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 hyperpara thyroidism resulting from vitamin D deprivation. PTH/PTH-related prote in (PTHrp) receptor messenger RNA (mRNA) expression, assayed by ribonu clease protection analysis, was studied in the kidney, femoral epi/met aphysis, and diaphysis. In addition, in the kidney, PTH/PTHrp receptor mRNA expression was correlated to receptor function by measuring aden yl 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 in vestigate the effects of calcium, PTH, and/or vitamin D status. The fi rst 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 group s that also received a vitamin D-deficient diet, the hypocalcemia and the hyperparathyroidism were later corrected, by either vitamin D supp lementation (D-Di) or lactose and high calcium diet (D-Ca+), 1 week be fore death. The results revealed a 2-fold decrease in the PTH-induced adenyl cyclase activity of the renal membranes in the D-D- rats compar ed to those in the three other groups. There was no significant differ ence in the four groups in adenyl cyclase activity stimulated by forsk olin, NaF, and isoproterenol. The decrease in PTH-induced adenyl cycla se activity was associated with an approximately 2-fold increase in PT H/PTHrp receptor mRNA expression in the kidneys of the D-D- rats compa red to controls. Normalization of PTH/PTHrp receptor mRNA expression w as observed after vitamin D supplementation (D-D+ rats), but not after correction of the hypocalcemia and secondary hyperparathyroidism by o ral lactose and calcium supplementation. In the epi/metaphysis, an app roximately 2-fold increase in PTH/PTHrp receptor mRNA was also observe d in the D-D- rats compared to the controls; this increase was partial ly corrected upon normalization of the calcemia and PTH levels with ei ther vitamin D (D-D+ group) or lactose/calcium (D-Ca+ group). In the d iaphysis, no change in the expression of PTH/PTHrp receptor mRNA was o bserved in any group. These results show that 1) renal PTH desensitiza tion and high levels of PTH are associated with an increase, not a dec rease, in PTH/PTHrp receptor mRNA expression in vitamin D deprivation; 2) vitamin D status is a major factor controlling the expression of P TH/PTHrp mRNA in the kidney; and 3) the regulation of PTH/PTHrp recept or mRNA expression can be different in different tissues. These result s point to the importance of a posttranscriptional event in the contro l of PTH receptor function in vivo.