Pyridostigmine partially reverses dexamethasone-induced inhibition of the growth hormone response to growth hormone-releasing hormone

Pyridostigmine Bromide (PYR) Alters Immune Function in B6C3F1 Mice

Pyridostigmine bromide (PYR) is an anticholinesterase drug indicated for the treatment of myasthenia gravis and neuromuscular blockade reversal. It acts as a reversible cholinesterase inhibitor and was used as a pretreatment for soldiers during Operation Desert Storm to protect against possible nerve gas attacks. Since that time, PYR has been implicated as a possible causative agent contributing to Gulf War Illness. PYR's mechanism of action has been well-delineated with regards to its effects on the nervous system, yet little is known regarding potential effects on immunological function. To evaluate the effects of PYR on immunological function, adult female B6C3F1 mice were gavaged daily for 14 days with PYR (0, 1, 5, 10, or 20 mg/kg/day). Immune parameters assessed were lymphoproliferation, natural killer cell activity, the SRBC-specific antibody plaque-forming cell (PFC) response, thymus and spleen weight and cellularity, and thymic and splenic CD4/CD8 lymphocyte subpopulations. Exposure to PYR did not alter splenic and thymus weight or splenic cellularity. However, 20 mg PYR/kg/day decreased thymic cellularity with decreases in both CD4+/CD8+ (20 mg/kg/day) and CD4-/CD8- (10 and 20 mg/kg/day) cell types. Functional immune assays indicated that lymphocyte proliferative responses and natural killer cell activity were normal; whereas exposure to PYR significantly decreased primary IgM antibody responses to a T-cell dependent antigen at the 1, 5, 10 and 20 mg/kg treatment levels for 14 days. This is the first study to examine the immunotoxicological effects of PYR and demonstrate that this compound selectively suppresses humoral antibody responses.

Impaired growth hormone secretion in fibromyalgia patients: evidence for augmented hypothalamic somatostatin tone

OBJECTIVE

To determine whether female fibromyalgia (FM) patients exhibit a normal growth hormone (GH) response to an acute exercise stressor, and to assess the importance of somatostatin tone in the generation of this GH response.

METHODS

Twenty female FM patients were compared with 10 healthy female controls. All subjects exercised to volitional exhaustion on a treadmill. A standard metabolic cart was used to monitor pulse, blood pressure, electrocardiography, oxygen uptake, carbon dioxide output, anaerobic threshold, and maximal workload. Blood was drawn for GH and cortisol measurements 1 hour before exercise, immediately before exercise, immediately after exercise, and 1 hour after exercise. One month later, testing that was exactly similar was performed, except all subjects were given pyridostigmine bromide (Mestinon; 30 mg orally) 1 hour before exercise.

RESULTS

Compared with controls, FM patients failed to exhibit a GH or cortisol response to acute exercise (P = 0.003). After administration of pyridostigmine, 1 hour before exercise, the GH levels of FM patients increased 8-fold (P = 0.001), to a value comparable with that of controls. Pyridostigmine did not increase the cortisol response to exercise in FM patients. Pyridostigmine alone did not stimulate GH secretion in FM patients, nor did it improve exercise-induced GH secretion in controls. FM patients with normal insulin-like growth factor 1 (IGF-1) levels had an impaired GH response to exercise.

CONCLUSION

Three new findings are reported: 1) FM patients have a reduced GH response to exercise, 2) pyridostigmine reverses this impaired response, and 3) defective GH secretion in FM can occur in patients with normal IGF-1 levels. Because pyridostigmine is known to reduce somatostatin tone, it is surmised that the defective GH response to exercise in FM patients probably results from increased levels of somatostatin, a hypothalamic hormone that inhibits GH secretion.

Evaluation of immunotoxicity induced by single or concurrent exposure to N,N-diethyl-m-toluamide (DEET), pyridostigmine bromide (PYR), and JP-8 jet fuel

Approximately 5,000 to 80,000 of the US service personnel involved in the Persian Gulf War have complained of a variety of nonspecific symptoms since their return in 1991. These symptoms have been collectively labeled Gulf War Illness and include muscle fatigue, general malaise, myalgia, impaired cognition, ataxia, headaches, fever, joint pain, skin rash, gastrointestinal disturbances, sleep disturbances, and respiratory difficulties. Exposures of military and service personnel were diverse and included the prescribed anti-nerve gas agent pyridostigmine bromide (PYR), N.N-diethyl-m-toluamide (DEET) insect repellent, and environmental exposures to jet fuel. Thus, studies in our laboratory were undertaken to determine if concurrent exposure to these agents, singly or in combination, would contribute to significant alterations in immunological function and disease susceptibility. To assess immune status, eight-week old B6C3F1 female mice were exposed for 14 days to single compounds or tertiary mixtures of 15.5 mg/kg DEET, 2 mg/kg PYR, and 500 mg/kg JP-8 (termed low dose), or 31 mg/kg DEET, 5 mg/kg PYR, and 1,000 mg/kg JP-8 (termed high dose). Immunosuppression was assessed 24 h after the last exposure. No remarkable alterations were evident in hematological parameters, spleen and thymus organ weight and total cellularity, natural killer (NK) cell activity, cytotoxic T-cell activity, or mitogen-induced lymphocyte proliferation after exposure to either single or tertiary mixtures at low or high doses. A few changes in CD4/CD8 flow cytometric lymphocyte subpopulations were detected after exposure to the tertiary mixture at the high dose. Delayed type hypersensitivity (DTH) was decreased by 88% after exposure to the high-dose mixture, and suppression of antibody-specific IgM immune responses (plaque-forming cell, PFC) occurred after exposure to all single and tertiary mixtures at both dose levels. In the PFC response, antagonism was apparent in the mixture, while coexposure to these agents resulted in a synergistic effect in the DTH response. Susceptibility to B16F10 tumor or Listeria monocytogenes challenge was not affected after single or tertiary exposures. These data suggest that combined exposure to DEET, PYR, and JP-8 does not profoundly alter many immunological endpoints, but does selectively target functional endpoints such as the PFC and DTH response. This should be considered when assessing human health risks in the military environment.

