Altered hypothalamic-pituitary-adrenal axis responsiveness in myotonic dystrophy: in vivo evidence for abnormal dihydropyridine-insensitive calcium transport.

In persons with myotonic dystrophy (DM), the ACTH response to CRH is greater than normal, while it is delayed in response to arginine vasopressin. Since influx of extracellular Ca2+ ions is a common step in signal transduction by both of these secretagogues, an abnormality of cellular Ca2+ transport may underlie the disturbances of hypothalamic-pituitary-adrenal axis function in this condition. Seven myotonic patients were given naloxone, which stimulates endogenous CRH release, and nifedipine, which blocks L-type voltage-dependent Ca2+ channels. Each subject underwent three tests, using different drug combinations, in a single blind, placebo-controlled protocol. Pretreatment with nifedipine delayed the time of the peak plasma hormone responses after naloxone [ACTH, 32.1 +/- 2.1 vs. 51.4 +/- 4.5 min (P < 0.05); cortisol, 42.9 +/- 2.1 vs. 70.7 +/- 4.3 min (P < 0.02); for naloxone and nifedipine/naloxone, respectively]. Additionally, nifedipine significantly reduced the proportion of the mean integrated ACTH response that had occurred by 30 min after naloxone administration (32.0 +/- 4.0% for naloxone vs. 17.6 +/- 2.4% for nifedipine/naloxone; P < 0.02) and the proportion of the mean integrated cortisol response by 45 min after naloxone administration (34.7 +/- 3.5% for naloxone vs. 25.0 +/- 2.6% for nifedipine/naloxone; P < 0.02). However, the total integrated responses did not change [ACTH, 1182.6 +/- 548.9 vs. 905.5 +/- 157.0 pmol/min.L (P = NS); cortisol 17,353 +/- 2,984 vs. 18,469 +/- 3,561 nmol/min.L (P = NS); for naloxone and nifedipine/naloxone, respectively]. We conclude that nifedipine delays, but does not reduce, the ACTH and cortisol responses to naloxone in DM. Since nifedipine has a different effect on normal controls (reduced response with unchanged timing), these findings imply an abnormality of dihydropyridine-insensitive Ca2+ transport (such as T-type Ca2+ channels) in the corticotrophs of DM patients.

Aspirin increases the human hypothalamic-pituitary-adrenal axis response to naloxone stimulation.

Prostaglandins are believed to influence hypothalamic-pituitary-adrenal (HPA) axis function, but their specific effects on ACTH and cortisol secretion in humans are unclear. Acetylsalicylic acid (aspirin) blocks the synthesis of prostaglandins from arachidonic acid. We studied the effects of oral aspirin on the plasma ACTH and cortisol responses to iv naloxone, which increases endogenous CRH release, in six normal volunteers and on the adrenocortical response to synthetic ACTH boluses in seven other healthy subjects, using placebo-controlled, single blinded protocols. Aspirin pretreatment significantly increased the ACTH response to naloxone [mean peak increase from basal, 8.3 +/- 1.2 vs. 5.9 +/- 0.8 pmol/L (P < 0.05); mean integrated response, 431.9 +/- 51.5 vs. 295.1 +/- 26.6 pmol/L.min (P < 0.005); for aspirin/naloxone and placebo aspirin/naloxone, respectively]. However, the corresponding cortisol results did not show statistically significant differences (P < 0.20). The mean integrated ACTH and cortisol responses were 46% and 26% greater with aspirin, respectively. Aspirin did not influence the cortisol responses to synthetic ACTH administration given according to a dose-response protocol. We conclude that aspirin augments the HPA axis response to naloxone stimulation in normal humans without having a direct effect at the adrenal level. The action of aspirin on the human HPA axis is probably mediated via inhibition of cyclooxygenase, resulting in changes in arachidonic acid metabolites, which influence ACTH release from corticotrophs.

Alprazolam blocks the naloxone-stimulated hypothalamo-pituitary-adrenal axis in man.

