Current In Rabbit Ventricular Myocytes Research Articles

Capsaicin Inhibits Multiple Voltage-Gated Ion Channels in Rabbit Ventricular Cardiomyocytes in TRPV1-Independent Manner.

Capsaicin is a naturally occurring alkaloid derived from chili pepper which is responsible for its hot, pungent taste. It exerts multiple pharmacological actions, including pain-relieving, anti-cancer, anti-inflammatory, anti-obesity, and antioxidant effects. Previous studies have shown that capsaicin significantly affects the contractility and automaticity of the heart and alters cardiovascular functions. In this study, the effects of capsaicin were investigated on voltage-gated ion currents in rabbit ventricular myocytes. Capsaicin inhibited rapidly activated (IKr) and slowly activated (IKs) K+ currents and transient outward (Ito) K+ current with IC50 values of 3.4 µM,14.7 µM, and 9.6 µM, respectively. In addition, capsaicin, at higher concentrations, suppressed voltage-gated Na+ and Ca2+ currents and inward rectifier IK1 current with IC50 values of 42.7 µM, 34.9 µM, and 38.8 µM, respectively. Capsaicin inhibitions of INa, IL-Ca, IKr, IKs, Ito, and IK1 were not reversed in the presence of capsazepine (3 µM), a TRPV1 antagonist. The inhibitory effects of capsaicin on these currents developed gradually, reaching steady-state levels within 3 to 6 min, and the recoveries were usually incomplete during washout. In concentration-inhibition curves, apparent Hill coefficients higher than unity suggested multiple interaction sites of capsaicin on these channels. Collectively, these findings indicate that capsaicin affects cardiac electrophysiology by acting on a diverse range of ion channels and suggest that caution should be exercised when capsaicin is administered to carriers of cardiac channelopathies or to individuals with arrhythmia-prone conditions, such as ischemic heart diseases.

Curcumin, a Multi-Ion Channel Blocker That Preferentially Blocks Late Na+ Current and Prevents I/R-Induced Arrhythmias

Increasing evidence shows that Curcumin (Cur) has a protective effect against cardiovascular diseases. However, the role of Cur in the electrophysiology of cardiomyocytes is currently not entirely understood. Therefore, the present study was conducted to investigate the effects of Cur on the action potential and transmembrane ion currents in rabbit ventricular myocytes to explore its antiarrhythmic property. The whole-cell patch clamp was used to record the action potential and ion currents, while the multichannel acquisition and analysis system was used to synchronously record the electrocardiogram and monophasic action potential. The results showed that 30 μmol/L Cur shortened the 50 and 90% repolarization of action potential by 17 and 7%, respectively. In addition, Cur concentration dependently inhibited the Late-sodium current (INa.L), Transient-sodium current (INa.T), L-type calcium current (ICa.L), and Rapidly delayed rectifying potassium current (IKr), with IC50 values of 7.53, 398.88, 16.66, and 9.96 μmol/L, respectively. Importantly, the inhibitory effect of Cur on INa.L was 52.97-fold higher than that of INa.T. Moreover, Cur decreased ATX II-prolonged APD, suppressed the ATX II-induced early afterdepolarization (EAD) and Ca2+-induced delayed afterdepolarization (DAD) in ventricular myocytes, and reduced the occurrence and average duration of ventricular tachycardias and ventricular fibrillations induced by ischemia–reperfusion injury. In conclusion, Cur inhibited INa.L, INa.T, ICa.L, and IKr; shortened APD; significantly suppressed EAD and DAD-like arrhythmogenic activities at the cellular level; and exhibited antiarrhythmic effect at the organ level. It is first revealed that Cur is a multi-ion channel blocker that preferentially blocks INa.L and may have potential antiarrhythmic property.

