Parathyroid hormone messenger ribonucleic acid: effects of calcium on cellular regulation in vitro.

The effects of changes in the concentration of extracellular calcium the principal regulatory of PTH secretion, on the cellular levels of the mRNA encoding prepro-PTH, a precursor of PTH, were studied in bovine parathyroid slices in vitro using a sensitive radiodensitometric cDNA hybridization assay and mRNA translation in a wheat germ cell-free system. Although lowering the calcium from 5 to 0.5 mM during 5- to 70-h incubations stimulated the secretion of PTH 4- to 8-fold and pro-PTH synthesis 1.4-fold, the cellular levels of total hybridizable PTH mRNA did not change. However, a 2- to 3-fold increase in PTH mRNA, which could be hybridized to oligo-dT-cellulose [poly(A)-rich], was observed in the slices incubated in 5 mM calcium (suppressed slices). The poly(A)-rich PTH mRNA constituted 30% of the total PTH mRNA in suppressed gland slices and 10% of the total PTH mRNA in stimulated gland slices. PTH mRNA did not decay measurably when RNA synthesis in the gland slices was inhibited with actinomycin D and alpha-amanitin for 6 h. Both the stability and the lack of perturbation of the cellular levels of total of total hybridizable PTH mRNA by changes in the concentration of extracellular calcium suggest that the short term regulation of PTH biosynthesis involves posttranscriptional mechanisms.