The ‘funny’ current (If) of pacemaker cells is the main membrane mechanism generating the diastolic depolarization, the phase of the action potential responsible for cardiac spontaneous activity. If is also responsible for heart rate (HR) modulation by the autonomic nervous system (DiFrancesco, 2010). In the heart, both during development and in adulthood, the kinetic properties and expression levels of If match the need of the cells to perform pacemaker activity. While at immature stages of embryonic development all cardiomyocytes are autorhythmic and express a robust If current activated within the physiological range of voltages, shortly after birth the pacemaker activity and If expression become restricted to cells of the conduction system only, while in the working myocardium the current is downregulated and its activation is shifted to more negative, non-physiological voltages (DiFrancesco, 2010). The characteristics of native If are the results of a complex interplay of different factors. The kinetic properties and expression levels of the f-channels, encoded by the HCN genes (HCN1–4), are finely regulated by auxiliary proteins (caveolin-3, MiRP-1, KCR-1, SAP97, TRP8) and lipids (phosphatidylinositol 4,5-bisphosphate) and also by post-translational modifications (phosphorylation and glycosylation). Any alteration of these regulatory pathways may modify the contribution of If to the action potential and become arrhythmogenic (Baruscotti et al. 2010). It is known that lipopolysaccharides (LPS) released from Gram-negative bacteria during severe sepsis reduce both native If and HCN2-or HCN4-mediated currents and shift their activation to more negative potentials. However, the basic mechanism involved was still unexplored. In this issue of The Journal of Physiology, Klockner et al. have elucidated molecular aspects of the process underlying such modulation (Klockner et al. 2014). Using the cell-attached configuration of the patch-clamp technique, Klockner et al. have shown that the LPS effect is not mediated by any of the intracellular modulatory pathways affecting HCN channels, but is instead due to a direct interaction with the channel. Klockner et al. have also shown that alterations of HCN current require the integrity of the LPS molecule, since neither the pro-inflammatory lipid A alone nor the O-chain polysaccharidic region alone are effective (Klockner et al. 2014). The effect of LPS on HCN currents has some unusual features and may represent a novel channel modulatory mechanism. While the decrease of current amplitude may simply reflect a direct blocking action on the channel pore, the large (∼20 mV) negative shift of the activation curve implies an interaction with the voltage sensor or at least a distortion of the electrical field sensed by the voltage sensor. Another unusual feature of LPS action is that the HCN channel voltage dependence is modified by an extracellular agent. The extracellular action of LPS is confirmed by experiments showing that HCN channel glycosylation partly prevents LPS action, while de-glycosylation of HCN channels significantly amplifies it (Klockner et al. 2014). Further specific experiments will be required to more fully clarify the details of LPS action on the channel structure. HR and heart rate variability (HRV), the beat-to-beat change of the duration of the R–R interval, are important indicators of cardiovascular conditions, and it is well established that low HR and high HRV are associated with a lower risk of life-threatening cardiac events (Papaioannou et al. 2013). Two components affect HR and HRV: the balance between the sympathetic and parasympathetic outputs, and intrinsic cellular factors (Papaioannou et al. 2013). Because of its involvement in the generation of the heartbeat and HR modulation, the contribution of the funny current may be especially relevant to HRV. It is known for example that supplementation with fish oil (rich in ω-3 polyunsaturated fatty acids (ω-3 PUFAs)) decreases HR, an effect attributable to a reduced If current, and increases HRV (Billman, 2013). Also, the ivabradine-induced pharmacological blockade of If induces a HR reduction and a HRV increase (DiFrancesco & Camm, 2004; Papaioannou et al. 2013). During sepsis, If is specifically decreased by LPS but HRV is decreased too, rather than increased. This atypical behaviour can be explained since as well as inhibiting If, LPS also have a stimulating effect on sympathetic output. In the presence of LPS, the If current is essentially fully inhibited, since although the LPS-induced decrease of If conductance is quantitatively similar to that produced by ω-3 PUFAs or by ivabradine (about 30%), the concurrent hyperpolarizing shift in the voltage dependence effectively zeroes its contribution. An increased sympathetic drive and a concurrent abolishment of the If-mediated rate control narrow the range of physiological rate modulation and are likely to make the system more susceptible to failure under stressful conditions. In summary, the work of Klockner et al. indicates that LPS release during sepsis can modify the If properties by shifting them out of their physiological range, which may impair normal rate control and decrease the cardiac safety factor, making the system more prone to life-threatening perturbations.
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Jul 31, 2014
Yang Liu + 9
Open Access