Adrenomedullin Stimulates Porcine Sinus Node Automaticity

Adrenomedullin (AM) is a vasodilatory and chronotropic peptide that has cardioprotective properties in settings of ischemia-reperfusion. Specifically, the 52-amino acid peptide hormone has been shown to reduce infarct size and potentially be protective against malignant arrhythmias. AM reduces L-type calcium currents in rabbit ventricular myocytes, and can possibly be utilized as an antiarrhythmic drug. However, at present it is unclear whether AM has cardiac electrophysiological effects in vivo. In the present study, we investigated whether AM affects sinus and atrioventricular nodal functions, cardiac pacing thresholds, refractory properties and intracardiac conduction intervals in an intact porcine model (Norwegian landrace and Yorkshire hybrids; n=12). Hemodynamic and electrophysiological parameters were recorded at baseline and following 60 min of AM-infusion (100 ng/kg/min). The hemodynamic effects of AM were characterized by a reduced mean arterial pressure (76 ± 9 vs. 57 ± 6; p<0.05; mmHg), tachycardia (91 ± 13 vs. 112 ± 15; p<0.05; beats/min) and an augmented cardiac index (131 ± 20 vs. 176 ± 28; p<0.05; ml/min/kg). The sinus cycle length was reduced (674 ± 89 vs. 546 ± 73; p<0.05; ms), sinoatrial conduction time was unaltered (79 ± 24 vs. 72 ± 19; ns; ms), and the sinus node recovery time (SNRT) was reduced at paced cycle lengths of 425 ms, 375 ms and 325 ms (average SNRT 1034 ± 449 vs.704 ± 141; p<0.05; ms). Diastolic pacing thresholds, effective refractory periods, Wenckebach cycle length and intracardiac conduction intervals were unaffected by AM. In conclusion, AM stimulates sinus node function with a reduced sinus cycle length and increased automaticity but has no further detectable intracardiac electrophysiological effects. Whether the increased automaticity is a direct effect on the sinus node or an indirect effect mediated through the autonomic nervous system remains to be determined.

Retraction: Calmodulin kinase and a calmodulin-binding ’IQ’ domain facilitate L-type Ca2+ current in rabbit ventricular myocytes by a common mechanism.

Retraction: Calmodulin kinase and a calmodulin-binding ’IQ’ domain facilitate L-type Ca2+ current in rabbit ventricular myocytes by a common mechanism.

Protein kinaseC and Ca(2+) -calmodulin-dependent protein kinaseII mediate the enlarged reverse INCX induced by ouabain-increased late sodium current in rabbit ventricular myocytes.

What is the central question of this study? What are the effects of protein kinaseC (PKC) and Ca(2+) -calmodulin-dependent protein kinaseII (CaMKII) on late sodium current (INaL ), reverse Na(+) -Ca(2+) exchange current (reverse INCX ) or intracellular Ca(2+) levels changed by ouabain? What is the main finding and its importance? Ouabain, even at low concentrations (0.5-8.0μm), can increase INaL and reverse INCX , and these effects may contribute to the effect of the glycoside to increase Ca(2+) transients and contractility. Both PKC and CaMKII activities may mediate or modulate these processes. It has been reported that the cardiac glycoside ouabain can increase the late sodium current (INaL ), as well as the diastolic intracellular calcium concentration and contractile shortening. Whether an increase of INaL participates in a pathway that can mediate the positive inotropic response to ouabain is unknown. We therefore determined the effects of ouabain on INaL , reverse Na(+) -Ca(2+) exchange current (reverse INCX ), intracellular Ca(2+) ([Ca(2+) ]i ) levels and contractile shortening in rabbit isolated ventricular myocytes. Ouabain (0.1-8μm) markedly increased INaL and reverse INCX in a concentration-dependent manner, with significant effects at concentrations as low as 0.5 and 1μm. These effects of ouabain were suppressed by the INaL inhibitors TTX and ranolazine, the protein kinaseC inhibitor bisindolylmaleimide and the Ca(2+) -calmodulin-dependent protein kinaseII inhibitor KN-93. The enhancement by 0.5μm ouabain of ventricular myocyte contractility and intracellular Ca(2+) transients was suppressed by 2.0μm TTX. We conclude that ouabain, even at low concentrations (0.5-8.0μm), can increase INaL and reverse INCX , and these effects may contribute to the effect of the glycoside to increase Ca(2+) transients and contractility. Both protein kinaseC and Ca(2+) -calmodulin-dependent protein kinaseII activities may mediate or modulate these processes.

