02 December 2008
Subjects with Cushing’s disease have diminished growth hormone (GH) response to growth hormone-releasing hormone (GHRH). The aim of our study was to investigate the underlying mechanism of this diminished GH response in these patients using pyridostigmine (PD), an acetylcholinesterase inhibitor, which is reported to increase GH secretion by reducing somatostatin tone. Eight subjects with untreated Cushing’s disease (caused by a pituitary adenoma) and 6 control subjects received GHRH 100 µg in 1 ml of saline, as intravenous bolus injection 60 min after (1) placebo (2 tablets, p.o.) or (2) PD (120 mg, p.o.). After GHRH plus placebo, the GH peak (mean ± SEM) was significantly lower in subjects with Cushing’s disease (2.4 ± 0.5 µg/l) compared to control subjects (25.1 ± 1.8 µg/l, p < 0.05). After GHRH plus PD, the GH peak was significantly enhanced both in subjects with Cushing’s disease (7.1 ± 2.3 µg/l, p < 0.05) and in control subjects (42.3 ± 4.3 µg/l, p < 0.05). In patients with Cushing’s disease, the GH response to GHRH plus PD was lower with respect to the GH response to GHRH alone in normal subjects. We conclude that hypercortisolism may cause a decrease in central cholinergic tone which is in turn hypothesized to be responsible of an enhanced somatostatin release from the hypothalamus. However, other metabolic or central nervous system alterations may act synergistically with hypercortisolism in causing GH inhibition in patients with Cushing’s disease.