Alprazolam (APZ) is a benzodiazepine with unique antidepressant activity for a drug of its class. There is some evidence of inhibition of the unstimulated hypothalamo-pituitary-adrenal axis by APZ which may be important in its therapeutic action, and could be detrimental in APZ-treated subjects who encounter stressful stimuli. To assess the effect of APZ on stimulated ACTH and cortisol secretion, we studied 14 normal subjects in a randomized, double-blind, placebo-controlled design. APZ or placebo capsule was administered orally in doses of 0.5 mg and 2 mg, 90 min before either naloxone, 125 micrograms/kg body weight i.v. bolus dose, a known stimulator of ACTH and cortisol release, or placebo. After naloxone stimulation, the area under the plasma ACTH/time curves was significantly reduced by APZ, in both the 2 mg (P < 0.0005) and 0.5 mg (P < 0.005) doses, compared to their respective placebo studies; similar reductions in area under the plasma cortisol/time curves occurred after 2 mg (P < 0.00002) and 0.5 mg (P < 0.0005) APZ doses. We conclude that APZ is a potent inhibitor of naloxone-stimulated ACTH and cortisol release in humans. Since APZ has been shown to inhibit CRH release in vitro, and naloxone-induced ACTH secretion is likely to be caused through CRH release, this suggests that APZ inhibition of naloxone action is via the parvocellular CRH neurons of the paraventricular nucleus and/or central neurotransmitter pathways impinging directly or indirectly on these CRH neurons. Thus APZ may exert at least some of its clinical effects through inhibition of central CRH release. APZ treatment could lead to a relative hyporesponse of the pituitary-adrenal axis during stress. APZ may be an important tool for manipulation of hypothalamic CRH release in studies of pituitary-adrenal function.

Alprazolam attenuates vasopressin-stimulated adrenocorticotropin and cortisol release: evidence for synergy between vasopressin and corticotropin-releasing hormone in humans.

Alprazolam (APZ), a triazolobenzodiazepine with unique clinical utility, has potent inhibitory effects on the human hypothalamic-pituitary-adrenal axis. Because APZ inhibits CRH secretion from isolated rat hypothalami and inhibits the probable CRH-mediated effect of naloxone on ACTH release, it is likely APZ acts as an inhibitor of hypothalamic CRH release in humans. The two principal physiological ACTH secretagogues are CRH and arginine vasopressin (AVP). We studied the ACTH and cortisol responses to an ACTH-releasing dose of AVP with and without preadministration of APZ in humans. Our hypothesis was that acute CRH deprivation by APZ would attenuate the ACTH response to vasopressin, as CRH and AVP act synergistically to control ACTH release. This synergy may depend on activation of subpopulations of corticotropes, some of which require both CRH and AVP together to elicit an ACTH response and/or intracellular "cross-talk" between second messenger pathways stimulated by the secretagogues. APZ (2 mg, orally) was given to eight healthy volunteers 90 min before AVP (0.0143 IU/kg BW, iv) in a randomized, double blind, placebo-controlled design during afternoon studies. ACTH and cortisol levels were measured at frequent intervals from 60 min before to 120 min after AVP injection. APZ reduced the mean integrated ACTH and cortisol responses to AVP by 67% and 70% respectively [ACTH, 161.6 +/- 59.7 vs. 53.0 +/- 20.9 pmol/min.L (P = 0.022); cortisol, 9314 +/- 3310 vs. 2763 +/- 1472 nmol/min.L (P = 0.020, AVP vs. APZ/AVP, respectively)]. APZ reduced the mean peak ACTH and cortisol responses to AVP by 57% (P = 0.023) and 40% (P = 0.0012), respectively. AVP levels were not significantly different in those who received APZ or placebo. This study provides further evidence of the potent inhibitory effects of APZ on ACTH and cortisol release in humans and is the first to find that APZ inhibits AVP-stimulated ACTH and cortisol release. This study also suggests that CRH/AVP synergy is an important physiological mechanism for ACTH release in humans, as indicated by the blunted ACTH response to AVP after APZ-mediated acute CRH deprivation. Inhibition of the pituitary-adrenal axis by APZ may explain its unique efficacy in psychiatric disorders thought to be associated with dysregulation of hypothalamic CRH release.

Aspirin inhibits vasopressin-induced hypothalamic-pituitary-adrenal activity in normal humans.

