PARATHYROID-HORMONE (PTH)/PTH-RELATED PROTEIN-RECEPTOR MESSENGER-RIBONUCLEIC-ACID EXPRESSION AND PTH RESPONSE IN A RAT MODEL OF SECONDARY HYPERPARATHYROIDISM ASSOCIATED WITH VITAMIN-D DEFICIENCY

The aim of the present work was to characterize at the molecular level the mechanism of PTH resistance in a rat model of secondary hyperparathyroidism resulting from vitamin D deprivation. PTH/PTH-related protein (PTHrp) receptor messenger RNA (mRNA) expression, assayed by ribonuclease protection analysis, was studied in the kidney, femoral epi/metaphysis, and diaphysis. In addition, in the kidney, PTH/PTHrp receptor mRNA expression was correlated to receptor function by measuring adenyl cyclase activity in crude renal membranes after stimulation by PTH (10(-10)-10(-6) M), forskolin (0.1 and 0.2 mM), NaF (5 and 10 mM), and isoproterenol (1 and 10 mu M). Four groups of rats were studied to investigate the effects of calcium, PTH, and/or vitamin D status. The first group received a control diet (D+D+). The second group received a diet deficient in vitamin D until death (D-D-). In the two other groups that also received a vitamin D-deficient diet, the hypocalcemia and the hyperparathyroidism were later corrected, by either vitamin D supplementation (D-Di) or lactose and high calcium diet (D-Ca+), 1 week before death. The results revealed a 2-fold decrease in the PTH-induced adenyl cyclase activity of the renal membranes in the D-D- rats compared to those in the three other groups. There was no significant difference in the four groups in adenyl cyclase activity stimulated by forskolin, NaF, and isoproterenol. The decrease in PTH-induced adenyl cyclase activity was associated with an approximately 2-fold increase in PTH/PTHrp receptor mRNA expression in the kidneys of the D-D- rats compared to controls. Normalization of PTH/PTHrp receptor mRNA expression was observed after vitamin D supplementation (D-D+ rats), but not after correction of the hypocalcemia and secondary hyperparathyroidism by oral lactose and calcium supplementation. In the epi/metaphysis, an approximately 2-fold increase in PTH/PTHrp receptor mRNA was also observed in the D-D- rats compared to the controls; this increase was partially corrected upon normalization of the calcemia and PTH levels with either vitamin D (D-D+ group) or lactose/calcium (D-Ca+ group). In the diaphysis, no change in the expression of PTH/PTHrp receptor mRNA was observed in any group. These results show that 1) renal PTH desensitization and high levels of PTH are associated with an increase, not a decrease, in PTH/PTHrp receptor mRNA expression in vitamin D deprivation; 2) vitamin D status is a major factor controlling the expression of PTH/PTHrp mRNA in the kidney; and 3) the regulation of PTH/PTHrp receptor mRNA expression can be different in different tissues. These results point to the importance of a posttranscriptional event in the control of PTH receptor function in vivo.