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Objective: To explore the effects of endoplasmic reticulum stress (ERS) related proteins and their mediated apoptosis in the formation of deep tissue injury of pressure ulcer in rats., Methods: Forty male Sprague-Dawley rats were divided into normal control group and groups A, B, C, D according to the random number table, with 8 rats in each group. Rats in group A were loaded with 22.47 kPa pressure with a special pressure apparatus for 2.0 h in the region over gracilis, and then unloaded for 0.5 h. Rats in group B were treated with the same manoeuvre as that in group A for 3 times in one day. Rats in groups C and D were treated with the same manoeuvre as that in group B for 2 and 3 days. Rats in normal control group were free from pressure loading. Rats in groups A, B, C, and D were sacrificed after pressure loading, and then the central part of pressure loaded muscular tissues were harvested for observation of histomorphological change with HE staining; apoptotic nucleoli per millimeter pressure loaded muscular tissue were counted with Hoechst 33258 staining; the levels of binding protein (BIP), protein disulfide isomerase (PDI), C/EBP homologous protein (CHOP), and caspase-12 were assessed with Western blotting (denoted as gray level ratio of target protein to GAPDH). The same parts of gracilis of rats in normal control group were harvested for determination of all the indexes as above. Data were processed with one-way analysis of variance, LSD-t test was applied for paired comparison., Results: (1) Histomorphological observation. Some pathological changes, including inflammatory cell infiltration, myofibers lysis, and vacuolar degeneration, etc. were observed in pressure loaded muscular tissue of rats in groups A, B, C, and D, but not in the same parts of gracilis muscle of rats in normal control group. Compared with those in normal control group [(2.7 ± 1.4) per millimeter muscular tissue], the number of apoptotic nuclei was significantly increased in pressure loaded muscular tissue of rats in groups A, B, C, and D [(14.5 ± 4.4), (11.0 ± 2.9) , (13.8 ± 5.1), (21.3 ± 6.0) per millimeter pressure loaded muscular tissue, with t values from 4.223 to 6.000, P values all below 0.01). (2) Western blotting. The protein expressions of BIP and PDI in rats of normal control group and groups A, B, C, D were respectively 0.64 ± 0.12, 1.20 ± 0.34, 1.59 ± 0.24, 1.17 ± 0.28, 1.44 ± 0.33; 0.48 ± 0.15, 0.61 ± 0.19, 1.23 ± 0.38, 0.37 ± 0.19, 0.29 ± 0.15, and they showed significant statistical difference (with F values respectively 5.32, 7.95, P < 0.05 or P < 0.01). The protein expressions of CHOP and caspase-12 in rats of normal control group and groups A, B, C, D were respectively 0.58 ± 0.18, 1.48 ± 0.27, 1.03 ± 0.21, 0.95 ± 0.30, 1.69 ± 0.34; 0.55 ± 0.12, 1.08 ± 0.31, 0.69 ± 0.24, 1.79 ± 0.20, 2.06 ± 0.47, with significant statistical difference (with F values respectively 8.17, 15.48, P values all below 0.01)., Conclusions: ERS related proteins and their apoptotic pathway may play an important role in the formation of deep tissue injury of pressure ulcer in rats.