PGs influence ACTH secretion. However, their specific role in modulating the activity of the human hypothalamic-pituitary-adrenal (HPA) axis remains unclear. Acetylsalicylic acid (aspirin) inhibits the synthesis of PGs from arachidonic acid by blocking the cyclooxygenase pathway. In this study we administered a single, clinically relevant dose of aspirin before HPA axis stimulation by a bolus dose of iv arginine vasopressin (AVP) to seven normal males using a randomized, placebo-controlled, single blinded design. Aspirin significantly reduced the cortisol response to AVP [mean peak increase from basal, 221.1 +/- 20.1 vs. 165.4 +/- 22.5 nmol/L (P = 0.0456); mean integrated response, 11,199.3 +/- 1,560.0 vs. 6,162.3 +/- 1,398.6 nmol.min/L (P = 0.0116) for placebo aspirin/AVP and aspirin/ AVP, respectively]. The ACTH response was reduced, but did not reach statistical significance [mean peak increase from basal, 7.5 +/- 2.2 vs. 4.3 +/- 0.3 pmol/L (P = 0.0563); mean integrated response, 142.6 +/- 36.0 vs. 96.2 +/- 8.7 pmol.min/L (P = 0.12) for placebo aspirin/ AVP and aspirin/AVP, respectively]. PGs may influence ACTH secretion by being stimulatory or inhibitory to the HPA axis at different levels, such as hypothalamic or pituitary. Which effect predominates in vivo during dynamic activation of the axis may depend on the level at which the secretory stimulus acts. We showed that when normal male volunteers were treated with the PG synthesis inhibitor, aspirin, they had a blunted HPA axis response to the pituitary corticotroph stimulator, AVP.

Paradoxical inhibition by aspirin of naloxone-induced adrenocorticotropin secretion in myotonic dystrophy.

The ACTH response to endogenous or exogenous CRH is increased in patients with myotonic dystrophy (DM), possibly because of abnormal function of cAMP-dependent protein kinases in this condition. Arachidonic acid (AA) metabolites are believed to interact with the cAMP-dependent second messenger system activated by CRH; therefore, drugs that interfere with AA metabolism may alter ACTH secretion in DM. In this study, seven DM patients were given naloxone, which stimulates endogenous CRH release, and aspirin, which inhibits the synthesis of prostaglandins from AA via the cyclooxygenase metabolic pathway. Pretreatment with aspirin reduced the mean integrated ACTH response to naloxone by 33% (P < 0.05). However, the corresponding 18% reduction in cortisol levels was not statistically significant (P > 0.10). These findings are in contrast to those of a previous study using an identical protocol, in which aspirin increased the ACTH response to naloxone in six normal volunteers. This difference between DM and control subjects is consistent with the hypothesis that the interaction between AA metabolites and the cAMP-dependent protein kinase-A second messenger system is abnormal in the corticotrophs of persons with DM.

The effect of desipramine on basal and naloxone-stimulated cortisol secretion in humans: interaction of two drugs acting on noradrenergic control of adrenocorticotropin secretion.

Desipramine (DMI), a tricyclic antidepressant and norepinephrine (NE) reuptake blocker, is reported to induce ACTH and cortisol release acutely in humans, probably by facilitating central NE neurotransmission. Tricyclic antidepressant therapy, including DMI, normalizes the ACTH and cortisol hypersecretion that often accompanies depression. The mechanism of hypothalamic-pituitary-adrenal (HPA) axis inhibition by DMI in humans is unknown. In rats, DMI reduces the activity of the locus ceruleus, a major source of NE innervation of the hypothalamic paraventricular nucleus, the site of CRH neurons. Naloxone induces ACTH and cortisol release in humans through a noradrenergic-mediated mechanism and a probable consequent stimulation of hypothalamic CRH release. To study the interaction of these drugs on NE neurotransmission and, hence, HPA axis activity in humans, we administered DMI alone and with naloxone in a randomized, double blind, placebo-controlled protocol in eight healthy male volunteers. DMI (75 mg, orally) was given 180 min before naloxone (125 micrograms/kg BW, i.v.). Plasma ACTH and cortisol were measured at frequent intervals from 60 min before to 120 min after naloxone treatment. Plasma cortisol levels were 77% higher 180 min after DMI compared to those after placebo treatment (287 +/- 17 vs. 162 +/- 14 nmol/L; P = 0.000005). DMI reduced the naloxone-induced rise in cortisol (P = 0.02), but there was no change in the integrated cortisol response. The increase in basal plasma ACTH levels after DMI treatment did not reach statistical significance. DMI significantly increased systolic blood pressure and heart rate consistent with an effect on the noradrenergic control of the cardiovascular system. In summary, DMI increased basal cortisol levels consistent with facilitation of NE neurotransmission and, hence, hypothalamic CRH release. However, DMI had no enhancing effect on naloxone-induced cortisol release. This contrasts with the synergy observed when non-antidepressant agents that increase NE neurotransmission are given with naloxone to humans. DMI increases glucocorticoid feedback sensitivity in the rat HPA axis after several weeks through up-regulation of central corticosteroid receptors. However, this slowly developing effect is unlikely to occur during these acute studies. The effect of DMI on naloxone-induced cortisol release is consistent with an inhibitory effect on central noradrenergic control of ACTH release, perhaps at the locus ceruleus. This is the first human study to suggest an inhibitory effect of DMI on central noradrenergic control of ACTH release.