Effect of didrovaltrate on l-calcium current in rabbit ventricular myocytes

ObjectiveTo investigate the effect of didrovaltrate on L-type calcium current (Ica-L) in rabbit ventricular myocytes. MethodsWe used the whole cell patch clamp recording technique. ResultsDidrovaltrate at concentrations of 30 μg/L and 100 μg/L significantly decreased peak ICa-L (ICa-Lmax) from (6.01 ± 0.48) pA/pF to (3.45±0.27) pA/pF and (2.16 ± 0.19) pA/pF (42.6% and 64.1%, n=8, P<0.01), respectively. Didrovaltrate shifted upwards the current-voltage curves of ICa-L without changing their active, peak and reverse potentials. Didrovaltrate affected the steady-state inactivation of ICa-L. The half activation potential (V1/2) was significantly shifted from (−26 ± 2) to (−36 ± 3) mV (n=6, P<0.05), with a significant change in the slope factor (k) (from 8.8 ± 0.8 to 11.1 ± 0.9, n=6, P<0.05). Didrovaltrate did not affect the activation curve. ConclusionDidrovaltrate blocks ICa-L in a concentration-dependent manner and probably inhibits ICa-L in its inactive state, which may contribute to its cardiovascular effect.

Calmodulin kinase II and protein kinase C mediate the effect of increased intracellular calcium to augment late sodium current in rabbit ventricular myocytes

An increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) augments late sodium current (I(Na.L)) in cardiomyocytes. This study tests the hypothesis that both Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) mediate the effect of increased [Ca(2+)](i) to increase I(Na.L). Whole cell and open cell-attached patch clamp techniques were used to record I(Na.L) in rabbit ventricular myocytes dialyzed with solutions containing various concentrations of [Ca(2+)](i). Dialysis of cells with [Ca(2+)](i) from 0.1 to 0.3, 0.6, and 1.0 μM increased I(Na.L) in a concentration-dependent manner from 0.221 ± 0.038 to 0.554 ± 0.045 pA/pF (n = 10, P < 0.01) and was associated with an increase in mean Na(+) channel open probability and prolongation of channel mean open-time (n = 7, P < 0.01). In the presence of 0.6 μM [Ca(2+)](i), KN-93 (10 μM) and bisindolylmaleimide (BIM, 2 μM) decreased I(Na.L) by 45.2 and 54.8%, respectively. The effects of KN-93 and autocamtide-2-related inhibitory peptide II (2 μM) were not different. A combination of KN-93 and BIM completely reversed the increase in I(Na.L) as well as the Ca(2+)-induced changes in Na(+) channel mean open probability and mean open-time induced by 0.6 μM [Ca(2+)](i). Phorbol myristoyl acetate increased I(Na.L) in myocytes dialyzed with 0.1 μM [Ca(2+)](i); the effect was abolished by Gö-6976. In summary, both CaMKII and PKC are involved in [Ca(2+)](i)-mediated augmentation of I(Na.L) in ventricular myocytes. Inhibition of CaMKII and/or PKC pathways may be a therapeutic target to reduce myocardial dysfunction and cardiac arrhythmias caused by calcium overload.

