Objective To observe the anisotropy of myocardial fiber orientation and conduction cells in canine pulmonary vein , and to investigate the expression of funny current (If ) channel subunit HCN1 ,HCN2 ,HCN4 mRNA in cadiocytes of pulmonary veins and left atrial in acute atrial fibrillation (AF) modle of caine .Methods Fourteen adult mongrel canines were randomly divided into atrial fibrillation group(n=7) and control group(n=7) .A model of acute AF has been developed in rapid atrial pacing (RAP) . The cardiocytes in control group were detect using hematoxylin hematoxylin and eosin (HE) and Periodic acid-Schiff(PAS) stain in the pulmonary vein .The mRNA expressions of HCN1 ,HCN2 ,HCN4 in two group were quantified by semiquantitative reverse transcription transcription-polymerase chain reaction(RT-PCR) .Results There were complex myocardial fiber orientation in pul-monary veins especially in PV-LA junction .PAS-positive cells could be seen in the endomembrane of pulmonary veins muscle sleeve .There were mRNA expression of HCN2 and HCN4 in pulmonary veins and left arial in two group ,but there was no mRNA expression of HCN1 .The mRNA levels of HCN4 from high to low were:pulmonary veins of atrial fibrillation group ,left atrial of atrial fibrillation group ,pulmonary veins of control group ,left atrial of control group(P<0 .05) .HCN2 mRNA levels from high to low were:pulmonary veins of atrial fibrillation group ,left atrial of atrial fibrillation group ,control group(P<0 .05) .There were no difference in the HCN2 mRNA level in pulmonary veins and left atrial in control group (P>0 .05) .Conclusion There are anisotro-py ,conduction cells in pulmonary veins .Up-regulation of pulmonary veins and left atrial HCN 2 ,HCN4 mRNA in atrial fibrillation caine ,may be correlated with the atrial fibrillation and maintenance .