Hypocalcemia increases and hypercalcemia decreases the steady-state level of parathyroid hormone messenger RNA in the rat.

To examine the effects of serum calcium concentrations on PTH biosynthesis, rats were made hyper- (serum total calcium, approximately 3.5 mM) or hypocalcemic (approximately 1.25 mM) and steady-state levels of PTH mRNA in parathyroid cells were measured by the primer extension method using a 32P-labeled synthetic oligomer. PTH mRNA levels increased about twofold in the rats made slightly hypocalcemic by infusion of calcium-free solution and decreased slightly in those made hypercalcemic by CaCl2 infusion (120-150 mumol/h) compared with the levels present in nonfasting control rats. Infusion of calcitonin (0.5 U/h) or EGTA (90 mumol/h) with calcium-free solution increased PTH mRNA levels further (two- to sevenfold) above the levels present in animals infused with calcium-free solution alone. These changes in PTH mRNA levels were observed after 48- but not 24-h infusion, and there was an inverse correlation between PTH mRNA levels and serum calcium concentrations. The results suggest that changes in serum calcium concentrations in the near physiological range regulate the biosynthesis of PTH by affecting steady-state levels of PTH mRNA when hypercalcemia or hypocalcemia continues for a relatively long period.