ACUTE PHOSPHATE-DEPLETION DISSOCIATES HORMONAL STIMULATED 2ND MESSENGERS IN OSTEOBLAST-LIKE CELLS

The acute effect (24 h) of either phosphate depletion or phosphate surfeit on hormonal stimulated signal transduction systems was studied in the osteoblastic cell line UMR-106. Elevation of intracellular Ca2+ ([Ca2+]in), induced by different calciotropic hormones (PTH, prostaglandin E2, endothelin) was blunted by acute phosphate depletion, whereas at high inorganic phosphate (P(i)) concentrations the rise in [Ca2+]in was augmented. Basal [Ca2+]in was not altered by either P(i) depletion or P(i) excess. The effect of acute phosphate depletion on hormonal mediated [Ca2+]in rise was not observed in the absence of extracellular Ca2+ suggesting that under these conditions, the release of Ca2+ from intracellular stores, is not affected. Also, nonhormonal calcium entry pathways such as depolarization-activated calcium channels or protein kinase C - activated Ca2+ channels were not affected by acute phosphate depletion. cAMP accumulation in the cells, either through receptor or nonreceptor- mediated mechanisms, increased under low P(i) conditions and decreased as P(i) concentration in the culture media was progressively increased from 0 to 2 mM during 24 h of incubation. Changes in P(i) concentration had no effect on basal cAMP generation by the cells. The facilitative effect of acute P(i) depletion on agonist-induced cAMP accumulation could be demonstrated in both the presence and absence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.2 mM). PTH receptor binding assessed with [Nle8 Nle18 Tyr34] bovine PTH (1-34)NH2 was not altered by phosphate depletion. We conclude that exposure of osteoblasts to different P(i) environments modulates the second messenger responses to hormones in a reciprocal fashion so that acute phosphate depletion down-regulates [Ca2+]in signals while augmenting cAMP generation and vice versa. Inasmuch as bone resorption processes can be modulated by Ca2+ and cAMP the data presented herein suggest that the altered bone resorptive response to calciotropic hormones (e.g. PTH), under surfeit or deficit of phosphate, is mediated by changes in [Ca2+]in and cAMP.