P greater than 0.05), whereas small elevations occurred after CRF challenge (184.6 +/- 40 pg/ml; P less than or equal to 0.01). Five-day-old adrenalectomized rats, which had elevated basal ACTH concentrations, increased ACTH secretion after exposure to ether or CRF. Thus, negative feedback appears to mediate critically the SNRP. Furthermore, sensitivity to such feedback was enhanced during the SNRP since the capacity of CORT to inhibit urethane-induced ACTH secretion in vivo declined with age; 1 mg/kg BW was the minimal dose that inhibited ACTH secretion at day 10, whereas at day 18, the threshold for a similar inhibition was 5 mg/kg BW. In contrast, at both ages, a dose of 10 micrograms/kg BW DEX inhibited ACTH release. In vitro dose response studies in whole pituitaries further demonstrated the enhanced pituitary sensitivity to GC feedback during the SNRP since the IC50 for CORT inhibition of CRF-induced ACTH release increased from days 3-5 to days 22-23. A similar, although not statistically significant trend was observed for DEX inhibition. Thus, neonatal rats exhibit an enhanced pituitary sensitivity to GC during the SNRP and removal of this inhibition allows ACTH secretion in response to ether stress.     

Corticotropin response to combined administration of human corticotropin-releasing hormone and small-dose arginine vasopressin in normal subjects.

Arginine vasopressin (AVP) is known to potentiate corticotropin (ACTH) secretion by human corticotropin-releasing hormone (hCRH), and a combined administration of hCRH and AVP appears useful as a pituitary ACTH reserve test. This study was designed to evaluate the appropriate dose of AVP and its route of administration, for better estimation of pituitary ACTH reserve in humans, when used in combination with a conventional hCRH stimulation test. First, intravenous (IV) doses of hCRH (100 micrograms) and AVP (0, 0.1, and 0.3 U) were administered simultaneously in six normal subjects. Second, IV hCRH was administered with intramuscular (IM) AVP (0, 1.0, 3.0, and 5.0 U) in 10 normal subjects. Blood samples for measurement of plasma ACTH were obtained at 0, 15, 30, 45, 60, 90, and 120 minutes after the hCRH with and without AVP administration. The order of AVP doses was randomly chosen in each subject. The peak plasma ACTH level was 65.0 +/- 16.0 pg/mL (30 minutes) with hCRH alone and 139.5 +/- 35.6 pg/mL (15 minutes) with hCRH plus 0.3 U IV AVP in six normal subjects. Similarly, the peak plasma ACTH level was 43.5 +/- 5.6 pg/mL (30 minutes) with hCRH alone and 116.0 +/- 19.6 (15 minutes) and 96.6 +/- 24.0 pg/mL (15 minutes) with hCRH plus 3.0 and 5.0 U IM AVP in 10 normal subjects, respectively. The hCRH-induced ACTH responses (delta ACTH) with both IV and IM AVP were significantly (P less than .05) greater than the respective control values with hCRH alone. The responses (delta ACTH) were comparable between the two phases of 3.0 and 5.0 U IM AVP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Corticotropin-releasing factor secretion increases in rat hypophysial portal blood during insulin-induced hypoglycemia

To study the possible involvement of hypothalamic corticotropin-releasing factor (CRF) in the stimulation of adrenocorticotropic hormone (ACTH) release caused by insulin-induced hypoglycemia (IIH), we measured CRF secretion in hypophysial portal blood (HPB) in rats anesthetized with sodium thiopental after injection of insulin. Before treatment, systemic ACTH levels (952 +/- SE 143 pg/ml; n = 12) were well above normal values, probably reflecting the anesthetic and surgical stress consecutive to the preparation for portal blood collection. Insulin injection induced a significant increase of ACTH release within 15 min (1,588 +/- 168 vs. 741 +/- 144 pg/ml; n = 6, in vehicle-injected rats) which lasted for at least 1 h. CRF levels in HPB were 857 +/- SE 168 pg/ml (n = 13) during the first-hour pretreatment collection. Vehicle injection did not modify CRF secretion (759 +/- 142 pg/ml; n = 6). Insulin injection provoked a significant increase in CRF release (1,449 +/- 257 pg/ml; n = 7). These data suggest that an increased hypothalamic CRF secretion is responsible for the stimulation of pituitary ACTH release following IIH. The possible involvement of central neuromediators in the IIH-induced CRF production is discussed.     