Enhanced growth hormone responses to pyridostigmine challenge in patients with panic disorder

BACKGROUND

Panic disorder is associated with neuroendocrinological abnormalities, some of which overlap with those seen in major depression. To date, there has been little assessment of the role of cholinergic mechanisms in this disorder.

METHOD

Sixteen patients with DSM-III-R panic disorder and an age and gender-matched comparison group were administered 120 mg of the acetylcholinesterase inhibitor pyridostigmine. Growth hormone (GH) responses over a three-hour period were monitored.

RESULTS

Mean delta GH, the difference between basal and the maximum pyridostigmine levels, was significantly greater in patients with panic disorder than in the comparison group.

CONCLUSIONS

This may reflect increased cholinergic responsivity in panic disorder.

Effect of glucocorticoids on the paradoxical growth hormone response to thyrotropin-releasing hormone in patients with acromegaly

It has been hypothesized that in acromegalic patients, as well as in normal subjects, acute increases in serum cortisol levels may cause an enhancement of hypothalamic somatostatin secretion, which in turn may be responsible for the glucocorticoid-mediated growth hormone (GH) inhibition. The aim of this study was to investigate short-term effects of an intravenous (i.v.) infusion of hydrocortisone on the GH response to thyrotropin-releasing hormone (TRH) in acromegaly. We studied six adult patients with active acromegaly. The group was composed of four women and two men with a mean age of 55.8 +/- 6.4 years (range, 27 to 68) and a mean body mass index of 26.7 +/- 1 kg/m2 (range, 23.3 to 30). All patients underwent the following treatments: (1) hydrocortisone alone: a bolus i.v. injection of hydrocortisone succinate 100 mg in 2 mL saline at time -60 minutes, followed by a 120-minute i.v. infusion of hydrocortisone succinate 250 mg in 250 mL saline from -60 to 60 minutes; (2) TRH+hydrocortisone: a bolus i.v. injection of TRH 200 micrograms 60 minutes after initiation of a 2-hour hydrocortisone infusion; (3) TRH alone: a bolus i.v. injection of TRH at time 0, 60 minutes after initiation of a 2-hour saline infusion. In all six patients, TRH induced large GH increases (absolute peak GH level, 58.1 +/- 23.2 micrograms/L; maximum % GH change with respect to baseline, 1,397.8% +/- 807.8%; range, 205% +/- 5,219%).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of a single administration of dexamethasone on basal and growth hormone-releasing hormone stimulated GH secretion in acromegaly

A single administration of dexamethasone causes both an early stimulatory and a late inhibitory effect on GH secretion in normal subjects.

OBJECTIVE

We investigated the effects of a single administration of dexamethasone on basal and GH-releasing hormone-stimulated GH secretion in eight patients with active acromegaly.

DESIGN

On three different days the patients received 4 mg i.v. dexamethasone, 1 microgram/kg body weight GH-releasing hormone 1-29, or matched placebos in different order.

PATIENTS

Eight subjects with active acromegaly, five of whom had not been treated previously, while the other three had received octreotide therapy which was stopped at least 7 days before testing.

MEASUREMENTS

Serum GH levels were measured in duplicate by a commercially available RIA kit.

RESULTS

Dexamethasone administration caused a significant decline of mean +/- SE GH levels from 51.8 +/- 13.8 to 30.0 +/- 9.2 mU/I at 180 minutes, that was not influenced by placebo administration at 180 minutes. On the contrary, when GH-releasing hormone substituted placebo administration, GH levels increased from 34.0 +/- 9.8 mU/I at 180 minutes to 56.0 +/- 15.6 mU/I at 195 minutes. The GH increase was higher when GH-releasing hormone was given without dexamethasone pretreatment (from 52.4 +/- 13.0 mU/I at 180 minutes to 86.4 +/- 25.4 mU/I at 195 minutes). Analysis of the GH area under the curve confirmed the significant inhibition of GH secretion after dexamethasone administration and the significant reduction of the GH response to GH-releasing hormone in the study with dexamethasone pretreatment.

CONCLUSIONS

At variance with data in normal subjects, acute i.v. administration of dexamethasone inhibits basal GH secretion and partially suppresses the GH response to GH-releasing hormone in acromegaly. Both alterations in the regulatory mechanism of adenomatous cells and perturbations of hypothalamic regulatory influences, induced by the state of chronic GH hypersecretion, are likely explanations of the different response to dexamethasone.