Nucleoporin 50 mediates Kcna4 transcription to regulate cardiac electrical activity

Emerging evidence has demonstrated that nucleoporins (Nups) play a pivotal role in cell-type-specific gene regulation, but how they control the expression and activity of ion channel genes in the heart remains unclear. Nup50, which is localized in the nucleus of cardiomyocytes, selectively induced an increase in the transcription and translation of Kcna4. The Kcna4 gene encodes a K+ voltage gated channel of shaker-related subfamily member 4 and is essential for regulating the action potential in cardiac membranes. Using immunofluorescence imaging, luciferase assays, and chromatin immunoprecipitation assays, we identified that the direct binding of the FG-repeat domain within Nup50 to the proximity of the Kcna4 promoter was required to activate the transcription and subsequent translation of Kcna4. Functionally, Nup50 overexpression increased the currents of KCNA4-encoded Ito,s channels, and reverse knockdown of Nup50 resulted in a remarkable decrease in the amplitude of Ito,s currents in cardiomyocytes. Moreover, a positive correlation between Nup50 and Kcna4 mRNA and protein expression was observed in heart tissues subjected to ischemic insults. These findings provide insights into the homeostatic control of cardiac electrophysiology through Nup-mediated regulation.

Autoantibodies to the β1-Adrenoceptor from Patients with Periodontitis as a Risk Factor for Cardiac Dysfunction

The presence of serum autoantibodies in periodontitis (P) patients against β1-adrenoceptor (β1-AR), using cardiac membranes or a synthetic β1-AR peptide corresponding to the second extracellular loop of human β1-AR as antigens, permit us to detect circulating antibody from 40 P patients but not in 20 normal individuals (control). Simultaneously, the P patients exhibited a decrease in HRV. Anti-β1-AR IgG titters correlated with the decrease in HRV of the same patients and the anti-β1-AR peptide IgG displayed partial agonist-like activity and modified the contractility of isolated atria, produced cyclic nucleotides, and inhibited the β1-AR agonistic activity of isoproterenol. We demonstrated in this study an association between periodontitis infection and an increased risk of cardiac disease, thereby highlighting the role of anti-β1-AR autoantibodies in alteration of myocardial contractility.

Effects of ω-3 polyunsaturated fatty acids on cardiac sarcolemmal Na+/H+ exchange

Myocardial ischemia-reperfusion activates the Na+/H+ exchanger, which induces arrhythmias, cell damage, and eventually cell death. Inhibition of the exchanger reduces cell damage and lowers the incidence of arrhythmias after ischemia-reperfusion. The ω-3 polyunsaturated fatty acids (PUFAs) are also known to be cardioprotective and antiarrhythmic during ischemia-reperfusion challenge. Some of the action of PUFAs may occur via inhibition of the Na+/H+ exchanger. The purpose of our study was to determine the capacity for selected PUFAs to alter cardiac sarcolemmal (SL) Na+/H+exchange. Cardiac membranes highly enriched in SL vesicles were exposed to 10–100 μM eicosapentanoic acid (EPA) or docosahexanoic acid (DHA). H+-dependent 22Na+ uptake was inhibited by 30–50% after treatment with ≥50 μM EPA or ≥25 μM DHA. This was a specific effect of these PUFAs, because 50 μM linoleic acid or linolenic acid had no significant effect on Na+/H+ exchange. The SL vesicles did not exhibit an increase in passive Na+ efflux after PUFA treatment. In conclusion, EPA and DHA can potently inhibit cardiac SL Na+/H+ exchange at physiologically relevant concentrations. This may explain, in part, their known cardioprotective effects and antiarrhythmic actions during ischemia-reperfusion.

A highly potent peptide analgesic that protects against ischemia-reperfusion-induced myocardial stunning

We recently discovered an opioid peptide analgesic, 2′,6′-dimethyltyrosine (Dmt)-d-Arg-Phe-Lys-NH2([Dmt1]DALDA), that can protect against ischemia-induced myocardial stunning. In buffer-perfused hearts, 30-min global ischemia followed by reperfusion resulted in a significant increase in norepinephrine (NE) overflow immediately upon reperfusion and significant decline in contractile force (45%). Pretreatment with [Dmt1]DALDA before ischemia completely abolished myocardial stunning and significantly reduced NE overflow (68%). In contrast, pretreatment with morphine before ischemia only provided brief protection against myocardial stunning and no reduction in NE overflow. [Dmt1]DALDA inhibited [3H]NE uptake into cardiac synaptosomes in vitro (IC50 = 3.9 μM), whereas morphine had no effect. Surprisingly, protection against myocardial stunning was apparent even when hearts were perfused with [Dmt1]DALDA only upon reperfusion, whereas reperfusion with morphine had no effect. Binding studies with [3H][Dmt1]DALDA revealed no high-affinity specific binding in cardiac membranes, suggesting that the cardioprotective actions of [Dmt1]DALDA are not mediated via opioid receptors. These findings suggest that [Dmt1]DALDA is a potent analgesic that may be useful for myocardial stunning resulting from cardiac interventions or myocardial ischemia.

Differential alterations in cardiac adrenergic signaling in chronic hypoxia or norepinephrine infusion

Norepinephrine (NE)-induced desensitization of the adrenergic receptor pathway may mimic the effects of hypoxia on cardiac adrenoceptors. The mechanisms involved in this desensitization were evaluated in male Wistar rats kept in a hypobaric chamber (380 Torr) and in rats infused with NE (0.3 mg · kg−1 · h−1) for 21 days. Because NE treatment resulted in left ventricular (LV) hypertrophy, whereas hypoxia resulted in right (RV) hypertrophy, the selective hypertrophic response of hypoxia and NE was also evaluated. In hypoxia, α1-adrenergic receptors (AR) density increased by 35%, only in the LV. In NE, α1-AR density decreased by 43% in the RV. Both hypoxia and NE decreased β-AR density. No difference was found in receptor apparent affinity. Stimulated maximal activity of adenylate cyclase decreased in both ventricles with hypoxia (LV, 41%; RV, 36%) but only in LV with NE infusion (42%). The functional activities of Gi and Gs proteins in cardiac membranes were assessed by incubation with pertussis toxin (PT) and cholera toxin (CT). PT had an important effect in abolishing the decrease in isoproterenol-induced stimulation of adenylate cyclase in hypoxia; however, pretreatment of the NE ventricle cells with PT failed to restore this stimulation. Although CT attenuates the basal activity of adenylate cyclase in the RV and the isoproterenol-stimulated activity in the LV, pretreatment of NE or hypoxic cardiac membranes with CT has a less clear effect on the adenylate cyclase pathway. The present study has demonstrated that 1) NE does not mimic the effects of hypoxia at the cellular level, i.e., hypoxia has specific effects on cardiac adrenergic signaling, and 2) changes in α- and β-adrenergic pathways are chamber specific and may depend on the type of stimulation (hypoxia or adrenergic).