The cardiac electrical impulse depends on an orchestrated interplay of transmembrane ionic currents in myocardial cells. Two critical ionic current mechanisms are the inwardly rectifying potassium current (I _K1 ), which is important for maintenance of the cell resting membrane potential, and the sodium current (I _Na ), which provides a rapid depolarizing current during the upstroke of the action potential. By controlling the resting membrane potential, I _K1 modifies sodium channel availability and therefore, cell excitability, action potential duration, and velocity of impulse propagation. Additionally, I _K1 –I _Na interactions are key determinants of electrical rotor frequency responsible for abnormal, often lethal, cardiac reentrant activity. Here, we have used a multidisciplinary approach based on molecular and biochemical techniques, acute gene transfer or silencing, and electrophysiology to show that I _K1 –I _Na interactions involve a reciprocal modulation of expression of their respective channel proteins (Kir2.1 and Na _V 1.5) within a macromolecular complex. Thus, an increase in functional expression of one channel reciprocally modulates the other to enhance cardiac excitability. The modulation is model-independent; it is demonstrable in myocytes isolated from mouse and rat hearts and with transgenic and adenoviral-mediated overexpression/silencing. We also show that the p ost synaptic density, d iscs large, and z onula occludens-1 (PDZ) domain protein SAP97 is a component of this macromolecular complex. We show that the interplay between Na _v 1.5 and Kir2.1 has electrophysiological consequences on the myocardium and that SAP97 may affect the integrity of this complex or the nature of Na _v 1.5–Kir2.1 interactions. The reciprocal modulation between Na _v 1.5 and Kir2.1 and the respective ionic currents should be important in the ability of the heart to undergo self-sustaining cardiac rhythm disturbances.

Histone H3 lysine 4 (H3K4me) methyltransferases and their cofactors are essential for embryonic development and the establishment of gene expression patterns in a cell-specific and heritable manner. However, the importance of such epigenetic marks in maintaining gene expression in adults and in initiating human disease is unclear. Here, we addressed this question using a mouse model in which we could inducibly ablate PAX interacting (with transcription-activation domain) protein 1 (PTIP), a key component of the H3K4me complex, in cardiac cells. Reducing H3K4me3 marks in differentiated cardiomyocytes was sufficient to alter gene expression profiles. One gene regulated by H3K4me3 was Kv channel-interacting protein 2 (Kcnip2), which regulates a cardiac repolarization current that is downregulated in heart failure and functions in arrhythmogenesis. This regulation led to a decreased sodium current and action potential upstroke velocity and significantly prolonged action potential duration (APD). The prolonged APD augmented intracellular calcium and in vivo systolic heart function. Treatment with isoproterenol and caffeine in this mouse model resulted in the generation of premature ventricular beats, a harbinger of lethal ventricular arrhythmias. These results suggest that the maintenance of H3K4me3 marks is necessary for the stability of a transcriptional program in differentiated cells and point to an essential function for H3K4me3 epigenetic marks in cellular homeostasis.

Genetic hypertension results from numerous phenotypic expressions. We hypothesized that increased calcium current in vascular smooth muscle of genetically hypertensive animals is partly responsible for observed increases in agonist sensitivity, contractility, and calcium influx. Using adult, spontaneously hypertensive stroke-prone rats (SHRSP) and normotensive Wistar-Kyoto (WKY) controls from an inbred colony, we characterized calcium current in smooth muscle cells isolated from cerebral arteries. Calcium current in WKY cells reached a maximum of -27.7 +/- 2.7 pA (n = 32) at +20 mV. Peak inward current at +20 mV in SHRSP cells had a mean amplitude of -44.4 +/- 3.0 pA (n = 72, P < .05). SHRSP cells exhibited a higher calcium current density. Maximal inward current normalized to cell capacitance yielded mean values of 2.07 +/- 0.11 pA/pF for WKY (n = 32) and 2.80 +/- 0.12 pA/pF (n = 79) for SHRSP (P < .05) cells. Transient-type Ca2+ channel current had the same magnitude and current-voltage relation in both cell types, giving an L-type/T-type ratio of 3.85 for WKY and 6.25 for SHRSP cells. The voltage-dependent inactivation curve for SHRSP calcium current was shifted to the right only over the range of -50 to -30 mV, but the half-maximal inactivation voltages and Boltzmann coefficients were not significantly different between cell types. Increased calcium inward current in this model of genetic hypertension could account in part for altered calcium homeostasis and increased vascular reactivity, contributing to hypertension and vasospasm.

