Effects of pyridostigmine and naloxone on the abnormal TSH response to TRH during starvation in humans

BACKGROUND

Starvation is associated with a blunted TSH response to thyrotropin-releasing hormone (TRH) (peak minus baseline < 5 mIU/L), despite basal TSH and thyroid hormone levels within the normal range. In light of the inhibitory effect of somatostatin on TSH secretion, we examined whether this condition is caused by an increased hypothalamic somatostatinergic tone in starving subjects. The possible involvement of endogenous opioids in the mechanism underlying the abnormal TSH response to TRH was also evaluated.

METHODS

The TSH response to TRH (25 micrograms in an intravenous bolus), serum total and free T4 and T3 levels, and 24-hour urinary-free cortisol levels were measured in 28 normal men (age 27-35 years) within 10% of their ideal body weight. They were randomly divided into 4 groups of 7. In 21 subjects (groups 1, 2, and 3), TRH tests were performed after an overnight (8 hours) fast, placebo administrations (control test), and after prolonged (56 hours) starvation. TRH tests after prolonged starvation were performed either after placebos (in all subjects) or the administration of pyridostigmine (180 mg orally) (in 7 subjects, group 1); naloxone (0.8 mg in an i.v. bolus injection) (in 7 subjects, group 2); or the combination of pyridostigmine and naloxone (in 7 subjects, group 3). The remaining 7 subjects (group 4) were tested at weekly intervals with TRH plus placebo, TRH plus naloxone, TRH plus pyridostigmine, and TRH plus naloxone plus pyridostigmine after a fasting period of 8 hours.

RESULTS

In all subjects of groups 1, 2, and 3, TRH-induced TSH rise was significantly lower after prolonged starvation than after overnight fast. Neither pyridostigmine nor naloxone, given alone, changed the basal levels of TSH and the TSH response to TRH after prolonged starvation. In contrast, the concomitant administration of naloxone and pyridostigmine significantly enhanced the TRH-induced TSH rise. After overnight fasting, naloxone administration in group 4 subjects did not change the TSH response to TRH, whereas pyridostigmine significantly enhanced the TSH response to TRH. When naloxone was given together with pyridostigmine and TRH the TSH response was similar to that observed in the TRH plus pyridostigmine test.

CONCLUSIONS

These data indicate that naloxone-sensitive endogenous opioids exert an inhibitory effect on the cholinergic stimulatory control of TSH secretion during prolonged starvation. This suggests that an enhanced hypothalamic somatostatinergic activity is involved in the mechanism underlying the reduced TSH response to TRH.

Effect of nitric oxide on basal and TRH-or metoclopramide-stimulated prolactin release in normal men. volpi@ipruniv.cce.unipr.it

In order to establish whether nitric oxide (NO) is involved in the regulation of basal and/or TRH- or metoclopramide (MCP)-stimulated PRL secretion, normal male subjects were treated i.v. with the NO-synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) (40 mg/kg injected plus 50 mg/kg infused over 60 min) in basal conditions (N.7 subjects) or just before the PRL releasing hormone TRH (20 or 200 microg iv) (N.7 subjects) or the antidopaminergic agent MCP (1 or 10 mg iv) (N.7 subjects). In control experiments, subjects received normal saline instead of L-NAME. The administration of L-NAME modified neither the basal secretion of PRL, nor the PRL release induced by TRH (20 or 200 microg) or MCP (1 or 10 mg). These data suggest that in humans, NO is not involved in the control of PRL release at the anterior pituitary level.

Alcoholism abolishes the gamma-aminobutyric acid (GABA)ergic control of GH secretion in humans

In order to establish possible alterations in the gamma-aminobutyric acid (GABA)ergic control of growth hormone (GH) secretion in alcoholics, 800 mg sodium valproate (a drug enhancing endogenous GABA activity), 10 mg baclofen (a GABA(B) receptor agonist) or a placebo were given orally to nine normal men (age 38-48 years) and nine 2-4-week-abstinent alcoholics (age 35-50 years; duration of alcohol consumption 3-6 years). Blood samples for GH assay were taken every 30 min for the next 150 min. Both drugs induced a significant increment in serum GH levels in the normal controls; mean peak was 7.2 and 3.27 times higher than baseline after sodium valproate and baclofen, respectively. In contrast, GH secretion in alcoholic patients did not change after baclofen or sodium valproate administration. Placebo administration did not modify GH secretion in any subject. In concurrence with previous reports showing alterations of GABAergic neurotransmission in the central nervous system of patients affected by alcoholism, these data show the loss of the GABAergic mechanism(s) underlying the GH response to sodium valproate and/or baclofen action in alcoholism.

