Comparative study of collecting tubules and vasopressin binding capacity in the renal medulla of developing hypothyroid rat

The effects of congenital hypothyroidism on both the structure and function of the renal medulla were studied by comparing, in 1-month old rats, the structural features of collecting tubules with the capacity of vasopressin to bind membrane preparations and the related adenylate cyclase activation. With the exception of a reduced caliber, hypothyroidism had no effect on the density, total number, distribution of tubules according to epithelial thickness, or on the number of epithelial cells, or their area. The binding capacity of vasopressin and the related adenylate cyclase activation were equally reduced by about 50%, without changes in (i) the basal or guanylyl-imidodiphosphate (Gpp(NH)p)-stimulated adenylate cyclase activities, (ii) the apparent dissociation constant (KD) of labelled vasopressin from its specific receptor or (iii) the apparent activation constant (Kact) of vasopressin for adenylate cyclase. Taken together, these results clearly demonstrate that congenital hypothyroidism exerts a direct influence on the developing responsiveness of the renal medulla, mainly by reducing the density of active hormone receptors per cell, instead of reducing cell number or cell membrane area.