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Objective: Our previous study has proved the protective effect of human tissue kallikrein 1(hKLK1)on erectile function of aged rats with the transgenic Sprague-Dawley(SD)rats harboring bKLK1 gene(TGR),but its role on diabetic erectile dysfunction(ED)is still unknown.Methods: An 8-week-old male wild type SD rats(WTR)and age matched TGR were randomly divided into five groups(n=8 in each group):(Ⅰ)untreated WTR as the control;(Ⅱ)untreated TGR;streptozotocin-induced diabetic(Ⅲ)wild type rats(WTDM);and(Ⅳ)transgenic rats(TGDM);(Ⅴ)TGDM with bradykinin receptor 2 inhibitor HOE140(TGDMH).Erectile response,represented with the ratio of peak intracavernous pressure to systemic mean arterial pressure and area under the curve,were measured by cavernous nerve electrostimulation after 12 weeks.Penile corpus cavernosum(CC)tissues from all rats were harvested for bKLK1 gene identification and mechanism exploration.The related genes expressions of PI3 kinase(PI3K)/endothelial nitric oxide synthase(eNOS)pathway in CC of each group were assayed to detect the effect of hKLK1 in vivo.CC smooth muscle cell(CCSMC)isolated from TGR was administrated with pathway inhibitors and the intracellular concentration of cGMP and Ca2+ were measured to confirm the potential signaling in vitro.In addition,the level of nicotinamide adenine dinucleotide phosphate(NADPH)oxidase,malondialdehyde(MDA)and activity of superoxide dismutase(SOD)were tested to indicate the local oxidative stress level in penile tissues from all groups.We also examined the reactive oxygen species(ROS)formation in CCSMC under the stimulation of high glucose to evaluate the anti-oxidative stress capacity of hKLK1 in vitro.Results: hKLK1 attenuated the decrease of erectile function of WTDM while the protective effect was abolished in TGDMH.The decreased protein expression of PI3K(p55),AKT,eNOS,phospho-Enos(Ser1177),cyclic guanosine monophosphate(cGMP)and increased level of phosphoeNOS(Thr495),phosphodiesterase type 5(PDE5),L-type calcium channel(ICa-L)in CC of WTDM were improved in that of TGDM,but deteriorated again with HOE140 in TGDMH.Meanwhile,hKLK1 could elevate the level of cGMP and reduce the concentration of Ca2+ in CCSMC in vitro,but these changes were minified by inhibiting B2R,PI3K or eNOS.In addition,the CC of WTDM had higher protein expression of NADPH oxidase(p22phox,p47phox and NOX2),MDA level and lower activity of SOD when compared to WTR.However,hKLK1 improved these oxidative stress related parameters in TGDM and inhibition with HOE140 in TGDMH would abolish the advantages existed in TGDM.Meanwhile,the level of ROS elevated in high glucose treated CCSMC in a concentrationdependent manner and hKLK1 could considerably inhibit the generation of ROS in vitro.Conclusions: These results indicate that hKLK1 could restore diabetes-induced ED through activation of PI3K/eNOS signaling and inhibition of oxidative stress in the CC in vivo and in vitro.