Influence of residual insulin secretion and duration of diabetes mellitus on the control of luteinizing hormone secretion in women

BACKGROUND

The aim of the present study was to establish whether the persistence of residual beta-cell activity after long-term diabetes mellitus (DM) exerts a protective role on luteinizing hormone (LH) secretion.

METHODS

The LH responses to stimulation with gonadotropin-releasing hormone (Gn-RH) (100 microg in an i.v. bolus) or naloxone (4 mg injected in an i.v. bolus, followed by the constant infusion of 8 mg in 2 h) were measured in C-peptide-positive (CpP) and C-peptide-negative (CpN) normally menstruating women with short-term (group 1 < 3 years, CpP n = 11, CpN n = 11) or long-term (group 2 > 10 years, CpP n = 11, CpN n = 11) DM and in age-matched normal control subjects (n = 11).

RESULTS

Gn-RH induced significant increments in LH secretion in all groups. Significant LH responses to naloxone were observed in all groups, except in group 2 CpN patients. However, the LH response to either Gn-RH or naloxone was significantly lower in group 1 CpN, group 2 CpP and group 2 CpN patients than in the normal control subjects. Furthermore, the LH response was significantly lower in group 2 CpP than in group 1 CpP patients and in group 2 CpN than in group 1 CpN subjects.

CONCLUSIONS

These results indicate a role for both deficiency in residual endogenous insulin secretion and duration of diabetes in the derangement of LH secretory control. The data suggest that the protective role exerted by residual beta-cell activity on LH secretion during the early years of DM diminishes with time elapsed after the onset of diabetes mellitus.

Oxytocin enhances the prolactin response to vasoactive intestinal polypeptide in healthy women

OBJECTIVE

To establish whether oxytocin modifies the stimulatory effect of vasoactive intestinal polypeptide (VIP) on prolactin secretion in healthy women.

DESIGN

Controlled clinical study.

SETTING

Healthy women in an academic research environment.

PATIENT(S)

Seven healthy women (aged 24-32 years) were tested on the 22nd day of two consecutive normal menstrual cycles.

INTERVENTION(S)

Vasoactive intestinal polypeptide (4 pmol x kg(-1) x min(-1) in 50 mL of normal saline infused i.v. for 60 minutes) was administered in either the presence or absence of concurrent treatment with oxytocin (2 IU injected plus 0.07 IU/min infused for 60 minutes).

MAIN OUTCOME MEASURE(S)

Serum prolactin levels.

RESULT(S)

The administration of VIP induced a significant increase in serum prolactin levels, with a mean peak response 1.6 times higher than baseline at 45 minutes after injection. In the presence of oxytocin, the prolactin response to VIP was significantly higher, with a mean peak response 2 times higher than baseline.

CONCLUSION(S)

These data suggest that in healthy women, oxytocin facilitates the regulation of the stimulating effect of VIP on prolactin secretion.

Involvement of nitric oxide in vasoactive intestinal peptide-stimulated prolactin secretion in normal men

To establish whether nitric-oxide (NO) participates in the regulation of prolactin (PRL) secretion in humans in basal conditions and/or under stimulation with vasoactive intestinal polypeptide (VIP), seven normal men were treated with a placebo (normal saline) or the NO synthase (NOS) inhibitor L-NAME (40 microg/kg injected plus 50 microg/kg infused intravenously over 60 minutes), which in previous studies has been found able to modify other pituitary hormone secretions. Experiments were performed either in basal conditions or during stimulation of PRL secretion with an intravenous infusion of VIP (4 pmol/kg min over 60 minutes). The administration of L-NAME was unable to change the basal secretion of PRL. In contrast, L-NAME significantly enhanced the PRL increase induced by VIP. These data argue against an involvement of NO in regulation of basal PRL secretion. In contrast, the stimulatory effect of L-NAME on VIP-induced PRL secretion suggests that NO exerts an inhibitory control of the PRL response to VIP.

