A sensitive and specific radioimmunoassay for antidiuretic hormone (ADH) has been applied to the measurement of ADH excreted in the urine. ADH is extracted from urine and concentrated using cation exchange chromatography, with mean recovery through the procedure of 89.4% for 125I-LVP and 98.5% for unlabeled AVP. Urine specimens were collected from Long-Evans rats and rats of the Brattleboro strain which were either heterozygous or homozygous for diabetes insipidus (DI). Immunoassay and antidiuretic bioassay performed on aliquots of the same urine specimens were in excellent agreement. Under conditions of ad lib. fluid and food intake, mean urinary excretion of immunoreactive material was 4.5 mU/24 hr in normal rats, 2.3 mU/24 hr in heterozygous DI rats and 0.16 mU/24 hr in homozygous DI rats. There was a positive relationship between urinary ADH excretion and urine concentration and an inverse relationship between urinary ADH excretion and urine volume. During water deprivation, normal rats promptly increased urinary ADH excretion to a mean peak value of 18.2 mU/24 hr, while heterozygous DI rats slowly increased urine ADH to only 5.5 mU/24 hr. Homozygous DI rats showed no increase in urinary immunoreactive material even though urine osmolality markedly increased. Following rehydration, normal and heterozygous DI rats underwent a prompt fall in urinary ADH excretion to control levels or less. Urinary ADH excretion is thus shown to parallel the ADH content of the neurohypophyseal system and to respond appropriately to stimuli known to influence ADH release. The response to dehydration of the rat heterozygous for DI suggests that the genetic abnormality in these rats results in impaired ability to release ADH as well as impaired ability to synthesize the hormone. The rat homozygous for DI has a complete deficiency in the ability to synthesize and release ADH.
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