Blocking effect of methylflavonolamine on human NaV1.5 channels expressed in Xenopus laevis oocytes and on sodium currents in rabbit ventricular myocytes

To investigate the blocking effects of methylflavonolamine (MFA) on human Na(V)1.5 channels expressed in Xenopus laevis oocytes and on sodium currents (I(Na)) in rabbit ventricular myocytes. Human Na(V)1.5 channels were expressed in Xenopus oocytes and studied using the two-electrode voltage-clamp technique. I(Na) and action potentials in rabbit ventricular myocytes were studied using the whole-cell recording. MFA and lidocaine inhibited human Na(V)1.5 channels expressed in Xenopus oocytes in a positive rate-dependent and concentration-dependent manner, with IC(50) values of 72.61 micromol/L and 145.62 micromol/L, respectively. Both of them markedly shifted the steady-state activation curve of I(Na) toward more positive potentials, shifted the steady-state inactivation curve of I(Na) toward more negative potentials and postponed the recovery of the I(Na) inactivation state. In rabbit ventricular myocytes, MFA inhibited I(Na) with a shift in the steady-state inactivation curve toward more negative potentials, thereby postponing the recovery of the I(Na) inactivation state. This shift was in a positive rate-dependent manner. Under current-clamp mode, MAF significantly decreased action potential amplitude (APA) and maximal depolarization velocity (V(max)) and shortened action potential duration (APD), but did not alter the resting membrane potential (RMP). The demonstrated that the kinetics of sodium channel blockage by MFA resemble those of class I antiarrhythmic agents such as lidocaine. MFA protects the heart against arrhythmias by its blocking effect on sodium channels.

Effects of β-adrenergic stimulation on L-type Ca2+ currents in rabbit ventricular myocytes during administration of volatile anesthetics

Effects of β-adrenergic stimulation on L-type Ca2+ currents in rabbit ventricular myocytes during administration of volatile anesthetics

Association between decreased intra-thoracic impedance and ventricular tachyarrhythmias

The increased risk of ventricular tachyarrhythmias (VT) in the congestive heart failure (CHF) population has been well described [ [1] Bigger Jr., J.T. Fleiss J.L. Kleiger R. Miller J.P. Rolnitzky L.M. The relationships among ventricular arrhythmias, left ventricular dysfunction, and mortality in the 2 years after myocardial infarction. Circulation. 1984; 69: 250-258 Crossref PubMed Scopus (1079) Google Scholar ]. The concept of mechanical–electrical feedback has been proposed as a potential mechanism for the observed clinical relationship between CHF and VT in multiple animal [ 2 Lab M.J. Mechanically dependent changes in action potentials recorded from the intact frog ventricle. Circ Res. 1978; 42: 519-528 Crossref PubMed Scopus (110) Google Scholar , 3 Franz M.R. Cima R. Wang D. Profitt D. Kurz R. Electrophysiological effects of myocardial stretch and mechanical determinants of stretch-activated arrhythmias. Circulation. 1992; 86: 968-978 Crossref PubMed Scopus (356) Google Scholar , 4 Hansen D.E. Craig C.S. Hondeghem L.M. Stretch-induced arrhythmias in the isolated canine ventricle. Evidence for the importance of mechanoelectrical feedback. Circulation. 1990; 81: 1094-1105 Crossref PubMed Scopus (253) Google Scholar , 5 Parker K.K. Lavelle J.A. Taylor L.K. Wang Z. Hansen D.E. Stretch-induced ventricular arrhythmias during acute ischemia and reperfusion. J Appl Physiol. 2004; 97: 377-383 Crossref PubMed Scopus (20) Google Scholar ] and computer [ 6 Healy S.N. McCulloch A.D. An ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes. Europace. 2005; 7: 128-134 Crossref PubMed Scopus (39) Google Scholar , 7 Kohl P. Hunter P. Noble D. Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models. Prog Biophys Mol Biol. 1999; 71: 91-138 Crossref PubMed Scopus (228) Google Scholar ] models. Despite these reports, the dynamic relationship between pulmonary congestion/decompensated heart failure and VT in the clinical environment is not well understood. A new cardiac defibrillator/cardiac resynchronization therapy (CRT-D) system is capable of ambulatory monitoring of ventricular arrhythmias as well as intra-thoracic fluid using thoracic impedance. The system records a daily mean impedance value and calculates an index of net pulmonary fluid accumulation [ [8] Yu C.M. Wang L. Chau E. et al. Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization. Circulation. 2005; 112: 841-848 Crossref PubMed Scopus (555) Google Scholar ]. An acute decrease in intra-thoracic impedance has been previously correlated with an increase in left ventricular filling pressure [ [8] Yu C.M. Wang L. Chau E. et al. Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization. Circulation. 2005; 112: 841-848 Crossref PubMed Scopus (555) Google Scholar ]. We report a case of repeated VT episodes preceded by acutely lowered thoracic impedance.

