Objective: The fibrosis and apoptosis of corpus cavernosum(CC)tissue are the major histomorphological changes associated with diabetes-induced erectile dysfunction(ED).Our previous studies have demonstrated that human tissue kallikrein 1(hKLK1)could partly prevent streptozotocin-induced diabetic ED.However,its effect on the histomorphological changes of penile CC in diabetic rats is still unknown.Methods: The 8-week-old male wild type SD rats(WTR)and age matched TGR were randomly divided into five groups:(Ⅰ)untreated WTR as the control;(Ⅱ)untreated TGR,streptozotocin-induced diabetic(Ⅲ)wild type rats(WTDM)and(Ⅳ)transgenic rats(TGDM)for 3 months;(Ⅴ)TGDM with bradykinin receptor 2 inhibitor HOE140(TGDMH).CC tissues from all rats were collected in liquid nitrogen or embedded in paraffin.Western blot was used to detect the protein level of transforming growth factor β1(TGF-β1),smad2/3,phospho-smad2/3,collagen Ⅰ and collagen Ⅳ in CC.Immunohistochemical staining and massons trichrome staining were conducted to detect the expression of TGF-β1,α-SMA and the ratio of smooth muscle to collagen.In addition,terminal deoxynucleotidyl transferase dUTP nick end labeling(TUNEL)assay was conducted to calculate the apoptosis index(AI)of cells in CC.Besides,the protein content of caspase3,B-cell lymphoma 2(Bcl-2)and Bcl-2-associated X(Bax)protein were also measured in CC and high concentration glucose treated CC smooth muscle cells(CCSMC)isolated from WTR and TGR.Results: The protein expressions of TGFβ1,smad2,collagen Ⅳ and the ratio of phospho-smad2/smad2 were increased in WTDM when compared to the control group,while these parameters were significantly attenuated in TGDM.However,the protective effect of hKLK1 was abolished by HOE140 in TGDMH.Furthermore,the contrary trends of α-SMA content and the smooth muscle/collagen ratio changes in CC of each group were also observed.No significant difference in collagen Ⅰ level was found between the five groups.In addition,when compared to the heightened level of AI,caspase3 and Bax/Bcl2 ratio in the CC of WTDM,these parameters were slightly decreased in TGDM.Meanwhile,hKLK1 could effectively inhibit the increase of the apoptosis related indexes in high glucose treated CCSMC in vitro.Conclusions: hKLK1 could exert anti-fibrosis and apoptosis effect in penile tissue of streptozotocin-induced diabetic rats as well as in CCSMC treated with high glucose.