STIMULATION BY PARATHYROID-HORMONE OF (CA-2+)-CA-45 UPTAKE IN OSTEOBLAST-LIKE CELLS - POSSIBLE INVOLVEMENT OF ALKALINE-PHOSPHATASE

We have investigated the actions of human PTH [hPTH-(l-34)] on the association of45Ca2+with two human (SaOS-2 and MG-63) and two rat (ROS 17/2.8 and UMR-106) osteoblast-like cell types. In SaOS-2 cells, hPTH-(l–34) binds to specific membrane receptors to activate adenylate cyclase. Treatment of SaOS-2 cells with hPTH-(l–34) resulted in an increase in45Ca2+uptake, in a dose-dependent fashion, up to 2-to 4-fold above control values. The increase was first evident at 10 min and persisted for at least 30 min. Treatment with nimodipine, a calcium channel antagonist, was without effect on the stimulatory action of PTH. A similar enhancement of cell-associated 48Ca2+ was observed when the cells were incubated with vasoactive intestinal peptide, which acts via different receptors to activate adenylate cyclase in SaOS-2 cells. Treatment with (Bu)2cAMP also induced an increase in cell-associated 46Ca2+. Pretreatment of SaOS-2 cells with hPTH-(1–34) for 4 h, which induced homologous desensitization to a second challenge with the same peptide for stimulation of cAMP production, did not attenuate the further enhancement of cell-associated 45Ca2+ by a second treatment with hPTH-(1–34). We then examined a possible relationship between alkaline phosphatase (ALPase) and 46Ca2+ uptake. SaOS-2 cells contained high levels of alkaline phosphatase activity and continuously released the enzyme into the medium. Release was enhanced by treatment with hPTH-(1-34) for 10 min. Incubation of cells with levamisole (an inhibitor of the liver/bone/kidney type of ALPase) resulted in a rapid decrease in basal and PTH-stimulated 45Ca2+ uptake, while treatment with L-Phe-Gly-Gly (an inhibitor of human placental ALPase) was without effect. Treatment of the cells with ALPase (bovine kidney) enhanced 45Ca2+ uptake. In MG-63 cells, a stimulatory effect of hPTH-(1–34) on cell-associated45Ca2+was also observed; however, hPTH-(1–34) did not stimulate cAMP production in MG-63 cells. In ROS 17/2.8 cells, neither hPTH-(1–34) nor rat PTH-(l–34) stimulated an increase in cell-associated45Ca2+, while in UMR-106 cells, rat PTH-(1–34) and (Bu)2cAMP did enhance45Ca2+uptake, although hPTH-(1–34) was without effect. We conclude that PTH can stimulate an increase in cell-associated45Ca2+in several osteoblast-like cell lines, possibly by modulating local ALPase activity; however, this action of PTH does not appear to be obligatorily dependent on the adenylate cyclase-stimulating action of PTH. © 1990 by The Endocrine Society.