CaM Kinase Inhibition

See related article, pages 1092–1099 Calcium/calmodulin-dependent protein kinases (CaM kinases) encompass a family of serine/threonine kinases (CaMKI-CaMKIV) that are regulated by calcium bound to calmodulin.1,2 Four separate genes encode the CaMKII subunits (α, β, γ, δ), and 6 to 12 subunits homo- or heteromultimerize to form the active enzyme. Each subunit contains an N-terminal catalytic domain which binds ATP and substrate, a regulatory domain which includes an autoinhibitory domain and a Ca/Calmodulin binding domain, and a C-terminal association (multimerization) domain. In the heart, a number of splice variants of CaMKIIδ are expressed, including CaMKIIδ2 (CaMKIIδC) and CaMKIIδ3 (CaMKIIδB) which differ from each other by the presence of a nuclear localization signal between the regulatory domain and the association domain in CaMKIIδ3.3 CaMKII functions as a local calcium sensor in the heart. At baseline, the autoinhibitory domain prevents substrate binding to the enzyme.1,2 In cardiac myocytes, intracellular Ca2+ [Ca2+]i rises because of transmembrane influx through L-type Ca2+ channels ( I Ca) or the Na/Ca exchanger and release from internal stores such as the sarcoplasmic reticulum (SR). When intracellular Ca2+ rises, it binds to the EF-hand motifs at the N- and C-terminals calmodulin, an ≈150 amino acid ubiquitous intracellular protein. The Ca/CaM complex then binds to the regulatory domain of CaMKII and removes …

Protein kinase C inhibitor peptides depress hydrogen peroxide-induced increase of L-type calcium current in rabbit ventricular myocytes

Protein kinase C inhibitor peptides depress hydrogen peroxide-induced increase of L-type calcium current in rabbit ventricular myocytes

A nonselective cation current in rabbit ventricular myocytes

Currents contributing repolarization in rabbit ventricular myocytes are very complex because the I(to.s) covers almost the whole repolarization phase of the action potential. The other components of repolarizing currents, such as I(Kr) and I(Ks), are small. In the present work, with whole-cell patch-clamp technique, a clear evidence was provided for the existence of a hitherto unreported, voltage-dependent, nonselective cation current (NSCC) in rabbit ventricular myocytes. Na(+), K(+), and Cs(+) can permeate through the nonselective cation channel and the NSCC can be blocked by Gd(3+). The channels are sensitive to Ca(2+), Mg(2+)-free, and insulin in bathing solution. Activation of NSCC may provide complex effects on action potential configuration depending on the basal conditions and the experimental situations. Considering the voltage dependence and rapid activation kinetics of this current, we speculate that this current can provide an important influence on all repolarization phases of the action potential as well as contribute to the resting membrane potential in rabbit ventricular myocytes. In addition, it is conceivable that, under certain pathophysiological conditions (e.g., ischemia or excessive mechanical stress), the sensitivity of the channels could be altered in such a way that the conductance opens even in the presence of physiological Ca(2+), Mg(2+), and insulin. It might be an important factor on the repolarization of the action potential. Changes in NSCC may lead to an induction or inhibition of arrhythmia.

An ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes

To develop an ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes consistent with experimental observations, that can be used to investigate the role of these currents in intact myocardium. A non-specific cation-selective stretch-activated current I(ns), was incorporated into the Puglisi-Bers ionic model of epicardial, endocardial and midmyocardial ventricular myocytes. Using the model, we predict a reduction in action potential duration at 20% repolarization (APD(20)) and action potential amplitude, an elevated resting transmembrane potential and either an increase or decrease in APD(90), depending on the reversal potential of I(ns). A stretch-induced decrease in I(K1) (70%), plus a small I(ns) current (g(ns) = 10 pS), results in a reduction in APD(20) and increase in APD(90), and a reduced safety factor for conduction. Increasing I(K1) (150%) plus a large I(ns) current (g(ns) = 40 pS), also leads to a reduction in APD(20) and increase in APD(90), but with a greater safety factor. Endocardial and midmyocardial cells appear to be the most sensitive to stretch-induced changes in action potential. The addition of the K(+)-specific stretch-activated current (SAC) I(Ko) results in action potential shortening. Transmural heterogeneity of I(Ko) may reduce repolarization gradients in intact myocardium caused by intrinsic ion channel densities, nonuniform strains and electrotonic effects.

Effect of lactic acidosis on L-type Ca2+ current in rabbit ventricular myocytes

Effect of lactic acidosis on L-type Ca2+ current in rabbit ventricular myocytes

Effect of isoflurane with/without nitrous oxide on L-type calcium current in rabbit ventricular myocytes and influence of phosphorilation

Effect of isoflurane with/without nitrous oxide on L-type calcium current in rabbit ventricular myocytes and influence of phosphorilation

Sera from patients with idiopathic dilated cardiomyopathy decrease ICa in cardiomyocytes isolated from rabbits.

Autoantibodies against muscarinic and adrenergic receptors have been found in the sera of patients with idiopathic dilated cardiomyopathy (IDC) and Chagas disease, but it is still unclear whether they can functionally interact with their respective receptors to modulate cardiac functions. In this study, our goal was to detect the presence of those antibodies in the sera of patients with IDC and characterize their electrophysiological effects on cardiomyocytes from rabbits. By using ELISA immunoassays, we detected high titers of antibodies against muscarinic M2 receptors in the sera of all IDC patients, whereas the detection of antibodies against the beta1-receptor occurred in 50% of them. Electrophysiological experiments using the whole cell configuration of the patch-clamp technique showed that sera from 43% of IDC patients induced a significant decrease (approximately 26%) in isoproterenol-stimulated L-type Ca2+ currents in rabbit ventricular myocytes, whereas the sera from healthy blood donors failed to do so. As expected, IDC sera also decreased the action potential duration (by 10.5%) due to a shortening of the plateau phase. Sera that reduced isoproterenol-stimulated L-type Ca2+ currents did not cause any effect on K+ currents. We conclude that sera from IDC patients have autoantibodies, which interact with muscarinic M2 receptors of rabbit cardiomyocytes, acting in an agonist-like fashion. This action results in changes in electrogenesis, which, as often observed in patients with IDC, could initiate ventricular arrhythmias that lead to sudden death.

Inhibition of fast sodium current in rabbit ventricular myocytes by protein tyrosine kinase inhibitors.

The present study investigated the effects of protein tyrosine kinase inhibitors on the fast sodium current ( I(Na)) in rabbit ventricular myocytes. Single rabbit ventricular myocytes were isolated enzymatically using Langendorff perfusion. I(Na) was recorded using the whole-cell patch-clamp technique at room temperature. The protein tyrosine kinase inhibitors genistein, AG957, ST638, and PP2 reversibly inhibited I(Na) in a concentration-dependent manner. At a test pulse potential of -30 mV, genistein (n=7) inhibited I(Na) by 37.7+/-3.2%, 53.4+/-2.5%, and 71.8+/-2.7% at concentrations of 15, 50, and 100 microM, respectively, without changing the voltage dependence of activation, while 100 microM AG957, 100 microM ST638, and 30 microM PP2 inhibited I(Na) by 38.7+/-2.4, 35.8+/-3.4, and 21.1+/-3.9%, respectively. Genistein (100 microM) and AG957 (100 microM) shifted the voltage for half-maximal inactivation of I(Na) from -76.7+/-2.0 mV (n=10) in control to -88.37+/-2.6 mV (n=6, P<0.05), and -82.9+/-1.7 (n=4, P<0.05), respectively, without changing the slope factor. Genistein and AG957 also significantly prolonged the time course of I(Na) recovery from inactivation. Daidzein and PP3, inactive analogs of genistein and PP2, respectively, did not inhibit I(Na) significantly. We conclude that protein tyrosine kinase signaling pathways may play an important role in regulation of I(Na) in cardiac myocytes.

