The effects of vasopressin on pituitary oxytocin content and plasma renin activity in rats with hypothalamic diabetes insipidus (Brattleboro strain)

Drinking, urine flow and osmolarity, plasma renin activity (PRA), and pituitary oxytocin have been studied in male and female Brattleboro rats, homozygous [with diabetes insipidus (DI)] and heterozygous (non-DI) for the phenotype of hypothalamic diabetes insipidus. The polydipsia and polyuria as well as the extreme hypotonicity of the urine was confirmed for male and female DI rats: urine to plasma osmolar ratios averaged 0.5 in DI and 5.0 in non-DI. Under similar conditions female DI rats concentrated the urine slightly more than males. PRAs were higher in DI than in non-DI rats, a difference especially noticeable in male animals. Pituitary oxytocin content was much reduced in male and female DI rats compared with equivalent non-DIs. In male DI rats, vasopressin (1.0 U/rat/day for 7 days) produced a profound fall in PRA and an increased pituitary oxytocin content as well as inducing the formation of an hypertonic urine with renal reabsorption of osmotically free water. In female DI rats, graded does of vasopressin (0.1, 0.25, 0.5 or 1.0 U/rat/day for 8 days) all reduced the pituitary oxytocin concent to the same degree, while the reduction in fluid turnover and the marginal increases in PRA were roughly proportional to the dose employed. Female DI rats were less sensitive to vasopressin than male DI animals as judged by the urinary osmolarity achieved for a given dose of hormone.