Effects of sex and age on pyridostigmine potentiation of growth hormone-releasing hormone-induced growth hormone release

Growth Hormone Responses to Low-Dose Physostigmine in Elderly vs. Young Women and Men

BACKGROUND

Growth hormone (GH) secretion is a sensitive measure of CNS cholinergic neurotransmission, and GH decreases considerably with age. Cholinesterase inhibitors, which increase acetylcholine concentrations, have been used in elderly subjects to investigate the neuroendocrine effects of aging and Alzheimer's disease. However, there have been only a few studies of a potential sex difference in GH responses to cholinesterase inhibitors in elderly subjects, with mixed results.

OBJECTIVE

We therefore administered low-dose physostigmine (PHYSO), a cholinesterase inhibitor, to normal, non-hormone-replaced, elderly women and men, to ascertain a potential sex difference in GH response. We hypothesized: (1) elderly women and men would have similar hormone responses, because of relatively low circulating estrogen in the women, and (2) the elderly women would have significantly lower baseline GH and GH responses to cholinergic challenge than the young women we studied previously.

METHODS

Normal elderly women and men > or =65 years of age meeting stringent inclusion and exclusion criteria were studied on three test days, 4-7 days apart, by serial blood sampling for several hours for baseline GH, followed by administration of low-dose PHYSO (first and third days) or saline (second day) at 18:00 h. Frequent blood sampling was continued for several hours. Plasma GH and hypothalamo-pituitary-adrenal cortical hormones were measured in each sample.

RESULTS

PHYSO administration produced no side effects in about half the elderly subjects and mild side effects in the other half, with no significant female-male differences and no significant relationship between the presence or absence of side effects and GH response. PHYSO significantly increased GH compared to saline, to a similar degree in the elderly women and men. The elderly women had a significantly greater GH response to PHYSO than did the young women, whereas GH responses were similar in the elderly and young men.

CONCLUSIONS

These results indicate similar GH responses to low-dose PHYSO in elderly women compared to elderly men, and a significantly greater GH response in elderly women compared to young women. A likely mechanism is increased sensitivity of central cholinergic systems that inhibit somatostatin and/or enhance GHRH release from the hypothalamus.

Influence of body mass index and gender on growth hormone (GH) responses to GH-releasing hormone plus arginine and insulin tolerance tests

The aim of this study is to assess whether gender and body mass index (BMI) should be considered in developing thresholds to define GH deficiency, using GH responses to GHRH + arginine (ARG) stimulation and insulin tolerance test (ITT). Thirty-nine healthy subjects (19 males, 20 females; ages 21-50 yr) underwent GHRH + ARG, and another 27 subjects (19 males, 8 females; ages 20-49 yr) underwent ITT. Peak GH response was significantly higher (P = 0.005) after GHRH + ARG than with ITT, and this difference could not be explained by age, gender, or BMI. Peak GH response was negatively correlated with BMI in both tests (GHRH + ARG, r = -0.76; and ITT, r = -0.65). Peak GH response to GHRH + ARG was higher in females than males (P = 0.004; ratio = 2.4), but it was attenuated after eliminating the influence of BMI (P = 0.13; ratio = 1.6). No significant gender differences were found in peak GH responses to ITT, which could be due to the smaller number of female subjects studied. GH response to GHRH + ARG and ITT stimulation is sensitive to BMI differences and less so to gender differences. A higher BMI is associated with a depressed GH response to both stimulation tests. BMI should therefore be considered as a factor when defining the diagnostic cut-off points in the assessment of GH deficiency, whereas whether gender should be likewise used is inconclusive from this study.

Variability in growth hormone stimulation tests

The diagnosis of growth hormone deficiency (GHD) in adulthood has become increasingly important because of the approved indication for growth hormone (GH) substitution therapy in such patients. While GH stimulation tests are superior to single measurements of other growth factors or spontaneous GH secretion in the diagnosis in adults, the reproducibility and specificity of GH stimulatory tests are often described to be low. This is also the case with the insulin tolerance test. Many external factors, such as fasting, physical activity, heat exposure and sleep, are known to influence GH secretion. The stimulatory or inhibitory effect of these factors on GH secretion might, therefore, influence the GH provocative test and contribute to the variability in response. Age and body composition are also known to influence GH secretion, and these factors must be considered when evaluating GH test responses. However, age-related cut-off levels for GHD have not been defined. Obesity is still a complicating factor in the diagnosis of GHD, even though some GH tests have been able to distinguish between obesity and true GHD. Based on these complicating factors, the parameters of GH stimulatory tests are recommended to be defined and standardized to optimize reproducibility and specificity. Furthermore, such tests should be performed only in patients with firm evidence of pituitary disease.