Studies were performed to assess whether ATP-sensitive K+ (KATP) channels on rabbit preglomerular vessels can influence afferent arteriolar (AA) tone. K+ channels with a slope conductance of 258 +/- 13 (n = 7) pS and pronounced voltage dependence were demonstrated in excised patches from vascular smooth muscle cells of microdissected preglomerular segments. Channel activity was markedly reduced by 1 mM ATP and in a dose-dependent fashion by glibenclamide (10(-9) M to 10(-6) M), a specific antagonist of KATP channels. 10(-5) M diazoxide, a K+ channel opener, activated these channels in the presence of ATP, and this effect was also blocked by glibenclamide. To determine the role of these KATP channels in the control of vascular tone, diazoxide was tested on isolated perfused AA. After preconstriction from a control diameter of 13.1 +/- 1.1 to 3.5 +/- 2.1 microns with phenylephrine (PE), addition of 10(-5) M diazoxide dilated vessels to 11.2 +/- 0.7 microns, which was not different from control. Further addition of 10(-5) M glibenclamide reconstricted the vessels to 5.8 +/- 1.5 microns (n = 5; P less than 0.03). In support of its specificity for KATP channels, glibenclamide did not reverse verapamil induced dilation in a separate series of experiments. To determine whether intracellular ATP levels can effect AA tone, studies were conducted to test the effect of the glycolytic inhibitor 2-deoxy-D-glucose. After preconstriction from 13.4 +/- 3.2 to 7.7 +/- 1.3 microns with PE, bath glucose was replaced with 6 mM 2-deoxy-D-glucose. Within 10 min, the arteriole dilated to a mean value of 11.8 +/- 1.4 microns (n = 6; NS compared to control). Subsequent addition of 10(-5) M glibenclamide significantly reconstricted the vessels to a diameter of 8.6 +/- 0.5 micron (P less than 0.04). These data demonstrate that KATP channels are present on the preglomerular vasculature and that changes in intracellular ATP can directly influence afferent arteriolar tone via these channels.

Evidence supports the expression of brain‐type sodium channels in the heart. Their functional role, however, remains controversial. We used global Na _V 1.6‐null mice to test the hypothesis that Na _V 1.6 contributes to the maintenance of propagation in the myocardium and to excitation‐contraction (EC) coupling. We demonstrated expression of transcripts encoding full‐length Na _V 1.6 in isolated ventricular myocytes and confirmed the striated pattern of Na _V 1.6 fluorescence in myocytes. On the ECG, the PR and QRS intervals were prolonged in the null mice, and the Ca ²⁺ transients were longer in the null cells. Under patch clamping, at holding potential (HP) = –120 mV, the peak I _Na was similar in both phenotypes. However, at HP = –70 mV, the peak I _Na was smaller in the nulls. In optical mapping, at 4 mM [K ⁺ ] _o , 17 null hearts showed slight (7%) reduction of ventricular conduction velocity (CV) compared to 16 wild‐type hearts. At 12 mM [K ⁺ ] _o , CV was 25% slower in a subset of 9 null vs . 9 wild‐type hearts. These results highlight the importance of neuronal sodium channels in the heart, whereby Na _V 1.6 participates in EC coupling, and represents an intrinsic depolarizing reserve that contributes to excitation.—Noujaim, S. F., Kaur, K., Milstein, M., Jones, J. M., Furspan, P., Jiang, D., Auerbach, D. S., Herron, T., Meisler, M. H., Jalife, J. A null mutation of the neuronal sodium channel Na _V 1.6 disrupts action potential propagation and excitation‐contraction coupling in the mouse heart. FASEB J. 26, 63–72 (2012). www.fasebj.org

Rationale:: The rapid delayed rectifier potassium current, I _Kr , which flows through the human ether-a-go-go-related (hERG) channel, is a major determinant of the shape and duration of the human cardiac action potential (APD). However, it is unknown whether the time dependency of I _Kr enables it to control APD, conduction velocity (CV), and wavelength (WL) at the exceedingly high activation frequencies that are relevant to cardiac reentry and fibrillation. Objective:: To test the hypothesis that upregulation of hERG increases functional reentry frequency and contributes to its stability. Methods and Results:: Using optical mapping, we investigated the effects of I _Kr upregulation on reentry frequency, APD, CV, and WL in neonatal rat ventricular myocyte (NRVM) monolayers infected with GFP (control), hERG ( I _Kr ), or dominant negative mutant hERG G628S. Reentry frequency was higher in the I _Kr -infected monolayers (21.12±0.8 Hz; n=43 versus 9.21±0.58 Hz; n=16; P 50%) than control during pacing at 1 to 5 Hz. CV was similar in both groups at low frequency pacing. In contrast, during high-frequency reentry, the CV measured at varying distances from the center of rotation was significantly faster in I _Kr -infected monolayers than controls. Simulations using a modified NRVM model predicted that rotor acceleration was attributable, in part, to a transient hyperpolarization immediately following the AP. The transient hyperpolarization was confirmed experimentally. Conclusions:: hERG overexpression dramatically accelerates reentry frequency in NRVM monolayers. Both APD and WL shortening, together with transient hyperpolarization, underlies the increased rotor frequency and stability.