Arginine vasopressin and oxytocin responses to angiotensin II are mediated by AT1 receptor subtype in normal men

This study was performed to determine whether the stimulatory effect of plasma angiotensin II (ANG II) on arginine vasopressin (AVP) and oxytocin (OT) secretion in humans is mediated by AT1 subtype receptors. For this purpose, the effects of the AT1 receptor antagonist losartan (50 mg orally) or a placebo on the AVP and OT responses to ANG II (intravenous infusion for 60 minutes of successively increasing doses of 4, 8, and 16 ng/kg min; each dose for 20 minutes) administration were evaluated in seven normal men. In additional experiments, the same subjects were tested with losartan (50 mg orally) alone or placebo alone. Neither losartan nor placebo given alone modified the basal levels of AVP and OT. ANG II infusion induced significant increments in both serum AVP and OT levels (mean peaks were 1.55 and 1.41 times higher than baseline, respectively). Both hormonal responses to ANG II were completely abolished by pretreatment with losartan. These data provide evidence of AT1 receptor involvement in mediation of the ANG II-stimulating effect on AVP and OT secretion.

Melatonin inhibits oxytocin response to insulin-induced hypoglycemia, but not to angiotensin II in normal men

In order to establish whether melatonin alters basal and/or stimulated oxytocin secretion, 18 normal men were treated (p.o.) with 6 or 12 mg melatonin or placebo in basal conditions (N-6 subjects) or concomitantly to the administration of insulin (O.15 IU/kg body weight in an i.v. bolus) (N-6 subjects) or angiotensin II (increasing doses of 4, 8 and 16 ng/kg/min, at intervals of 20 min). The administration of 6 or 12 mg melatonin did not change basal and angiotensin II-stimulated oxytocin secretion. In contrast, the oxytocin response to insulin-induced hypoglycemia was significantly reduced by melatonin treatment. In fact, the mean peak oxytocin response to hypoglycemia was 2.2 times higher than baseline in the absence of melatonin, whereas it was 1.6 times higher than basal value after administration of 6 or 12 mg melatonin. These data indicate an involvement of melatonin in the regulation of the oxytocin response to hypoglycemia in normal men. The lack of effects of melatonin on basal and angiotensin II-stimulated oxytocin secretion argues against the possibility that melatonin exerts an overall modulatory role on oxytocin secretion in humans.

Different effects of naloxone on the growth hormone response to melatonin and pyridostigmine in normal men

The effect of melatonin (MEL) (12 mg orally), pyridostigmine (60 mg orally), the combination of MEL and pyridostigmine, or placebo on growth hormone (GH) secretion was tested in seven normal men. In addition, MEL tests and pyridostigmine tests were repeated after pretreatment with naloxone (1.2-mg bolus followed by intravenous [i.v.] infusion of 1.6 mg/h for 3 hours). Serum GH levels increased fivefold after MEL and sixfold after pyridostigmine administration. The concomitant administration of MEL did not change the GH response to pyridostigmine. In the presence of naloxone, the GH response to MEL was completely abolished, whereas naloxone did not modify the pyridostigmine-induced GH increase. These data suggest that MEL and pyridostigmine stimulate GH secretion through a common mechanism, which is probably represented by the inhibition of somatostatin activity. However, in contrast to pyridostigmine, the action of MEL appears to be exerted through a naloxone-sensitive opioid mediation.

Effect of residual endogenous insulin secretion on the abnormal oxytocin response to hypoglycaemia in insulin-dependent diabetics

OBJECTIVES

Arginine-vasopressin (AVP) and oxytocin (OT) secretions are abnormally stimulated by hypoglycaemia in patients with IDDM. Since previous studies showed that AVP secretion is influenced by the persistence of residual endogenous insulin secretion, we wondered whether this factor also regulates OT secretion.

DESIGN

Case-control study: the OT response to insulin-induced hypoglycaemia was measured in normal and diabetic patients with or without residual endogenous insulin secretion.

SUBJECTS

Ten normal male subjects, 10 C-peptide positive (CpP) and 11 C-peptide negative (CpN) male diabetic patients.

