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G protein-coupled inwardly rectifying K⁺ channels (GIRK) are important in the regulation of heart rate and atrial electrophysiology. GIRK channels are activated by G protein-coupled receptors, including muscarinic M₂ receptors and adenosine A₁ receptors. The aim of this study was to characterize and compare the electrophysiological effects of acetylcholine (ACh) and adenosine on GIRK channels in rat atria. Action potential duration at 90% repolarization (APD₉₀), effective refractory period (ERP), and resting membrane potential (RMP) were investigated in isolated rat atria by intracellular recordings. Both the adenosine analog N6-cyclopentyladenosine (CPA) and ACh profoundly shortened APD₉₀ and ERP and hyperpolarized the RMP. No additive or synergistic effect of CPA and ACh coapplication was observed. To antagonize GIRK channel activation, the specific inhibitor rTertiapin Q (TTQ) was applied. The coapplication of TTQ reversed the CPA and ACh-induced effects. When TTQ was applied without exogenous receptor activator, both APD₉₀ and ERP were prolonged and RMP was depolarized, confirming a basal activity of the GIRK current. The results reveal that activation of A₁ and M₂ receptors has a profound and equal effect on the electrophysiology in rat atrium. This effect is to a major extent mediated through GIRK channels. Furthermore, these results support the notion that atrial GIRK currents from healthy hearts have a basal component and additional activation can be mediated via at least 2 different receptor mechanisms.

Background— Atrial fibrillation (AF) is the most common cardiac arrhythmia. The cardiac sodium channel, Na V 1.5, plays a pivotal role in setting the conduction velocity and the initial depolarization of the cardiac myocytes. We hypothesized that early-onset lone AF was associated with genetic variation in SCN5A . Methods and Results— The coding sequence of SCN5A was sequenced in 192 patients with early-onset lone AF. Eight nonsynonymous mutations (T220I, R340Q, T1304M, F1596I, R1626H, D1819N, R1897W, and V1951M) and 2 rare variants (S216L in 2 patients and F2004L) were identified. Of 11 genopositive probands, 6 (3.2% of the total population) had a variant previously associated with long QT syndrome type 3 (LQTS3). The prevalence of LQTS3-associated variants in the patients with lone AF was much higher than expected, compared with the prevalence in recent exome data (minor allele frequency, 1.6% versus 0.3%; P =0.003), mainly representing the general population. The functional effects of the mutations were analyzed by whole cell patch clamp in HEK293 cells; for 5 of the mutations previously associated with LQTS3, patch-clamp experiments showed an increased sustained sodium current, suggesting a mechanistic overlap between LQTS3 and early-onset lone AF. In 9 of 10 identified mutations and rare variants, we observed compromised biophysical properties affecting the transient peak current. Conclusions— In a cohort of patients with early-onset lone AF, we identified a high prevalence of SCN5A mutations previously associated with LQTS3. Functional investigations of the mutations revealed both compromised transient peak current and increased sustained current.

KCNQ4-encoded voltage-dependent potassium (Kv7.4) channels are important regulators of vascular tone that are severely compromised in models of hypertension. However, there is no information as to the role of these channels in responses to endogenous vasodilators. We used a molecular knockdown strategy, as well as pharmacological tools, to examine the hypothesis that Kv7.4 channels contribute to β-adrenoceptor–mediated vasodilation in the renal vasculature and underlie the vascular deficit in spontaneously hypertensive rats. Quantitative PCR and immunohistochemistry confirmed gene and protein expression of KCNQ1, KCNQ3, KCNQ4, KCNQ5, and Kv7.1, Kv7.4, and Kv7.5 in rat renal artery. Isoproterenol produced concentration-dependent relaxation of precontracted renal arteries and increased Kv7 channel currents in isolated smooth muscle cells. Application of the Kv7 blocker linopirdine attenuated isoproterenol-induced relaxation and current. Isoproterenol-induced relaxations were also reduced in arteries incubated with small interference RNAs targeted to KCNQ4 that produced a ≈60% decrease in Kv7.4 protein level. Relaxation to isoproterenol and the Kv7 activator S-1 were abolished in arteries from spontaneously hypertensive rats, which was associated with ≈60% decrease in Kv7.4 abundance. This study provides the first evidence that Kv7 channels contribute to β-adrenoceptor–mediated vasodilation in the renal vasculature and that abrogation of Kv7.4 channels is strongly implicated in the impaired β-adrenoceptor pathway in spontaneously hypertensive rats. These findings may provide a novel pathogenic link between arterial dysfunction and hypertension.

