Effects of aldosterone, methylprednisolone and triiodothyronine on the response to water loading in the conscious hypothyroid rat with diabetes insipidus.

The role of the antidiuretic hormone-independent mechanisms underlying the impairment of water excretion in hypothyroidism remains controversial. We examined the excretion of an oral water load (50 ml/kg b.w.) over a 3-hour period before and after the administration of aldosterone, methylprednisolone, both aldosterone and methylprednisolone, and triiodothyronine in 12 hypothyroid and 7 age-matched control Wistar rats of the homozygous Brattleboro strain (DI). Conscious animals were studied to circumvent the adverse influence of anesthetics and to facilitate the re-examination of the same animals. At baseline, the hypothyroid DI rats exhibited a marked impairment in their response to water administration. In comparison to control DI rats they had a lower urine volume, CH2O, CH2O/V and creatinine clearance, and higher urine osmolality (p less than 0.005-less than 0.001). Because of the possibility of decreased delivery of filtrate due to dehydration in the DI animals, a separate group of DI rats received saline in place of the water load. Despite improvement in urine flow with saline, CH2O generation remained decreased in the hypothyroid DI rats. Methylprednisolone increased both urine volume (p less than 0.001) and CH2O (p less than 0.001) in the hypothyroid group, but CH2O formation remained below the values obtained in the control animals. While aldosterone and methylprednisolone alone and in combination improved utilization of distally-delivered filtrate for the formation of CH2O (p less than 0.02-0.005), neither hormone, either alone or in combination, fully corrected the dilution defect. In contrast, the administration of triiodothyronine fully normalized all the observed abnormalities in the hypothyroid rats, suggesting that thyroid hormone deficiency is directly responsible for the abnormal water-excretion in experimental hypothyroidism.