Parathyroid hormone stimulation of mitosis in rat thymic lymphocytes is independent of cyclic AMP

The in vitro mitotic response of rat thymic lymphocytes to hPTH(1–34), hPTH (1–38), and 8,18 NIe hPTH(1–34) exhibits a dependency upon extracellular calcium. Removal of extracellular calcium or the addition of Verapamil (5 μg/ml) or trifluoroperazine (10 μM) abrogated the mitotic response. Mitogenic concentrations of 8,18 NIe hPTH(1–34) increased calcium 45 uptake from 4.49 ± 0.25 to 8.23 ± 0.75 pMol/10⁶ cells/min. The intracellular calcium concentration, measured by Quin 2 fluorescence, also increased after addition of 8,18 NIe hPTH(1–34). Parathyroid hormone‐induced activation could not be demonstrated in an otherwise responsive thymocyte membrane adenylate cyclase. In intact cells mitogenic levels of 8,18 NIe hPTH(1–34) decreased intracellular cyclic AMP content. This response was blocked by both 3‐isobutyl 1‐methyl xanthine and trifluoroperazine, and may indicate activation of calcium‐dependent phosphodiesterase. We conclude that PTH stimulates thymic lymphocyte proliferation independently of cyclic AMP, and that changes in cellular calcium homeostasis are intimately involved in the action of PTH. In all of the assays employed, the hitherto antagonistic analogue 8,18 NIe 34 Tyr bPTH(3–34)amide proved to be an agonist. We postulate that the receptor utilized for this PTH action may not exhibit classical PTH structure‐activity specificities.