HCN4 pacemaker channels attenuate the parasympathetic response and stabilize the spontaneous firing of the sinoatrial node

Abstract

Key points The contribution of HCN4 pacemaker channels in the autonomic regulation of the sino‐atrial node (SAN) has been a matter of debate.The transgenic overexpression of HCN4 did not induce tachycardia, but reduced heart rate variability, while the conditional knockdown of HCN4 gave rise to sinus arrhythmia.The response of the SAN to β‐adrenergic stimulation was not affected by overexpression or knockdown of HCN4 channels.When HCN4 channels were knocked down, the parasympathetic response examined by cervical vagus nerve stimulation (CVNS) was enhanced; the CVNS induced complete sinus pause. The overexpression of HCN4 attenuated bradycardia induced by CVNS only during β‐adrenergic stimulation.We concluded that HCN4 pacemaker channels stabilize the spontaneous firing by attenuating the parasympathetic response of the SAN. AbstractThe heart rate is dynamically controlled by the sympathetic and parasympathetic nervous systems that regulate the sinoatrial node (SAN). HCN4 pacemaker channels are the well‐known causative molecule of congenital sick sinus syndrome. Although HCN4 channels are activated by cAMP, the sympathetic response of the SAN was preserved in patients carrying loss‐of‐function mutations of the HCN4 gene. In order to clarify the contribution of HCN4 channels in the autonomic regulation of the SAN, we developed novel gain‐of‐function mutant mice in which the expression level of HCN4 channels could be reversibly changed from zero to ∼3 times that in wild‐type mice, using tetracycline transactivator and the tetracycline responsive element. We recorded telemetric ECGs in freely moving conscious mice and analysed the heart rate variability. We also evaluated the response of the SAN to cervical vagus nerve stimulation (CVNS). The conditional overexpression of HCN4 did not induce tachycardia, but reduced heart rate variability. The HCN4 overexpression also attenuated bradycardia induced by the CVNS only during the β‐adrenergic stimulation. In contrast, the knockdown of HCN4 gave rise to sinus arrhythmia, and enhanced the parasympathetic response; complete sinus pause was induced by the CVNS. In vitro, we compared the effects of acetylcholine on the spontaneous action potentials of single pacemaker cells, and found that similar phenotypic changes were induced by genetic manipulation of HCN4 expression both in the presence and absence of β‐adrenergic stimulation. Our study suggests that HCN4 channels attenuate the vagal response of the SAN, and thereby stabilize the spontaneous firing of the SAN.

Should heart rate variability be “corrected” for heart rate? Biological, quantitative, and interpretive considerations

“…These results can all be explained by the funny channel counteracting the effectiveness of vagal activity without invoking an intrinsic dependency of HRV on heart period. Kozasa et al (2018) also provide direct support for the relative angle scenario of Figure 5b. In isolated pacemaker cells, the basal diastolic depolarization rate in HCN4 knockdown mice was much slower than in the wild type animals (Kozasa et al, 2018, their figure D, p. 821).…”

“…This appears at first sight to be most compatible with the fixed angle scenario of Figure 5b. However, using a murine genetic knockdown of HCN4, Kozasa et al (2018) reported findings that were at odds with a fixed angle scenario. HCN4 is a main component of the funny channel, and this knockdown model mimics the bradycardic effects of ivabradine, as well as its positive, increasing effects on HRV.…”

“…Funny channel blockade prolongs heart period, and many studies show that this bradycardia is coupled to a parallel increase in HRV (Borer & Le Heuzey, 2008;Kurtoglu et al, 2014). Vagal activity is still widely regarded to be the primary driver of HRV under ivabradine because atropine completely prevents the increase in HRV (Kozasa et al, 2018;Mangin et al, 1998). Nonetheless, the increase in HRV is counterintuitive, as ivabradine-evoked bradycardia causes a parallel decrease in blood pressure.…”

A Brief History of Pacemaking

“…Also importantly, GIRK4 channel inactivation has been shown to rescue bradyarrhythmias of HCN4-deficient mice (Mesirca et al, 2014), highlighting a previously unsuspected role of I KACh in counterbalancing I f changes. Complexities in the autonomic modulation of rate have also been described by use of genetically modified mice (Kozasa et al, 2018). These studies suggest that funny channels act to limit excessive bradycardia caused by potent parasympathetic activity.…”

Heart rate regulation in patients with anorexia nervosa

“…While non-neural mechanisms of bradycardia, such as HCN4 down-regulation or the HCN4 c.1737+1G>T mutation, presumably express themselves in HRV boost, a drug-induced parasympathetic dominance has shown to lead to a saturation effect of HRV metrics (de Geus, Gianaros, Brindle, Jennings, & Berntson;Goldberger, Challapalli, Tung, Parker, & Kadish, 2001;Kozasa et al, 2018).…”