Renal parathyroid hormone receptors in the chick: downregulation in secondary hyperparathyroid animal models

Abstract

We characterized the binding of 125I-[Nle8, Nle18, Tyr34]parathyroid hormone-(1-34) amide [125I-nlPTH-(1-34)] to renal plasma membranes prepared from chicks to determine the effects of secondary hyperparathyroid states on renal PTH receptors. This radioligand exhibited specific binding to membranes with high affinity (Kd, 2-3 X 10(-9) M). Agonists or competitive antagonists of PTH were effective in competing for binding sites labeled with 125I-nlPTH-(1-34), whereas an inactive fragment of PTH, salmon calcitonin, and bovine growth hormone did not compete with the radioligand for renal PTH receptors. Newly hatched chicks raised on control diet with adequate vitamin D and calcium or diets deficient in either vitamin D or calcium were used to study the regulation of renal PTH receptors in experimental models of secondary hyperparathyroidism. We found that both experimental diets resulted in marked hypocalcemia and progressive loss of renal cyclic AMP responsiveness to PTH in vitro. Associated with this refractoriness to the hormone was a marked reduction in PTH receptors in membranes from both vitamin D-deficient and calcium-deficient chick kidney. No change in the affinity of the PTH receptors was found. Vitamin D3, in a single dose of 250 micrograms, partially restored serum calcium of vitamin D-deficient birds toward normal by 72 h and also partly restored renal cyclic AMP responsiveness to PTH and the PTH receptor number toward control values. We conclude that renal refractoriness to PTH observed in experimentally hyperparathyroid animals models is due to a marked loss of plasma membrane receptor sites for PTH without an apparent change in the affinity of the receptors for the hormone.

Effects of dietary-induced hyperparathyroidism on the parathyroid hormone-receptor-adenylate cyclase system of canine kidney. Evidence for postreceptor mechanism of desensitization.

“…The results of the present studies differ from previous investigations by our laboratory (18) and others (12,13) on PTH desensitization in vitro or in vivo.…”

Dual Signaling and Ligand Selectivity of the Human PTH/PTHrP Receptor

“…The possible physiologic relevance of these observations is supported by the substantial changes in expression of the PTHR that are known to occur in cells of bone or kidney during repeated PTH exposure in vitro or in vivo. (33,(37)(38)(39)(40)(41) Moreover, variation of endogenous PTHR expression within the range that we have studied in LLC-PK1 cells has been described in other cell systems in which endogenous PTHR genes are expressed. (42,43) We were surprised to find no activation of PLC by hPTH in cells that expressed 1,000,000 hPTHRs/cell and in which hPTH (21) Various P T H ( 3 -3 4 ) a n a l o g s , m a i n l y b P T H ( 3 -3 4 ) o r [Nle 8,18 ,Tyr 34 ]bPTH(3-34) amide, were shown by some investigators, (15,25,44) though not by others (45,46) to increase cytosolic free calcium in rodent bone or opossum kidney cells.…”

Rapid development of renal resistance to low doses of synthetic bovine parathyroid hormone fragment 1-34. Dissociation of urinary cyclic adenosine monophosphate, phosphaturic, and calciuric responses.

“…The characteristics of PTH receptors and their functional relationship to adenyl cyclase are currently under active investigation (16,29). Two earlier studies in parathyroidectomized rats, which were published before the concept of homologous down-regulation appeared, presented data which suggest that tachyphylaxis occurs in phosphaturic (1,4) and urinary cAMP (1) responses to prolonged or repetitive exogenous PTH stimulation.…”

“…However, in the intact rat, even large doses of PTH given as prolonged infusions or as serial bolus injections failed to elicit refractoriness in vivo, despite the concurrent in vitro evidence of this phenomenon (2). Studies in chicks (29) and rats (30) that were made secondarily hyperparathyroid by dietary deficiency of vitamin D or calcium have shown evidence of tachyphylaxis both in vivo and in vitro; however, the possible influence of concomitant hypocalcemia on these results was not evaluated. The applicability of these findings to human studies is uncertain because there are significant interspecies differences in the effect of PTH on cAMP production and excretion (13,20,31) as well as intraspecies variability in the responses to different PTH preparations (13).…”