PRELIMINARY STUDIES

plasma C-peptide levels were measured after intravenous administration of 1 mg glucagon. Insulin tolerance test (ITT): diabetics were studied after optimization of their metabolic status by 3 days of treatment with constant subcutaneous insulin infusion. CpP and CpN diabetics and normal controls were tested with an intravenous administration of 0.15 IU per kg body weight insulin. Blood samples for OT assay were taken just before the rapid injection of insulin (time 0) and at time 15, 30, 45 and 60 min.

RESULTS

The basal concentrations of OT were similar in all groups. Insulin induced a similar hypoglycaemic nadir in all groups at 30 min, even though diabetic groups showed a delayed recovery in blood glucose levels. The glycaemic pattern was similar in all diabetic patients. Hypoglycaemia-induced OT rise was significantly higher in the two diabetic groups than in the normal group. However, CpN patients showed significantly higher OT increments than CpP subjects.

CONCLUSIONS

These data indicate that a residual endogenous insulin secretion exerts a partial protective action against the hypothalamic-pituitary disorder affecting the OT secretory system in IDDM.

Alcoholism abolishes the effects of melatonin on growth hormone secretion in humans

Chronic alcohol consumption profoundly affects hypothalamic-pituitary function. The present study was performed in order to establish whether alcoholism modifies the effects of melatonin (MEL) on the neuroendocrine control of growth hormone (GH) secretion. For this purpose, the effects of oral administration of 12 mg MEL or placebo on basal and hypoglycemia-stimulated GH secretion were tested in nine (40-52-year-old) alcoholic men after 10-31 days of abstinence and in nine age- and weight-matched normal controls. Hypoglycemia was induced with an intravenous bolus injection of 0.15 IU/kg body weight of insulin. MEL but not placebo administration induced a small, but significant increase in basal GH secretion in the normal controls. In contrast, neither MEL nor placebo treatment significantly changed the basal serum GH levels in alcoholic men. Both groups showed a similar hypoglycemic pattern after insulin administration. The GH response to insulin-induced hypoglycemia was significantly lower in alcoholic than in normal subjects. MEL administration significantly reduced hypoglycemia-induced GH rise in the normal controls, but not in alcoholic patients. These data show that alcoholism not only reduces the GH response to insulin-induced hypoglycemia, but also abolishes MEL actions on basal and hypoglycemia-stimulated GH secretion.

Stimulation of ACTH and GH release by angiotensin II in normal men is mediated by the AT1 receptor subtype

This study was performed in order to determine whether the stimulatory effect of plasma angiotensin II (ANG II) on Adrenocorticotropic hormone (ACTH) and growth hormone (GH) secretion in humans is mediated by AT1 subtype receptors. For this purpose, the effects of the administration of the AT1 receptor antagonist, losartan (50 mg p.o.) or a placebo on the ACTH and GH responses to ANG II (i.v. infusion for 60 min of successively increasing doses (4, 8 and 16 ng/kg/min); each dose for 20 min) were evaluated in eight normal men. ANG II infusion induced significant increases in both serum ACTH and GH levels (mean peaks were 1.6- and four-times higher than baseline, respectively). The ACTH response to ANG II was completely abolished by pretreatment with losartan. Also, the ANG II-induced GH rise was reduced by administration of losartan, but the GH response was still significantly higher than the basal value (mean peak was twice as high as the baseline). These data provide evidence of AT1 receptor involvement in mediation of the ANG-II stimulating effect on ACTH and GH secretion.

Effect of melatonin on arginine vasopressin secretion stimulated by physical exercise or angiotensin II in normal men

The present study was undertaken in order to establish the possible involvement of melatonin in the mechanisms underlying the arginine-vasopressin (AVP) responses to physical exercise and angiotensin II (ANG II). On two mornings at least 1 week apart, normal male subjects were tested with exercise on a bicycle ergometer (the workload was gradually increased at 3-min intervals until exhaustion and lasted about 15 min in all subjects) or ANG II (60-min infusion of ANG II (Asp 1, IIe 5 angiotensin II) dissolved in 5% glucose in successively increasing doses of 4, 8, 16 ng/kg/min; each dose for 20 min). Tests were carried out with the administration of either 6 mg melatonin or placebo. Melatonin treatment neither modified the basal concentrations of AVP nor changed the AVP response to ANG II. In contrast, plasma AVP levels rose 3.6 times during exercise in the absence of melatonin, but only 2.3 times in the presence of melatonin. These data indicate an involvement of melatonin in the mechanism underlying the AVP response to physical exercise, but not ANG II, in normal men.

