The generation of the mammalian heartbeat is a complex and vital function requiring multiple and coordinated ionic channel activities. The functional role of low-voltage activated (LVA) T-type calcium channels in the pacemaker activity of the sinoatrial node (SAN) is, to date, unresolved. Here we show that disruption of the gene coding for CaV3.1/alpha1G T-type calcium channels (cacna1g) abolishes T-type calcium current (I(Ca,T)) in isolated cells from the SAN and the atrioventricular node without affecting the L-type Ca2+ current (I(Ca,L)). By using telemetric electrocardiograms on unrestrained mice and intracardiac recordings, we find that cacna1g inactivation causes bradycardia and delays atrioventricular conduction without affecting the excitability of the right atrium. Consistently, no I(Ca,T) was detected in right atrium myocytes in both wild-type and CaV3.1(-/-) mice. Furthermore, inactivation of cacna1g significantly slowed the intrinsic in vivo heart rate, prolonged the SAN recovery time, and slowed pacemaker activity of individual SAN cells through a reduction of the slope of the diastolic depolarization. Our results demonstrate that CaV3.1/T-type Ca2+ channels contribute to SAN pacemaker activity and atrioventricular conduction.

''Pacemaker'' f-channels mediating the hyperpolarization-activated nonselective cation current I f are directly regulated by cAMP. Accordingly, the activity of f-channels increases when cellular cAMP levels are elevated (e.g., during sympathetic stimulation) and decreases when they are reduced (e.g., during vagal stimulation). Although these biophysical properties seem to make f-channels ideal molecular targets for heart rate regulation by the autonomic nervous system, the exact contribution of the major I f-mediating cardiac isoforms HCN2 and HCN4 to sinoatrial node (SAN) function remains highly controversial. To directly investigate the role of cAMP-dependent regulation of hyperpolarization activated cyclic nucleotide activated (HCN) channels in SAN activity, we generated mice with heart-specific and inducible expression of a human HCN4 mutation (573X) that abolishes the cAMP-dependent regulation of HCN channels. We found that hHCN4 -573X expression causes elimination of the cAMP sensitivity of I f and decreases the maximum firing rates of SAN pacemaker cells. In conscious mice, hHCN4 -573X expression leads to a marked reduction in heart rate at rest and during exercise. Despite the complete loss of cAMP sensitivity of If, the relative extent of SAN cell frequency and heart rate regulation are preserved. Our data demonstrate that cAMPmediated regulation of If determines basal and maximal heart rates but does not play an indispensable role in heart rate adaptation during physical activity. Our data also reveal the pathophysiologic mechanism of hHCN4 -573X-linked SAN dysfunction in humans.bradycardia ͉ hyperpolarization cyclic nucleotide-gated ion channels ͉ pacemaker activity ͉ sinoatrial node ͉ transgenic mice H eart automaticity is a fundamental physiological function in higher organisms. The dominant pacemaker of mammalian hearts is the sinoatrial node (SAN). Dysfunction or failure of SAN activity in humans may lead to an abnormally low heart rate, causing such symptoms as palpitations, fatigue, and syncope, which eventually require implantation of a pacemaker device (1). Elevated resting heart rate, in contrast, is associated with increased cardiovascular mortality and morbidity (2, 3). Despite the importance of cardiac pacemaking as a physiological process, the complex mechanisms underlying SAN automaticity and heart rate regulation remain incompletely understood. SAN activity is controlled by the autonomic nervous system; cholinergic and ␤-adrenergic stimulation either slows or accelerates spontaneous SAN activity. Several ionic currents contribute to cardiac automaticity (4-6). Some of these currents are targets of regulation by the autonomic nervous system, but their physiological importance is a matter of debate (4, 5). ''Pacemaker'' f-channels mediating the hyperpolarizationactivated nonselective cation current I f , in particular, are directly regulated by cAMP (7). These channels are homotetrameric or heterotetrameric complexes of hyperpolarization-activated cyclic nucleotide-gated (HCN) subunits (8...