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Funny current ( I f ), formed by hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels), is supposed to be crucial for the membrane clock regulating the cardiac pacemaker mechanism. We examined the presence and activity of HCN channels in the brown trout ( Salmo trutta fario) sinoatrial (SA) pacemaker cells and their putative role in heart rate ( f H ) regulation. Six HCN transcripts (HCN1, HCN2a, HCN2ba, HCN2bb, HCN3, and HCN4) were expressed in the brown trout heart. The total HCN transcript abundance was 4.0 and 4.9 times higher in SA pacemaker tissue than in atrium and ventricle, respectively. In the SA pacemaker, HCN3 and HCN4 were the main isoforms representing 35.8 ± 2.7 and 25.0 ± 1.5%, respectively, of the total HCN transcripts. Only a small I f  with a mean current density of −1.2 ± 0.37 pA/pF at −140 mV was found in 4 pacemaker cells out of 16 spontaneously beating cells examined, despite the optimization of recording conditions for I f  activity. I f  was not found in any of the 24 atrial myocytes and 21 ventricular myocytes examined. HCN4 coexpressed with the MinK-related peptide 1 (MiRP1) β-subunit in CHO cells generated large I f  currents. In contrast, HCN3 (+MiRP1) failed to produce I f  in the same expression system. Cs +  (2 mM), which blocked 84 ± 12% of the native I f , reversibly reduced f H  19.2 ± 3.6% of the excised multicellular pacemaker tissue from 53 ± 5 to 44 ± 5 beats/min ( P &lt; 0.05). However, this effect was probably due to the reduction of I Kr , which was also inhibited (63.5 ± 4.6%) by Cs + . These results strongly suggest that f H  regulation in the brown trout heart is largely independent on I f .

american journal of physiology. regulatory, integrative and comparative physiology/american journal of physiology. regulatory, integrative, and comparative physiology

Small functional If current in sinoatrial pacemaker cells of the brown trout (Salmo trutta fario) heart despite strong expression of HCN channel transcripts