5-HT3 serotonergic receptor mediation of hypoglycemia-induced arginine-vasopressin but not oxytocin secretion in normal men

The present study was undertaken in order to establish the possible involvement of 5-HT3 serotonergic receptors in the control of basal and/or hypoglycemia-stimulated arginine vasopressin (AVP) and/or oxytocin (OT) secretion. For this purpose, 12 normal men were injected intravenously with a bolus of 4 mg ondansetron, a specific 5-HT3 receptor antagonist, under basal conditions (n = 6) or 30 min before insulin (0.15 IU/kg body weight) administration (n = 6) (insulin tolerance test (ITT)). Control experiments with normal saline instead of ondansetron treatment were performed. Furthermore, on a different occasion, the same subjects were tested in identical experimental conditions with 8 mg ondansetron. Our results showed that the hypoglycemic response to insulin was similar during the ITT and ondansetron plus ITT. Inhibition of 5-HT3 serotonergic receptors with ondansetron (4 or 8 mg) did not modify the basal secretion of AVP and OT and the OT response to insulin-induced hypoglycemia. In contrast, the administration of 4 or 8 mg ondansetron significantly reduced in a similar manner hypoglycemia-induced AVP rise. Mean peak level at 45 min after insulin injection was 2.25 times higher than baseline in the control ITT and 1.5 times higher than basal value in the ondansetron (4 or 8 mg) plus ITT. These data demonstrate that 5-HT3 serotonergic receptors at least partially mediate the AVP response to hypoglycemia, without modifying the simultaneous OT response. On the other hand, 5-HT3 receptors do not appear to be involved in the control of basal posterior pituitary hormone secretions.

Different effects of pyridostigmine on the thyrotropin response to thyrotropin-releasing hormone in endogenous depression and subclinical thyrotoxicosis

Primary organic disorders of the thyroid gland must be excluded in interpreting the thyrotropin (TSH)-releasing hormone (TRH) test in affective disease. Both endogenous depression and subclinical thyrotoxicosis are frequently associated with low basal TSH levels and a blunted (<5 mIU/L) TSH response to TRH despite thyroid hormone levels within the normal range. The present study was performed to establish whether a reduction of the hypothalamic somatostatinergic tone by treatment with the acetylcholinesterase inhibitor pyridostigmine before TRH might be useful to distinguish endocrine from affective diseases. Twelve male depressed patients (aged 41.4 +/- 3.1 years) and 12 men (aged 43.4 +/- 4.1 years) with subclinical thyrotoxicosis because of autonomous thyroid nodules were selected according to the presence of a low basal TSH level and a blunted TSH response to 200 microg TRH intravenously (IV) (TSH increment was <5 mIU/L at 30 minutes [peak] after TRH) but thyroid hormone levels within the normal range. All patients were tested again with TRH 60 minutes after treatment with 180 mg pyridostigmine orally. Eleven normal men served as controls. Basal TSH levels were 0.2 +/- 0.2 mIU/L (mean +/- SE) in depression and 0.1 +/- 0.2 in subclinical thyrotoxicosis (normal controls, 1.4 +/- 0.3). In both groups, the mean peak response to TRH was significantly higher than baseline; however, according to selection, the TSH increase was less than 5 mIU/L. Pyridostigmine did not change basal TSH levels in any group, but significantly enhanced the TRH-induced TSH increase in normal controls and in depressed subjects (TSH increment became >7 mIU/L in all depressed subjects). In contrast, no significant change in the TSH response to TRH was observed in subclinical thyrotoxicosis after pyridostigmine treatment. Basal and TRH- and pyridostigmine + TRH-induced TSH levels were significantly higher in the normal controls than in the other groups. These data show a cholinergic involvement in the blunted TSH response to TRH in patients with endogenous depression, but not in subjects with subclinical thyrotoxicosis, suggesting that these diseases could be separated on the basis of the pyridostigmine + TRH-induced TSH response test.

