Background-HCN channels activate the pacemaker current I f , which is thought to contribute significantly to generation and regulation of heart rhythm. HCN4 represents the dominant isotype in the sinoatrial node and binding of cAMP was suggested to be necessary for autonomic heart rate regulation. Methods and Results-In a candidate gene approach, a heterozygous insertion of 13 nucleotides in exon 6 of the HCN4 gene leading to a truncated cyclic nucleotide-binding domain was identified in a 45-year-old woman with sinus bradycardia. Biophysical properties determined by whole-cell patch-clamp recording of HEK293 cells demonstrated that mutant subunits (HCN4-695X) were insensitive to cAMP. Heteromeric channels composed of wild-type and mutant subunits failed to respond to cAMP-like homomeric mutant channels, indicating a dominant-negative suppression of cAMP-induced channel activation by mutant subunits. Pedigree analysis identified 7 additional living carriers showing similar clinical phenotypes, that is, sinus node dysfunction with mean resting heart rate of 45.9Ϯ4.6 bpm (nϭ8) compared with 66.5Ϯ9.1 bpm of unaffected relatives (nϭ6; PϽ0.01). Clinical evaluation revealed no ischemic or structural heart disease in any family member. Importantly, mutant carriers exhibited normal heart rate variance and full ability to accelerate heart rate under physical activity or pharmacological stimulation. Moreover, mutant carriers displayed distinctive sinus arrhythmias and premature beats linked to adrenergic stress. Conclusions-In humans, cAMP responsiveness of I f determines basal heart rate but is not critical for maximum heart rate, heart rate variability, or chronotropic competence. Furthermore, cAMP-activated I f may stabilize heart rhythm during chronotropic response. (Circ Arrhythm Electrophysiol. 2010;3:542-552.)

Altered C-linker oligomerization in heteromeric channels is considered to promote familial tachycardia-bradycardia syndrome and persistent AF, indicating that f-channel dysfunction contributes to the development of atrial tachyarrhythmias.

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independent of cell size, and that calcium cycling protein density is independent of cell area and size. It's well documented that the gap junction protein, connexin-43(Cx43), is highly expressed in peripheral vs. central SAN area. Since it is possible that cell size doesn't completely discriminate between Cx43-negative and positive cells, we measured Cx43-immunolabeling, electrophysiological and Ca 2þ cycling properties of single isolated SANC. Freshly isolated adult rabbit Cx43-negative SANC are, on average, smaller (592.359.2mm 2 , n=579) than Cx43-positive SANC (747.8512.2mm 2 , n=571, p<0.001), but there is no difference in the spontaneous AP cycle length based on Cx43 expression (340.2513.6ms n=30 in Cx43-negative cells, vs. 326.559.0ms n=50 in Cx43-positive cells, p=0.39). AP parameters also do not differ between Cx43-negative and positive cells, but the AP of later has a shorter repolarization time (APD90: 136.958.6ms vs. 110.254.8ms, p<0.01), which is not related to cell size. No significant differences are detected in the major characteristics of basal AP-triggered Ca 2þ-transient or spontaneous Local-Ca 2þ-Releases during diastolic depolarization between Cx43-negative and positive SANC. Acute bAR stimulation (1mM isoproterenol) reduces the AP cycle length to the same level in both Cx43-negative and positive SANC (257.359.0ms n=9 vs. 271.657.7ms n=14, respectively, p=0.25), and no differences of APD90 are detected (p=0.17). Our results indicate that although different in size, there is no statistical difference between single isolated Cx43-negative and positive SANC of spontaneous AP cycle length or during maximal bAR stimulation.

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