Autoregulation in the parathyroid glands by PTH/PTHrP receptor ligands in normal and uremic rats

Autoregulation in the parathyroid glands by PTH/PTHrP receptor ligands in normal and uremic rats.BackgroundThe secretion of parathyroid hormone (PTH) from the parathyroid glands might be regulated by autocrine/paracrine factors. We have previously shown that N-terminal parathyroid hormone-related protein (PTHrP) enhanced the secretory PTH response to low calcium in vivo and in vitro in rat parathyroid glands. N-terminal PTHrP fragments are equipotent to N-terminal PTH as ligands for the PTH/PTHrP receptor that is demonstrated in parathyroid tissue. This supports the possibility that the parathyroid cells respond to PTH/PTHrP receptor ligands and as such are target for an autoregulatory action of PTH and PTHrP. Our aim was to search for the PTH/PTHrP receptor in rat parathyroid glands and to examine the effects of PTHrP 1-40 on PTH secretion in in vivo models of secondary hyperparathyroidism (HPT) in uremic rats.MethodsPTH secretion was examined during ethyleneglycol tetraacetic acid (EGTA)-induced hypocalcemia both with and without PTHrP. Five groups, each of six normal rats, received a bolus of increasing doses of 0.1, 1.0, 10, and 100 μg of PTHrP 1-40, or vehicle only. Chronic renal failure (CRF) was induced by 5/6 nephrectomy. One group of 12 CRF rats received a standard diet, while another CRF group of 18 rats received a high phosphorus diet to induce more severe HPT. After 8weeks of uremia, the rats were infused with EGTA and PTHrP 1-40 or vehicle. The presence of the PTH/PTHrP receptor in the rat parathyroid glands was examined by reverse transcription-polymerase chain reaction (RT-PCR) technique. PTH was measured by a rat PTH assay that does not cross-react with PTHrP.ResultsIn a dose-related manner, PTHrP enhanced the PTH response to hypocalcemia in normal rats. A similar rate of decrease of plasma Ca++ was induced by EGTA in all experimental groups. In CRF rats, plasma creatinine (0.99 ± 0.10mmol/L vs. 0.33 ± 0.01mmol/L, P < 0.001) and plasma PTH (226 ± 32pg/mL vs. 69 ± 16pg/mL, P < 0.001) levels were significantly increased. The CRF rats on high phosphorus diet had significant hypocalcemia (Ca++, 1.04 ± 0.02mmol/L vs. 1.28 ± 0.03mmol/L, P < 0.001), hyperphosphatemia (3.48 ± 0.3mmol/L vs. 2.25 ± 0.1mmol/L, P < 0.001) and severe secondary HPT, PTH (984 ± 52pg/mL vs. 226 ± 32pg/mL, P < 0.001) compared to CRF rats on a standard phosphorus diet. The maximal PTH response to hypocalcemia was enhanced in CRF rats (maximum PTH 382 ± 58pg/mL vs. 196 ± 29pg/mL in normal rats, P < 0.01) and further enhanced by PTHrP 1-40 to 826 ± 184pg/mL (P < 0.01). The secretory capacity of the parathyroid glands in response to low Ca++ was severely diminished in uremia. In CRF rats given a high phosphorus diet, the basal PTH levels were at the upper part of the calcium/PTH curve, and the induction of more marked hypocalcemia did not stimulate PTH secretion further (maximum PTH 1475 ± 208pg/mL vs. basal 1097 ± 69pg/mL, NS). PTHrP, however, further enhanced the maximal PTH levels significantly (maximum PTH 3142 ± 296pg/mL, P < 0.01). The presence of the PTH/PTHrP receptor in the rat parathyroid glands was confirmed by RT-PCR technique.ConclusionPTHrP enhanced significantly, in a dose-related manner, the low Ca++-stimulated PTH secretion in normal rats. The PTH/PTHrP receptor is present in rat parathyroid glands. The impaired secretory capacity of the parathyroid glands in uremic rats is significantly enhanced by PTHrP. An autocrine/paracrine role in the parathyroid glands of the PTH/PTHrP receptor targeting peptides, PTHrP and PTH, is suggested. Thus, it is hypothesized that PTH during hypocalcemia might have a positive auto-feedback regulatory role on its own secretion