Downregulation of parathyroid hormone receptors in renal membranes from aged rats

The mechanism of the inhibition or blunting of parathyroid hormone (PTH)-stimulated Na⁺-Ca²⁺ exchange activity in renal cortical cells from aged rats was examined. The number of PTH binding sites in basolateral membranes prepared from adult (6 mo) and old (24 mo) rats was quantitated by the binding of the synthetic analogue ¹²⁵I-labeled [Nle^8,18, Tyr³⁴]bPTH-(1 - 34) amide to the membrane. The maximum number of specific PTH binding sites, B_max, was 92.7 ± 9.3 and 36.7 ± 6.1 fmol/mg protein, respectively, in membranes prepared from adult and old rats. The affinity of the receptor to PTH was unaffected with age. The level of PTH binding components (68 and 70 kDa) estimated by a ligand affinity blot technique using biotinylated bPTH-(1 - 34) as the ligand was similarly reduced in membranes isolated from senescent rats. To test the hypothesis that change in the number of PTH binding sites and level of PTH binding components represented an adaptive response to a high serum PTH level, rats were parathyroidectomized (PTX) and the changes were reexamined. Decreases in the number of PTH binding sites and PTH binding components were either partially or completely negated by the surgery. These findings suggest that the blunting of both the PTH-stimulated Na⁺-Ca²⁺ exchange and adenylate cyclase activities in the kidneys of aged rats was due, in part, to the loss of PTH receptors in basolateral membranes and that this defect could be partially reversed by removal of the parathyroid gland.