Adrenocorticotropin hyperresponse to the corticotropin-releasing hormone-mediated stimulus of naloxone in patients with myotonic dystrophy.

We previously showed that CRH-mediated stimuli, including exogenous CRH, cause ACTH hypersecretion in many myotonic dystrophy (DM) patients. We confirmed this by giving naloxone, a stimulator of endogenous CRH release, to a large number of DM patients and controls. DM patients, first degree relatives, and normal controls received i.v. naloxone at 1400 h, and blood was taken for ACTH (RIA) and cortisol (high pressure liquid chromatography) measurements from 15 min before to 120 min after naloxone treatment. DM patients had basal ACTH levels approximately twice those of controls, and their ACTH responses were 4 times those of controls. In contrast, DM basal cortisol levels were not significantly different from those of relatives and were slightly higher than those of normal subjects. Cortisol responses were similar in the three groups, probably due to attenuation at high levels of adrenocortical stimulation, although some patients with inappropriately low cortisol responses for their level of ACTH stimulation warrant further investigation. Nineteen of the 36 patients whose ACTH responses were greater than 3 SD above the normal mean were classed as hyperresponders. Seven patients, who were tested more than once, had reproducible responses relative to those of the normal subjects. We conclude that ACTH hypersecretion after CRH-mediated stimuli, including naloxone, is an inherent, but variable, feature of DM, caused by expression of the genetic mutation at the anterior pituitary. The mechanism is probably a defect in the intracellular pathway initiated by CRH-receptor interaction as a result of abnormal levels of a cAMP-dependent kinase, DMPK, the product of the gene undergoing mutation in DM.

Androgen response to hypothalamic-pituitary-adrenal stimulation with naloxone in women with myotonic muscular dystrophy.

Myotonic muscular dystrophy (MMD) is a disease of autosomal dominant inheritance characterized by multisystem disease, including myotonia, muscle-wasting and weakness of all muscular tissues, and endocrine abnormalities attributed to a genetic abnormality causing a defective cAMP-dependent kinase. We have previously reported that MMD patients demonstrate ACTH hypersecretion after endogenous CRH release stimulated by naloxone administration while manifesting a normal cortisol (F) response. Additionally, others have reported a reduced adrenal androgen (AA) response to exogenous ACTH administration in MMD patients. As ACTH stimulates the secretion of both AAs and F, it is possible that the discordant relationship of these hormones in MMD patients results from a defect of adrenocortical ACTH receptor function or postreceptor signaling or subsequent biochemical events. Furthermore, the molecular abnormality seen in MMD patients may suggest that the mechanism underlying the frequently observed discordances in the secretion of glucocorticoids and AAs (e.g. adrenarche, surgical trauma, severe burns, or intermittent glucocorticoid administration) are explainable solely via an alteration in the function of the ACTH receptor or postreceptor signaling. To ascertain whether the responses of F and AAs to endogenous ACTH diverged in this disorder, we prospectively studied the responses of these hormones to naloxone-stimulated CRH release in nine premenopausal women with MMD and seven healthy age and weight-matched control women. After naloxone infusion (125 micrograms/kg, i.v.), blood sampling was performed at baseline (i.e. -5 min) and at 30 and 60 min. In addition to the absolute hormone level at each time, we calculated the net increment (i.e. change) at 30 and 60 min and the area under the curve (AUC) for F, ACTH, dehydroepiandrosterone (DHA), and androstenedione (A4). Consistent with our previous study, MMD patients demonstrated higher ACTH levels at all sampling times except [minud]5 min. AUC analysis revealed the ACTHAUC values were significantly higher in MMD than in control women (457 +/- 346 vs. 157 +/- 123 pmol/min.L; P < 0.03), whereas the FAUC response did not differ between MMD and controls (13860 +/- 3473 vs. 13375 +/- 3465 nmol/min.L; P > 0.5). Despite the greater ACTH secretion, the baseline circulating dehydroepiandrosterone sulfate levels were significantly lower in MMD compared with control women (18 +/- 23 vs. 61 +/- 23 mumol/L; P < 0.002). The serum concentrations of A4 at baseline, 30 min, and 60 min and DHA levels at 30 and 60 min were also significantly lower in MMD vs. control women. Additionally, the A4AUC and DHAAUC values were significantly lower in MMD patients than in controls. Furthermore, the net response of DHA at 60 min to the endogenous ACTH increase was also reduced in MMD patients compared with that in control subjects (2.3 +/- 2.1 vs. 5.6 +/- 2.6 nmol/L; P < 0.02). In conclusion, in addition to ACTH hypersecretion to CRH-mediated stimuli, these data suggest that MMD patients have a defect in the adrenocortical response to ACTH, reflected in normal F and reduced DHA and A4 secretion. Whether this defect is inherent to the disease or simply reflects adaptive changes to chronic disease remains to be demonstrated. However, it is possible that further studies of the response of MMD patients to ACTH may reveal a mechanism that explains the frequently observed dichotomy in the secretion of glucocorticoids and AAs.