Comparison of the Effect of Class IA Antiarrhythmic Drugs on Transmembrane Potassium Currents in Rabbit Ventricular Myocytes

The blockade of cardiac transmembrane potassium channels, which is commonly seen with various antiarrhythmic drugs, plays an important role in their mechanism of action. We studied and compared the less-explored effects of three Class IA antiarrhythmics on the transient outward current (I(to)) and on the inward rectifier (I(kl)), ATP sensitive (I(KATP)), and delayed rectifier (I(K)) potassium currents in rabbit ventricular myocytes. Transmembrane currents were measured by applying the whole-cell configuration of the patch-clamp technique at 37 degrees C in myocytes enzymatically isolated from rabbit ventricular preparations. Quinidine (10 microM), disopyramide (10 microM), and procainamide (50 microM) were studied at concentrations close to or exceeding the therapeutic plasma level. All studied drugs significantly decreased the amplitude of I(KATP) (activated by 50 microM pinacidil) and I(K) currents. None of them influenced significantly I(kl). The amplitude of I(to) was decreased by quinidine and disopyramide but was not considerably altered by procainamide. The fast inactivation of I(to) was not changed by procainamide and was significantly accelerated by quinidine and disopyramide. Although quinidine, disopyramide, and procainamide are all classified as Class IA antiarrhythmics, these drugs had different effects on various potassium currents, which may partially explain their distinct effect on repolarization in various cardiac tissues and on cardiac arrhythmias in clinical settings.

Calmodulin kinase and a calmodulin-binding 'IQ' domain facilitate L-type Ca2+ current in rabbit ventricular myocytes by a common mechanism.

1. Ca2+-calmodulin-dependent protein kinase II (CaMK) and a calmodulin (CaM)-binding 'IQ' domain (IQ) are both implicated in Ca2+-dependent regulation of L-type Ca2+ current (I(Ca)). We used an IQ-mimetic peptide (IQmp), under conditions in which CaMK activity was controlled, to test the relationship between these CaM-activated signalling elements in the regulation of L-type Ca2+ channels (LTCCs) and I(Ca) in rabbit ventricular myocytes. 2. A specific CaMK inhibitory peptide nearly abolished I(Ca) facilitation, but the facilitation was 'rescued' by cell dialysis with IQmp. 3. IQmp significantly enhanced I(Ca) facilitation and slowed the fast component of I(Ca) inactivation, compared with an inactive control peptide. Neither effect could be elicited by a more avid CaM-binding peptide, suggesting that generalized CaM buffering did not account for the effects of IQmp. 4. I(Ca) facilitation was abolished and the fast component of inactivation eliminated by ryanodine, caffeine or thapsigargin, suggesting that the sarcoplasmic reticulum (SR) is an important source of Ca2+ for I(Ca) facilitation and inactivation. IQmp did not restore I(Ca) facilitation under these conditions. 5. Engineered Ca2+-independent CaMK and IQmp each markedly increased LTCC open probability (P(o)) in excised cell membrane patches. The LTCC P(o) increases with CaMK and IQmp were non-additive, suggesting that CaMK and IQmp are components of a shared signalling pathway. 6. Both CaMK and IQmp induced a modal gating shift in LTCCs that favoured prolonged openings, indicating that CaMK and IQmp affect LTCCs through a common biophysical mechanism. 7. These findings support the hypothesis that CaMK is required for physiological I(Ca) facilitation in cardiac myocytes. Both CaMK and IQmp were able to induce a modal gating shift in LTCCs, suggesting that each of these signalling elements is important for Ca2+-CaM-dependent LTCC facilitation in cardiac myocytes.