Effect of exogenous glucocorticoid on osmotically stimulated antidiuretic hormone secretion and on water reabsorption in man

Objective: Glucocorticoids exert tonic suppression of antidiuretic hormone (ADH) secretion. Hypocortisolism in secondary adrenocortical insufficiency can result in a clinical picture similar to the syndrome of inappropriate ADH secretion. On the other hand, in vitro and in vivo results provide evidence for ADH suppression in states of hypercortisolism. To test the hypothesis that ADH suppression is of relevance during glucocorticoid therapy, we investigated the influence of prednisolone on the osmotic stimulation of ADH. Design and methods: Seven healthy men were subjected to water deprivation tests with the measurement of plasma ADH (pADH) and osmolality (posmol) before and after glucocorticoid treatment (5 days 30 mg prednisolone per day). Results: Before glucocorticoid treatment, the volunteers showed a normal test with an adequate increase of pADH (basal 0.54 +/- 0.2 to 1.9 +/- 0.72 pg/mI (mean +/- S.D.)) in relation to posmol(basal 283.3 +/- 8.5 to 293.7 +/- 6 mosmol/kg). After prednisolone intake, pADH was attenuated (< 0.4 pg/ml) in spite of an increase of posmol from 289.3 +/- 3.6 to 297.0 +/- 5.5 mosmol/kg. However, urine osmolar concentration increased normally during water deprivation after prednisolone. Urinary cAMP excretion increased during water deprivation without glucocorticoid treatment from 3.56 +/- 0.55 to 6.07 +/- 0.76 mu mol/l, reflecting the increased pADH levels. The rise in cAMP excretion was completely blunted by prednisolone treatment. Conclusions: We speculate that there may be an ADH-independent stimulation of the formation or function of aquaporin-2 channels by prednisolone and/or a direct osmotic stimulation of water reabsorption independent of ADH and glucocorticoid control.