Expression-level Dependent Activation of Recombinant Human Parathyroid Hormone/parathyroid Hormone-Related Peptide Receptor: Effect of Human Parathyroid Hormone (1-34), (1-31), and (28-48).

A stable recombinant chinese hamster ovary (CHO) cell model system expressing the human type-1 receptor for parathyroid hormone and parathyroid hormone-related peptide (hPTH-R) was established for the analysis of human PTH (hPTH) variants. The cell lines showed receptor expression in the range from 10(5) to 1.9 x 10(6) receptors per cell. The affinity of the receptors for hPTH-(1-34) was independent of the receptor number per cell (K-d congruent to 8 nmol/l). The induction of cAMP by hPTH-(1-34) is maximal in clones expressing >2 x 10(5) receptors per cell and Ca++ signals were maximal in cell lines expressing >1.4 x 10(6) receptors per cell. Second messenger specific inhibitors demonstrated that PTH-induced increases in intracellular cAMP and Ca++ are independent and Ca++ ions are derived from intracellular stores. The cAMP-specific receptor activator hPTH-(1-31) showed also an increase in intracellular Ca++. Even in cell lines expressing more than 10(6) receptors per cell the Ca++/PKC specific activator kPTH-(28-48) did not activate hPTH-Rs. Based on these results, synthesis of further derivatives of PTH is required to identify pathway-specific ligands for the type-1 hPTH-R.