Mechanisms of Down-Regulation of the Renal Parathyroid Hormone Receptor in Rats with Chronic Renal Failure

Hypocalcemia, hyperphosphatemia and resistance to the action of parathyroid hormone (PTH) are well-characterized features in advanced chronic renal failure (CRF). Their pathogenesis has been attributed to both PTH receptor (PTH-R) down-regulation and postreceptor abnormalities. In this study, we examined the renal expression of the PTH-R mRNA in CRF (5/6 nephrectomy) rats. Experiments were also performed to determine whether an acidic condition and PTH itself influence PTH-R mRNA expression. RT-competitive PCR was used to examine mRNA expression, and polyclonal antibody against PTH-R was used for Western blot. PTH-R mRNA expression was abundant in glomeruli, proximal convoluted and straight tubules (PCT, PST), small in medullary and cortical thick ascending limbs, and cortical collecting ducts and not detectable in outer and inner medullary collecting ducts. The expression was significantly decreased in PCT and PST in CRF rats. Decrease in PTH-R mRNA expression was observed 1 week after the induction of CRF. PTH-R protein was decreased at 2 (–23%) and 4 (–45%) weeks in renal cortex, but not in medulla in CRF rats. PTH-R mRNA expression in PST was decreased by low pH (7.1 or 6.7) incubation compared with that at pH 7.4. PTH(1–34) (10–9 M) increased PTH-R mRNA expression in PST from control rats by 250%. The stimulatory effect of PTH on PTH-R mRNA expression was decreased by the incubation at low pH medium. In summary, renal PTH-R is down-regulated in CRF rats. The decrease in mRNA expression in PCT and PST causes the decrease in PTH-R protein. Metabolic acidosis may participate in the down-regulation of PTH-R in early stage of CRF. This abnormality could be important in the pathogenesis of secondary hyperparathyroidism of CRF.