Effect of dexamethasone on growth hormone (GH) response to growth hormone releasing hormone in acromegaly.

The effect of dexamethasone on the GH response to GH-releasing hormone (GHRH) was studied in vivo in six patients with acromegaly as well as in vitro in monolayer cultures of GH-secreting pituitary tumor cells obtained from three of these patients. Oral administration of 9 mg/day dexamethasone for 2 days decreased plasma GH responses to iv injection of 100 micrograms GHRH-(1-44 amide) in all six patients. Blood glucose levels were significantly increased, while plasma somatomedin-C levels were significantly decreased by this regimen of dexamethasone treatment. On the other hand, 2-day pretreatment with 50 nM dexamethasone of monolayer cultures of pituitary adenoma cells potentiated GH release basally and/or in response to 100 pM to 1 nM GHRH in vitro. These results indicate that the potentiating action of 2-day treatment of dexamethasone in vitro is overwhelmed in vivo by some extra-pituitary action, probably on the central nervous system, of glucocorticoids in patients with acromegaly.