Abnormal response of adrenocorticotropin to corticotropin releasing factor in spontaneously hypertensive rats

The pituitary-adrenocortical response to ovine corticotropin-releasing factor (CRF) was investigated in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) anesthetized with pentobarbital. After intravenous administration of various doses of CRF (0.1, 1.0 and 10 micrograms), the plasma levels of ACTH were significantly increased in both groups. ACTH increments after various doses of CRF were dose-dependent in WKY. ACTH increments after injections of 0.1, 1.0, and 10 micrograms of CRF in SHR were 123.2 +/- 36.5 (SE), 123.0 +/- 52.2 and 60.3 +/- 48.5 pg/ml at 5 min, and 386.3 +/- 73.8, 243.4 +/- 86.6 and 220.0 +/- 31.4 pg/ml at 15 min, respectively. The ACTH increments in SHR at 15 min after administration of 0.1 microgram of CRF were higher (p less than 0.02) than those in WKY. However, ACTH increments in SHR after administration of 10 micrograms of CRF were lower than those in WKY at 5 min (p less than 0.05) and 15 min. These data suggest that the plasma ACTH responses to various doses of CRF represent a dose-unrelated plateau response in SHR. The plasma levels of corticosterone after administration of various doses of CRF did not change significantly and there were no significant differences between the two strains. The results of these experiments suggest that SHR have an abnormal response of the pituitary-adrenocortical axis to CRF, and the abnormal response may be attributable to desensitization of the pituitary to CRF.     

Prostaglandin E2 enhances AVP-stimulated but not CRF-stimulated ACTH secretion from cultured fetal sheep pituitary cells.

This study examined the ability of prostaglandin E2 (PGE2) to regulate ACTH secretion from cultured anterior pituitary cells of fetal sheep between days 130 and 140 of gestation (term = 145 days). Corticotrophin-releasing factor (CRF) and arginine vasopressin (AVP) induced dose-dependent (0.1-1000 nmol/l) increases in ACTH secretion from fetal sheep pituitary cells maintained in culture for 6 days, with AVP being significantly (P less than 0.01) more potent than CRF. PGE2 (1000 nmol/l) significantly (P less than 0.05) enhanced the ability of AVP, but not CRF, to stimulate ACTH secretion. However, PGE2 given alone (0.1-1000 nmol/l) had no effect on ACTH secretion. Concomitant administration of CRF and AVP induced a greater release of ACTH than after treatment with either peptide alone, a synergistic interaction which was unaffected by simultaneous administration of PGE2. These results provide evidence for a direct action of PGE2 on ACTH secretion from the fetal sheep pituitary gland via a specific interaction with AVP. This interaction may allow increased fetal plasma concentrations of PGE2, seen during late gestation, to stimulate fetal pituitary-adrenal maturation.     

ACTH response to desmopressin in a patient with acromegaly; expression of corticotropin-releasing factor, urocortins and vasopressin V1b receptor in GH-producing pituitary adenoma

GH-producing pituitary adenomas frequently co-produce other certain anterior pituitary hormones, such as prolactin (PRL). In contrast, GH-producing adenomas which express all of corticotropin-releasing factor (CRF), urocorin1 (Ucn1) and urocortin3 (Ucn3) have not been reported. A 39-year-old woman was admitted for evaluation of the pituitary tumor. The diagnosis of acromegaly was confirmed by elevated serum GH and IGF-I levels, and the absence of GH suppression by oral glucose tolerance test. ACTH response to desmopressin (DDAVP) was observed (plasma ACTH levels increased from 13.9 to 50.4 pg/ml at 90 min). Although it is known that ACTH response to DDAVP is considerably useful for the diagnosis of ACTH-dependent Cushing's syndrome, the diagnosis of Cushing's disease was not supported by the criteria. The patient underwent transsphenoidal resection of the pituitary tumor. Immunohistological examination confirmed a GH- and PRL-producing adenoma, whereas ACTH was negative. ACTH response to DDAVP disappeared after tumor removal. To determine the cause of preoperative ACTH response to DDAVP, we examined expression of CRF family peptides and vasopressin V1b receptor in the pituitary adenoma by immunohistochemistry. Immunohistochemistry revealed positive immunostaining for CRF, Ucn1, Ucn3 and vasopressin V1b receptor in the adenoma. These observations raised the possibility that DDAVP caused an ACTH response, perhaps via the paracrine effects of tumor-derived CRF and Ucn1. When ACTH response to DDAVP is observed in patients with pituitary tumor, not only the direct effect of DDAVP on ACTH secretion, but also a possible involvement of CRF and/or urocortins expressed in the pituitary adenoma, should be considered.