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Nitric oxide (NO) plays a crucial role in the physiological and pathophysiological regulations of osteoblast functions. This study is designed to evaluate the toxic effects of NO released by sodium nitroprusside (SNP), an NO donor, on neonatal Wistar rat calvarial osteoblasts from the analyses of cell viability, alkaline phosphatase (ALP) activity, cell morphology, apoptotic cells, terminal deoxynucleotidyl transferase-mediated dUTP nick end-label (TUNEL) assay, DNA ladder, and immunocytochemistry and Western blot for proapoptotic Bax protein. SNP increased the levels of nitrite, an oxidative product of NO, in the culture medium of osteoblasts in concentration-and time-dependent manners, and altered cell morphologies to round and shrinkage shapes. Administration of osteoblasts with SNP resulted in concentration-and time-dependent decreases of cell viability and ALP activity. Analysis of apoptotic cells revealed that SNP increased the percentages of osteoblasts processing apoptosis. Analyses of TUNEL and DNA ladder showed that SNP caused DNA fragmentation. Pretreatment with cycloheximide, an inhibitor of protein synthesis, partially blocked SNP-induced osteoblast apoptosis. Imunocytochemical and immunoblotting analyses revealed that SN P increased Bax protein in osteoblasts. This study suggests that SNP could increase the levels of NO in osteoblasts, and cause osteoblast

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