Comparison of adrenocorticotropin (ACTH) stimulation tests and insulin hypoglycemia in normal humans: low dose, standard high dose, and 8-hour ACTH-(1-24) infusion tests.

The efficacy of the standard high dose ACTH stimulation test (HDT), using a pharmacological 250-microg dose of synthetic ACTH-(1-24), in the diagnosis of central hypoadrenalism is controversial. The insulin hypoglycemia test is widely regarded as the gold standard dynamic stimulation test of the hypothalamo-pituitary-adrenal (HPA) axis that provides the most reliable assessment of HPA axis integrity and reserve. Alternatively, a prolonged infusion of ACTH causes a continuing rise in plasma cortisol levels that may predict the adrenals' capacity to respond to severe ongoing stress. In nine normal subjects, we compared plasma ACTH and cortisol levels produced by three i.v. bolus low doses of ACTH-(1-24) (0.1, 0.5, and 1.0 microg/1.73 m2; LDTs) with those stimulated by hypoglycemia (0.15 U/kg insulin) and with the cortisol response to a standard 250-microg dose of ACTH-(1-24). The normal cortisol response to an 8-h ACTH-(1-24) infusion (250 microg at a constant rate over 8 h) was determined using three modern cortisol assays: a high pressure liquid chromatography method (HPLC), a fluorescence polarization immunoassay (FPIA), and a standard RIA. In the LDTs, stepwise increases in mean peak plasma ACTH were observed (12.4 +/- 2.0, 48.2 +/- 7.2, 120.2 +/- 15.5 pmol/L for the 0.1-, 0.5-, and 1.0-microg LDTs, respectively; P values all <0.0022 when comparing peak values between tests). The peak plasma ACTH level after insulin-induced hypoglycemia was significantly lower than that produced in the 1.0-microg LDT (69.6 +/- 9.3 vs. 120.2 +/- 15.5 pmol/L; P < 0.0002), but was higher than that obtained during the 0.5-microg LDT (69.6 +/- 9.3 vs. 48.2 +/- 7.2 pmol/L; P < 0.02). In the LDTs, statistically different, dose-dependent increases in peak cortisol concentration occurred (355 +/- 16, 432 +/- 13, and 482 +/- 23 nmol/L; greatest P value is 0.0283 for comparisons between all tests). The peak cortisol levels achieved during the LDTs were very different from those during the HDT (mean peak cortisol, 580 +/- 27 nmol/L; all P values <0.00009. However, the mean 30 min response in the 1.0-microg LDT did not differ from that in the HDT (471 +/- 22 vs. 492 +/- 22 nmol/L; P = 0.2). In the 8-h ACTH infusion test, plasma cortisol concentrations progressively increased, reaching peak levels much higher than those in the HDT [995 +/- 50 vs. 580 +/- 27 nmol/L (HPLC) and 1326 +/- 100 vs 759 +/- 31 nmol/L (FPIA)]. Significant differences in the basal, 1 h, and peak cortisol levels as determined by the three different assay methods (HPLC, FPIA, and RIA) were observed in the 8-h infusion tests. Similarly, in the HDTs there were significant differences in the mean 30 and 60 min cortisol levels as measured by HPLC compared with those determined by FPIA. We conclude that up to 30 min postinjection, 1.0 microg/1.73 m2 ACTH-(1-24) stimulates maximal adrenocortical secretion. Similar lower normal limits at 30 min may be applied in the 1.0-microg LDT and the HDT, but not when lower doses of ACTH-(1-24) are administered. The peak plasma ACTH level produced in the 1.0-microg LDT is higher than in the insulin hypoglycemia test, but is of the same order of magnitude. The peak cortisol concentration obtained during an 8-h synthetic ACTH-(1-24) infusion is considerably higher than that stimulated by a standard bolus 250-microg dose, potentially providing a means of evaluating the adrenocortical capacity to maintain maximal cortisol secretion. Appropriate interpretation of any of these tests of HPA axis function relies on the accurate determination of normal response ranges, which may vary significantly depending on the cortisol assay used.