Beta-endorphin, adrenocorticotropic hormone and cortisol secretion in abstinent alcoholics

The circadian secretion of beta-endorphin, adrenocorticotropic hormone (ACTH), and cortisol was evaluated in 14 non-cirrhotic alcoholic men after 7 and 28 days of abstinence and in 12 sex- and age-matched normal subjects. A significant decrease in plasma levels of beta-endorphin, reduced ACTH levels, and increased cortisol levels were observed in samples taken at 08.00 h, 12.00 h, 18.00 h, and 23.00 h both after 7 and 28 days of abstinence. These data suggest the presence of a strong negative feedback on pro-opiomelanocortin synthesis by cortisol hypersecretion in abstinent alcoholics, which might be due to long-term stimulation of adrenal function by alcohol. The decreased plasma beta-endorphin levels might predispose to relapse in alcohol abuse.

Different control mechanisms of growth hormone (GH) secretion between gamma-amino- and gamma-hydroxy-butyric acid: neuroendocrine evidence in Parkinson's disease

The observation that baclofen stimulates growth hormone (GH) secretion in normal men, but not in parkinsonian patients led us to test the GH releasing effect of other gamma-amino-butyric acid (GABA)ergic agents with different mechanisms of action in Parkinson's disease. For this purpose 10 normal men and 10 de novo parkinsonian patients were tested with sodium valproate (800 mg PO), gamma-hydroxybutyric acid (GHB) (25 mg/kg body weight PO) and baclofen (10 mg PO). All drugs induced a significant increment in serum GH levels in the normal controls. On the other hand, GH secretion in parkinsonian patients did not change after baclofen or sodium valproate administration, whereas it showed normal responsiveness to GHB. These data suggest that the mechanism underlying the GH response to GHB is different from that (or those) mediating sodium valproate and/or baclofen action. In addition, the former, but not the latter mechanism appears to be preserved in the parkinsonian brain.

Hormonal (ACTH, cortisol, beta-endorphin, and met-enkephalin) and cardiovascular responses to hyperthermic stress in chronic alcoholics

Chronic alcohol drinking causes profound alterations in hypothalamic-pituitary function. In the present study, endocrine [corticotropin (ACTH), beta-endorphin, cortisol, and met-enkephalin] and cardiovascular (blood pressure) changes in response to hyperthermic stress (sauna at 90 degrees C for 30 min) were evaluated in 25 normal men (25 to 50 years old) and in 48 male alcoholic subjects (34 to 56 years old) after 5 weeks of abstinence. Significantly lower increments in systolic blood pressure were observed in alcoholics than in control subjects. Furthermore, alcoholics showed lower ACTH, beta-endorphin, and cortisol increments in response to sauna than normal controls. In contrast, sauna-induced hyperthermia did not change significantly the circulating met-enkephalin levels in either normal controls or chronic alcoholics. These data suggest that an impairment in the adaptive response to stress affects alcoholic men even after a few weeks of abstinence from alcohol.

Effect of pyridostigmine on the thyroid-stimulating hormone response to thyrotropin-releasing hormone in abstinent alcoholics

Alcoholism is sometimes associated with a blunted thyroid-stimulating hormone (TSH) response to thyrotropin-releasing hormone (TRH; peak minus baseline < 5 mIU/liter), despite basal TSH and thyroid hormone levels within the normal range. In light of the inhibitory effect of somatostatin on TSH secretion, we examined whether this condition is caused by an increased hypothalamic somatostatinergic tone in alcoholic subjects. To answer this question, 16 euthyroid male alcoholics (aged 38 to 50 years) with normal [n = 8; normal responder alcoholics (NRAs)] or blunted [n = 8; low responder alcoholics (LRAs)] TSH response to TRH were selected in a preliminary TRH test (200 micrograms in an intravenous bolus). In addition, 8 age- and weight-matched normal men were tested with TRH and used as normal controls (NCs). NCs and alcoholic patients showed similar basal serum TSH, free triiodothyronine, and free thyroxine levels. All subjects were tested again with TRH, 60 min after treatment with 180 mg of pyridostigmine orally. According to selection criteria, NCs and NRA groups showed similar TSH responses to TRH, whereas TRH-induced TSH rise was strikingly lower in LRAs than in NCs and the NRA group. Pyridostigmine did not change the basal levels of TSH in any group, whereas it enhanced in a similar manner the TRH-induced TSH rise in the NC and NRA groups. No significant change in the TSH response to TRH was observed in LRA patients after pyridostigmine treatment. These data argue against the possibility that an enhanced somatostatinergic tone is responsible for the blunted TSH response to TRH observed in some alcoholic patients. This phenomenon might be attributed to alcohol-related defects of stimulated pituitary thyrotroph secretory capacity in some individuals, possibly due to genetic vulnerability and/or the toxic effects of prolonged alcohol abuse.

