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Activated platelets release a multifaceted blend of growth factors that has stimulatory effects on mesenchymal cells, both in vitro and in vivo, which imply beneficial effects on wound repair and tissue regeneration. Previous studies on fibroblast cultures have revealed that more potent growth factors, with respect to cell proliferation, are released in acidic preparations of lysed platelet concentrates in comparison with neutral and alkaline preparations. The current study was intended to investigate the influence of pH on lysed platelet concentrates with respect to release of growth factors, cell proliferation and alkaline phosphatase (ALP) activity in human osteoblast-like cells (hFOB 1.19). Cell proliferation was assessed with the MTT kit, ALP activity by conventional enzymatic reaction kinetics and growth factors platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta) by enzyme-linked immunosorbent assays. Osteoblast-like cells were stimulated with lysed platelet concentrates preincubated at pH 4.4, 5.4, 7.4, and 7.6. A 3-13-fold increase of cell proliferation was found in comparison with controls and the most evident increase was observed with platelets activated at pH 5.4. The highest ALP activity was observed in preparations at pH 7.6. Platelets incubated in an acidic environment (pH 5.4) induced a higher proliferation compared with preincubation at neutral or alkaline pH and the level of PDGF was also found to be higher in acidic preincubations. The level of TGF-beta was, in contrast, lowest at pH 4.4. We suggest, based on these experimental findings, that acidic milieu influence platelets to release growth factors more potent to stimulate osteoblast proliferation than neutral and alkaline platelet preparations. Lysed platelet concentrates prepared at an alkaline pH might release additional components with stimulating effects resulting in other features than cell proliferation. This is the first report, to our knowledge, about a pH dependent stimulatory effect of lysed platelet concentrates on human osteoblast-like cell proliferation. Lysed platelet concentrates, preincubated in acidic or alkaline buffers, may benefit fracture healing, implant fixation and might also be advantageous in the treatment of wounds with platelet constituents; however, this has to be investigated in extended experimental and clinical settings. This is an electronic version of an article published in:Ola Wahlström, Cecilia Linder, Anders Kalén and Per Magnusson, Variation of pH in lysed platelet concentrates influence proliferation and alkaline phosphatase activity in human osteoblast-like cells., 2007, Platelets, (18), 2, 113-118.Platelets is available online at informaworldTM: http://dx.doi.org/10.1080/09537100600800537Copyright: Taylor & Francishttp://www.tandf.co.uk/journals/default.asp