The long QT syndrome is characterized by a prolongation of the QT interval measured on the surface electrocardiogram. Prolonging the QT interval increases the risk of dangerous ventricular fibrillations, eventually leading to sudden cardiac death. Pharmacologically induced QT interval prolongations are most often caused by antagonizing effects on the repolarizing cardiac current called IKr. In humans IKr is mediated by the human ether-a-go-go related gene (hERG) potassium channel. We recently presented NS3623, a compound that selectively activates this channel. The present study was dedicated to examining the in vivo effects of NS3623. Injection of 30 mg/kg NS3623 shortened the corrected QT interval by 25 +/- 4% in anaesthetized guinea pigs. Accordingly, 50 mg/kg of NS3623 shortened the QT interval by 30 +/- 6% in conscious guinea pigs. Finally, pharmacologically induced QT prolongation by a hERG channel antagonist (0.15 mg/kg E-4031) could be reverted by injection of NS3623 (50 mg/kg) in conscious guinea pigs. In conclusion, the present in vivo study demonstrates that injection of the hERG channel agonist NS3623 results in shortening of the QTc interval as well as reversal of a pharmacologically induced QT prolongation in both anaesthetized and conscious guinea pigs.

We examined the action of levosimendan, a new Ca2+-sensitizing inodilator, on isolated porcine coronary arteries. Vessel rings were studied in isometric myographs. Arterial cyclic adenosine monophosphate (cAMP) levels were determined by radioimmunoassay. Levosimendan (10(-7)-10(-3) M) completely relaxed arteries preconstricted by prostaglandin F2alpha (PGF2alpha) with a pD2 (-logEC50) value of 3.99 +/- 0.05 (n = 6-9 in all experiments). Pretreatment with levosimendan also prevented contraction induced by PGF2alpha. The vasorelaxation produced by levosimendan (10(-7)-10(-3) M) was not attenuated by removal of the endothelium. Levosimendan (10(-7)-10(-3) M) relaxed contractions induced by 30 mM K+ as well as 80 mM K+, whereas the K+ channel opener levcromakalim selectively relaxed contraction induced by 30 mM K+. Neither the cyclooxygenase inhibitor indomethacin nor the beta-adrenoceptor blocker propranolol influenced levosimendan-induced vasorelaxation. The Ca2+-entry blocker isradipine failed to relax arteries precontracted by endothelin-1 in Ca2+-free/EGTA medium. However, levosimendan (10(-7)-3 x 10(-3) M) completely relaxed endothelin-1-induced contractions in this medium. Levosimendan potentiated the relaxant effect of a cAMP-stimulating drug, isoprenaline, but also that of nitroglycerin and isradipine. At a maximal effective concentration, it increased arterial tissue contents of cAMP twofold. In conclusion, levosimendan produces coronary vasorelaxation by a mechanism that seems to be endothelium independent and not mediated by K+ channel opening, Ca2+-entry blockade, release of cyclooxygenase products, or beta-adrenoceptor stimulation. Accumulation of cAMP may possibly participate in vasorelaxation at high concentrations of levosimendan, but a cAMP-independent mechanism seems to be involved at lower concentrations.

This report presents the results of a patch-clamp study examining the role of intracellular ATP in the regulation of endothelial inward rectifier K+ channels. Administration of ATP to the cytosolic surface of inside-out patches reversibly activated the K+ channel within seconds. ATP (1 mM) increased the mean open probability by a factor of 3.5, primarily by increasing the number of openings. Administration of the nonhydrolyzable ATP analogue ATP-gamma-S failed to modulate channel activity. Inhibition of ATP synthesis by administration of cyanide plus iodoacetate resulted in channel closure within 1 to 6 minutes. In experiments in which ATP was coadministered with the metabolic blockers, the channel activity continued unchanged for up to 30 minutes, but when ATP was removed, the activity rapidly decayed. We propose that normal functioning of the inward rectifier K+ channel is ATP dependent. Phosphorylation of the channel molecule is probably essential for maintaining activity.

By virtue of its location at the interface of flowing blood and vascular tissue, the endothelial cell monolayer is in a unique position for interactions with soluble and cellular elements of the blood on one side and with component cells of the vascular tissue on the other. This brief review outlines humoral and contact-mediated endothelial communication with other cells, particularly the resident cells of the vessel wall. Evidence for gap junctional communication channels between endothelium and vascular cells is summarized and discussed in relation to endothelial ion channel activity. Myoendothelial gap junctional communication is proposed as a mechanism involved in vasorelaxation, either independent of or in concert with secreted endothelium-derived relaxing factor(s).