Hypersensitivity of the hypothalamic-pituitary-adrenal axis to naloxone in post-traumatic stress disorder.

Naloxone, which increases endogenous corticotropin-releasing hormone (CRH) release by blocking an inhibitory opioidergic tone on the hypothalamic-pituitary-adrenal (HPA) axis, was administered in a dose-response protocol to seven healthy volunteers and 13 patients with treated posttraumatic stress disorder (PTSD). Six of the PTSD patients showed an increased hormonal response to the lowest naloxone dose (6 micrograms/kg) compared to both the control subjects and the other PTSD patients. This difference persisted on detailed subgroup analysis, although it was less marked at the highest naloxone dose (125 micrograms/kg). The responses of the other seven PTSD patients were indistinguishable from those of the control group. The greater responses of the six PTSD patients could not be explained on the basis of associated psychiatric illnesses or psychotropic drug therapy, and did not correlate with standard psychological testing or severity of PTSD. The results of this preliminary study therefore suggest that a hypersensitivity of the HPA axis to endogenous CRH stimulation may occur in PTSD.

The insulin hypoglycemia test: hypoglycemic criteria and reproducibility.

The insulin hypoglycemia test (IHT) is widely regarded as the "gold standard" for dynamic stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. This study aimed to investigate the temporal relationship between a rapid decrease in plasma glucose and the corresponding rise in plasma adenocorticotropic hormone (ACTH), and to assess the reproducibility of hormone responses to hypoglycemia in normal humans. Ten normal subjects underwent IHTs, using an insulin dose of 0.15 U/kg. Of these, eight had a second IHT (IHT2) and three went on to a third test (IHT3). Plasma ACTH and cortisol were measured at 15-min intervals and, additionally, in four IHT2s and the three IHT3s, ACTH was measured at 2.5- or 5-min intervals. Mean glucose nadirs and mean ACTH and cortisol responses were not significantly different between IHT1, IHT2 and IHT3. Combined data from all 21 tests showed the magnitude of the cortisol responses, but not the ACTH responses, correlated significantly with the depth and duration of hypoglycemia. All subjects achieved glucose concentrations of of < or = 1.6 mmol/l before any detectable rise in ACTH occurred. In the seven tests performed with frequent sampling, an ACTH rise never preceded the glucose nadir, but occurred at the nadir, or up to 15 min after. On repeat testing, peak ACTH levels varied markedly within individuals, whereas peak cortisol levels were more reproducible (mean coefficient of variation 7%). In conclusion, hypoglycemia of < or = 1.6 mmol/l was sufficient to cause stimulation of the HPA axis in all 21 IHTs conducted in normal subjects. Nonetheless, our data cannot reveal whether higher glucose nadirs would stimulate increased HPA axis activity in all subjects. Overall, the cortisol response to hypoglycemia is more reproducible than the ACTH response but, in an individual subject, the difference in peak cortisol between two IHTs may exceed 100 nmol/l.

Interactions between the stimulated hypothalamic-pituitary-adrenal axis and leptin in humans.

Leptin, produced by adipocytes, has homeostatic effects on body fat mass through inhibition of appetite and stimulation of the sympathetic nervous system. Several studies have reported that high-dose exogenous glucocorticoids increase circulating leptin concentrations in humans. Conversely, leptin has inhibitory effects on the hypothalamic-pituitary-adrenal (HPA) axis, both at the hypothalamic and adrenal levels. We hypothesized that acute hypercortisolism, in the physiological range, may not alter leptin secretion. Four stimuli of the HPA axis were administered to eight healthy male volunteers in a placebo-controlled study. On separate afternoons, in a randomised order, fasting subjects received i.v. injections of saline, naloxone (125 microg/kg); vasopressin (0.0143 IU/kg); naloxone and vasopressin in combination; or insulin (0.15 U/kg; a dose sufficient to induce hypoglycaemia). Plasma concentrations of adrenocorticotrophic hormone (ACTH), cortisol and leptin were measured before and for 120 min after the injection. The cortisol secretory response was greatest after insulin-hypoglycaemia, this response was significantly greater than that following naloxone, naloxone/vasopressin, or vasopressin alone. Despite the cortisol release, leptin concentrations were not increased after any stimulus. Insulin-hypoglycaemia was associated with a decrease in leptin concentration at 60 and 90 min, while naloxone did not alter leptin concentrations. However, basal leptin concentrations were positively correlated with integrated ACTH and cortisol responses to naloxone, but did not correlate with ACTH or cortisol responses to the other stimuli. Thus acute elevations of plasma cortisol, in the physiological range, do not appear to influence plasma leptin concentrations. The fall in plasma leptin concentration after insulin-induced hypoglycaemia may reflect catecholamine secretion after this stimulus.

