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The activity of pacemaker cells in the sinoatrial node (SAN) is an indicator of normal sinus rhythm.Clinical studies have revealed that the dysfunction of the SAN progressively increases with aging.In this study,we determined the changes in hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) expression and the relationship between aging and canine SAN dysfunction.The results of cardiac electrophysiological determination revealed that the intrinsic heart rate decreased from 168 ±11 beats min-1 in young canines to 120 ± 9 beats min-1 in adults and to 88 ± 9 beats min-1 in aged canines.The sinus node recovery time (SNRT) increased from 412 ± 32 ms in young canines to 620 + 56 ms in adults and to 838 + 120 ms in aged canines.Corrected SNRT (CSNRT) increased from 55 + 12ms in young canines to 117 ± 27ms in adults and to 171 ± 37ms in aged canines.These results indicated that SAN function deteriorated with aging in the canine heart.However,histological staining illustrated that fibrosis was not significantly increased with aging in canine SAN.Real-time polymerase chain reaction indicated that the expression of HCN4 mRNA was downregulated in the elderly canine SAN.Similarly,we also verified that HCN4 protein expression within the SAN declined with aging via immunofluorescence staining and weste blot analysis.Taken together,our data show that electrical remodeling,related to the down-regulation of HCN4,is responsible for the gradually increased incidence of SAN dysfunction with aging.Our results provide further evidence for explaining the mechanisms of age-related deterioration in the SAN.