Influence of residual C-peptide secretion on nocturnal serum TSH peak in well-controlled diabetic patients

OBJECTIVE

Several alterations in hypothalamo-pituitary-thyroid (HPT) function have been described in diabetes mellitus and have been attributed to metabolic decompensation. The present study was performed in order to establish whether residual endogenous insulin secretion in patients with insulin-dependent diabetes mellitus (IDDM) may play a role in the control of HPT function.

DESIGN

The nocturnal (2230 h-0200 h) serum TSH surge, the TSH response to TRH (200 microgram as an i.v. bolus) and serum free thyroid hormone levels were evaluated in C-peptide positive (CpP) (subjects with residual detectable endogenous pancreatic beta-cell activity) and C-peptide negative (CpN) patients both before and after optimization of metabolic status by 3 days of treatment with continuous subcutaneous insulin infusion, and in normal controls. TSH response to TRH and serum free thyroid hormone levels were assessed in the morning.

SUBJECTS

Twenty male diabetic patients hospitalized to achieve a better control of hyperglycaemia were subdivided into 10 CpP (age: 33 +/- 1.5 years (mean +/- SE); body mass index (BMI): 22.6 +/- 0.3) and 10 CpN (age: 32 +/- 1.7 years; BMI: 22.5 +/- 0.4) patients. Nine normal men (age: 34.0 +/- 1.2 years; BMI: 23.1 +/- 0.4) served as controls.

MEASUREMENTS

The nocturnal serum TSH peak was measured by dividing the highest night-time TSH value by the next morning TSH value and then multiplying by 100. Serum TSH levels were measured in samples taken just before (time 0) and 30 minutes, after TRH administration. Serum free thyroid hormone levels were measured in samples taken at time 0 of the TRH test.

RESULTS

Before improvement of hyperglycaemia, CpP and CPN patients showed similar alterations in HPT function; i.e. serum free T3 levels and TSH responses to TRH were lower than normal; the nocturnal TSH surge was absent. Correction of hyperglycaemia normalized all examined HPT parameters in CpP diabetics, whereas normalization in serum free T3 levels and pituitary TSH responsiveness to TRH in CpN patients was not accompanied by restoration of the nocturnal TSH peak.

CONCLUSIONS

These data indicate that the absence of residual pancreatic beta-cell function in patients with insulin-dependent diabetes mellitus is associated with neuroendocrine dysfunction in the regulation of circadian TSH secretion, which is not reversible after restoration of good glycaemic control.

Effect of melatonin on hypoglycemia and metoclopramide-stimulated arginine vasopressin secretion in normal men

The present study was performed in order to establish whether melatonin (MEL) plays a role in the regulation of arginine vasopressin secretion (AVP) in normal human subjects. For this purpose, the effects of an oral administration of 6 or 12 mg MEL on basal and metoclopramide (MCP)- or hypoglycemia-stimulated AVP secretion was tested in 18 normal men. MCP was given at a dose of 20 mg as an intravenous (i.v.) bolus; hypoglycemia was induced with an i.v. bolus injection of 0.15 IU/kg body weight of insulin. In addition, in view of the well-known inhibitory effect of MEL on the growth hormone (GH) response to hypoglycemia, GH levels were measured during the insulin tolerance test (ITT), as an independent index of MEL activity. MEL did not produce any change in AVP secretory patterns in basal conditions or during the MCP test. In contrast, the mean peak AVP response to hypoglycemia was 2.33 times higher than baseline in the control ITT, whereas it was only 1.77 times higher than baseline in the ITT plus MEL tests. Also, the GH response to hypoglycemia was significantly lower in the presence than in the absence of MEL. For both AVP and GH, the inhibitory effect of MEL during ITT was similar, when either 6 or 12 mg MEL was given. These data indicate an involvement of MEL in the control of the AVP response to hypoglycemia, but not of basal and MCP-induced AVP secretion. In addition, the similar effects of MEL on GH and AVP secretions during ITT suggest that similar neuroendocrine mechanisms underlie these hormonal responses to hypoglycemia.