Inhibition of naloxone-stimulated adrenocorticotropin release by alprazolam in myotonic dystrophy patients.

Myotonic dystrophy (DM) is an autosomal dominant disorder causing myotonia, progressive muscle weakness, and endocrine abnormalities including hypothalamic-pituitary-adrenal (HPA) axis hyperresponsiveness to CRH-mediated stimuli. This ACTH hyperresponsiveness appears directly related to the underlying genetic abnormality. Naloxone (Nal)-mediated CRH release causes ACTH release in normal humans and an ACTH hyperresponse in DM. Alprazolam (APZ) attenuates the ACTH release in response to Nal in normal individuals, probably by inhibiting CRH release. This study investigates the effects of APZ on Nal-induced HPA axis stimulation in DM. The ACTH response to Nal in DM subjects was significantly reduced by APZ. Despite this DM patients have a relative resistance to APZ inhibition of Nal-induced ACTH/cortisol release. APZ caused a smaller percentage reduction in AUC for ACTH in DM compared with controls. These findings provide further insight into the mechanism(s) of the HPA axis abnormalities in DM. In DM, there may be an increase in tonic opioid inhibition to CRH release with compensatory increases in stimulatory pathways. Alternatively, these patients may have a basal increase in pituitary vasopressin levels or an enhanced AVP/CRH synergistic mechanism at the level of the corticotroph.

CRH-mediated pituitary-adrenal responses are inhibited by nifedipine in humans.

Sustained CRH-stimulated ACTH release in vitro depends on Ca2+ influx and is inhibited 30-40%, but not delayed, by dihydropyridine Ca2+ channel blockers. In five normal humans, we found that nifedipine pretreatment reduced integrated ACTH responses to the CRH-mediated stimulus of fenfluramine by 28% and cortisol responses by 34%, results comparable with those from in vitro reports. Nifedipine did not alter the timing of peak hormonal responses. We conclude that (1) in humans, nifedipine inhibits ACTH release by fenfluramine by blocking Ca2+ influx via L-type channels in corticotrophs; (2) the magnitude of fenfluramine-stimulated CRH release is probably unaltered by nifedipine and (3) because the timing is unaltered, nifedipine does not affect the rate of CRH delivery to the corticotroph.

Polyneuropathy in Australian outpatients with type II diabetes mellitus.

In order to determine the local prevalence of polyneuropathy among adult outpatients with type II (non-insulin-dependent) diabetes mellitus, we applied a series of standardised measures to patients attending a multidisciplinary diabetes clinic. The study group comprised 94 men and 15 women; mean age, 70.6+/-7.8 years; mean duration of diabetes, 11.7+/-10.1 years; and mean HbA1c 8.3%+/-1.7%. Neuropathy Symptom Scores > or = 1 were present in 97% of patients (mean, 3+/-2; range, 0-12), and 95% had Neuropathy Disability Scores > or = 2 (mean, 27+/-19; range, 0-87). 52% of men reported impotence. Autonomic dysfunction on cardiovascular reflex testing was present in 46% of patients (39/84). Finger and toe vibration perception thresholds were greater than 3SD higher than mean thresholds measured in control subjects without diabetes in 43% and 58% of patients, respectively. Polyneuropathy, defined as lower limb sensory and motor nerve conduction velocity or latency outside mean +/-2 SD of that measured in age-matched controls, was present in 49% of patients (53/109). These results suggest that there is a high prevalence of polyneuropathy in Australian out-patients with type II diabetes mellitus. In this study, clinical assessment using Neuropathy Disability Scores was not diagnostically useful since only five patients had a normal score. Using nerve-conduction studies as the "gold standard" diagnostic criteria, the best alternative test for the presence of polyneuropathy was toe vibration perception threshold (sensitivity 74%, specificity 56%). In view of the emerging evidence that intensive glycaemic control reduces the rate of progression of polyneuropathy, we recommend that patients with type II diabetes mellitus have nerve-conduction studies performed for early detection of this important complication.

Use of aminohydroxypropylidene bisphosphonate (AHPrBP, "APD") for the treatment of hypercalcemia in patients with renal impairment.

Aminohydroxypropylidene bisphosphonate (AHPrBP, "APD") is a relatively new bisphosphonate which has been shown to be effective for control of hypercalcemia due to a variety of causes. Renal impairment has been reported following the use of other bisphosphonates and pre-existing renal impairment has been regarded as a contraindication to the use of AHPrBP. We report the successful use of intravenous AHPrBP to control hypercalcemia in three patients with renal impairment, one of whom was dialysis-dependent. No significant side effects were noted; in particular, there was no further deterioration in renal function. Intravenous AHPrBP may be a safe and effective agent for the control of hypercalcemia in patients with renal impairment.

L-type calcium channels and CRH-mediated ACTH and cortisol release in humans.

1. The effect of pretreatment with nifedipine on naloxone-stimulated corticotrophin-releasing hormone (CRH)-induced adrenocorticotrophin (ACTH) release in humans was investigated. The mean peak plasma ACTH and cortisol levels and the mean peak change in cortisol levels from basal were significantly lower in the nifedipine/naloxone test than in the naloxone alone test. The integrated areas under the ACTH-time and cortisol-time curves were reduced by 33 and 49%, respectively, in the nifedipine/naloxone test compared with the naloxone alone test. These results correlate well with published in vitro studies. 2. Acute administration of oral nifedipine partially inhibited naloxone-stimulated ACTH and cortisol release, probably by blockade of plasma membrane voltage-dependent L-type calcium channels normally activated following binding of CRH to pituitary corticotroph receptors. 3. Naloxone-induced CRH release may replace insulin hypoglycaemia testing of pituitary ACTH reserve in humans.

Naloxone-induced ACTH release in man is inhibited by clonidine.

1. Adrenergic mechanisms play an important role in regulation of ACTH release. We used the alpha 2-adrenergic agonist, clonidine, as a central nervous system inhibitor of ACTH release to see if it would alter naloxone-induced ACTH secretion in normal human volunteers. 2. There was a significant blunting of the mean peak level of ACTH and the mean peak change of ACTH from basal as well as the area under the ACTH-time curve when clonidine was given prior to naloxone. 3. We conclude that clonidine, by blocking central noradrenergic pathways, which stimulate corticotropin-releasing hormone secretion, inhibits naloxone-induced ACTH secretion. This suggests that naloxone causes ACTH release through these same central noradrenergic pathways.

Naloxone stimulation of ACTH secretion during petrosal sinus sampling in Cushing's syndrome.

1. Petrosal sinus sampling has been used to establish the source of adrenocorticotropin (ACTH) in ACTH-dependent Cushing's syndrome. Naloxone, an opioid antagonist, stimulates ACTH secretion, probably via release of endogenous hypothalamic corticotropin releasing hormone (CRH). 2. Three patients with hypercortisolism were studied. Two showed suppressed (> 50%) urinary-free cortisol excretion with high-dose dexamethasone treatment (2 mg every 6 h for 2 days), one did not suppress. The patients were subjected to bilateral simultaneous inferior petrosal sinus sampling (BSIPSS) with simultaneous peripheral venous (forearm) samples. Basal (unstimulated) samples were taken and naloxone (125 micrograms/kg bodyweight) was given intravenously with subsequent simultaneous sampling. Plasma ACTH was measured by radio-immunoassay (RIA). 3. All cases exhibited a marked rise in immunoreactive (IR)-ACTH levels (pmol/L) after naloxone injection, basal to peak: case 1, left 11.5-22.1, right 9.8 with no rise, peripheral 9.1-9.5; case 2, left 456-863, right 125-501, peripheral 59-82; case 3, left 12.7-13.0, right 277-431, peripheral 12.1-11.7. All results indicate pituitary Cushing's syndrome, with a central to peripheral ratio > 2.3:1. Pituitary Cushing's syndrome was confirmed on the results of trans-sphenoidal pituitary surgery in cases 1 and 3. 4. It is suggested that naloxone injection during petrosal sinus sampling in Cushing's syndrome may assist in the diagnosis of ACTH source, by enhancing ACTH release from a pituitary micro-adenoma.