Vasopressin V(1) receptor-mediated activation of central sympatho-adrenomedullary outflow in rats

High-threshold Ca(2+) channels and tetrodotoxin-resistant Na(+) channels are highly expressed in small dorsal root ganglion neurons. In acutely isolated rat dorsal root ganglion neurons, the effects of neomycin, one of the aminoglycoside antibiotics, on high-threshold Ca(2+) currents and tetrodotoxin-resistant Na(+) currents were examined using whole-cell patch recording. We showed for the first time that neomycin dose-dependently inhibited peak high-threshold Ca(2+) currents and peak tetrodotoxin-resistant Na(+) currents with half-maximal inhibitory concentrations at 3.69 microM (n=20) and 1213.44 microM (n=25), respectively. Inactivation properties of high-threshold Ca(2+) currents and activation properties of tetrodotoxin-resistant Na(+) currents were also affected by neomycin with reduction of excitability of small dorsal root ganglion neurons. Half-maximal inactivation voltage of high-threshold Ca(2+) currents was -45.56 mV before and -50.46 mV after application of neomycin (n=10). Half-maximal activation voltage of tetrodotoxin-resistant Na(+) currents was -19.93 mV before and -11.19 mV after administration of neomycin (n=15). These results suggest that neomycin can inhibit high-threshold Ca(2+) currents and tetrodotoxin-resistant Na(+) currents in small dorsal root ganglion neurons, which may contribute to neomycin-induced peripheral and central analgesia.     

KB130015, a new amiodarone derivative with multiple effects on cardiac ion channels

KB130015 (KB015), a new drug structurally related to amiodarone, has been proposed to have antiarrhythmic properties. In contrast to amiodarone, KB015 markedly slows the kinetics of inactivation of Na(+) channels by enhancing concentration-dependently (K(0.5) asymptotically equal to 2 microM) a slow-inactivating I(Na) component (tau(slow) asymptotically equal to 50 ms) at the expense of the normal, fast-inactivating component (tau(fast) asymptotically equal to 2 to 3 ms). However, like amiodarone, KB015 slows the recovery from inactivation and causes a shift (K(0.5) asymptotically equal to 6.9 microM) of the steady-state voltage-dependent inactivation to more negative potentials. Despite prolonging the opening of Na(+) channels KB015 does not lengthen but often shortens the action potential duration (APD) in pig myocytes or in multicellular preparations. Only short APDs in mouse are markedly prolonged by KB015, which frequently induces early afterdepolarizations. KB015 has also an effect on other ion channels. It decreases the amplitude of the L-type Ca(2+) current (I(Ca-L)) without changing its time course, and it inhibits G-protein gated and ATP-gated K(+) channels. Both the receptor-activated I(K(ACh)) (induced in atrial myocytes by either ACh, adenosine or sphingosylphosphorylcholine) and the receptor-independent (GTPgammaS-induced or background) I(K(ACh)) are concentration-dependently (K(0.5) asymptotically equal to 0.6 - 0.9 microM) inhibited by KB015. I(K(ATP)), induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP), is equally suppressed. However, KB015 has no effect on I(K1) or on I(to). Consistent with the effects in K(+) currents, KB015 does not depolarize the resting potential but antagonizes the APD shortening by muscarinic receptor activation or by DNP. Intracellular cell dialysis with KB015 has marginal or no effect on Na(+) or K(+) channels and does not prevent the effect of extracellularly applied drug, suggesting that KB015 interacts directly with channels at sites more easily accessible from the extracellular than the intracellular side of the membrane. At high concentrations KB015 exerts a positive inotropic action. It also interacts with thyroid hormone nuclear receptors. Its toxic effects remain largely unexplored, but it is well tolerated during chronic administration.     

The effects of flavoxate hydrochloride on voltage-dependent L-type Ca2+ currents in human urinary bladder

The effects of flavoxate hydrochloride (Bladderon, piperidinoethyl-3-methylflavone-8-carboxylate; hereafter referred as flavoxate) on voltage-dependent nifedipine-sensitive inward Ba(2+) currents in human detrusor myocytes were investigated using a conventional whole-cell patch-clamp. Tension measurement was also performed to study the effects of flavoxate on K(+)-induced contraction in human urinary bladder. Flavoxate caused a concentration-dependent reduction of the K(+)-induced contraction of human urinary bladder. In human detrusor myocytes, flavoxate inhibited the peak amplitude of voltage-dependent nifedipine-sensitive inward Ba(2+) currents in a voltage- and concentration-dependent manner (K(i) = 10 microM), and shifted the steady-state inactivation curve of Ba(2+) currents to the left at a holding potential of -90 mV. Immunohistochemical studies indicated the presence of the alpha(1C) subunit protein, which is a constituent of human L-type Ca(2+) channels (Ca(V)1.2), in the bundles of human detrusor smooth muscle. These results suggest that flavoxate caused muscle relaxation through the inhibition of L-type Ca(2+) channels in human detrusor.     

Inhibition of recombinant low-voltage-activated Ca(2+) channels by the neuroprotective agent BW619C89 (Sipatrigine)

T-type Ca(2+) currents were recorded in 2 mM Ca(2+) from HEK 293 cells stably expressing recombinant low-voltage-activated Ca(2+) channel subunits. Current-voltage relationships revealed that these currents were low-voltage activated in nature and could be reversibly antagonised by mibefradil, a known T-type channel blocker. At a test potential of -25 mV alpha(1I)-mediated Ca(2+) currents were rapidly and reversibly inhibited by 1-100 microM BW619C89 (IC(50)=14 microM, Hill coefficient 1.3). In contrast to its actions on N-type Ca(2+) channels, a near IC(50) dose (10 microM) of BW619C89 produced no alterations in either the kinetics or voltage-dependence of T-type currents. In additional single dose experiments, currents mediated by rat alpha(1G), human alpha(1H) or human alpha(1I) channel subunits were also inhibited by BW619C89. Overall our data indicate that T-type Ca(2+) channels are more potently blocked by BW619C89 than either type-II Na(+) channels or N-type Ca(2+) channels. It seems, therefore, that inhibition of low-voltage-activated Ca(2+) channels is likely to contribute to the anticonvulsant and neuroprotective actions of this and related compounds.     

Electropharmacological effects of UK-1745, a novel cardiotonic drug, in guinea-pig ventricular myocytes

Effects of (2RS, 3SR)-2-aminomethyl-2,3,7,8-tetrahydro-2,3,5,8, 8-pentamethyl-6H-furo-[2,3-e] indol-7-one hydrochloride (UK-1745), a novel cardiotonic drug with beta-adrenoceptor blocking property and antiarrhythmic activity, on the action potentials of isolated papillary muscles and the membrane currents of single ventricular myocytes of guinea pigs were examined and compared with those of milrinone using conventional microelectrode and patch-clamp techniques. In papillary muscles, UK-1745 (3-100 microM) produced a mild positive inotropic response and depressed the maximum upstroke velocity of the action potential (V(max)) at 100 microM. Milrinone, a phosphodiesterase III inhibitor, markedly shortened the action potential duration with a significant increase in developed tension. In enzymatically-dissociated ventricular myocytes, UK-1745 failed to increase the L-type Ca(2+) current (I(Ca)) at 10 and 30 microM and rather decreased I(Ca) at 100 microM. The high concentration of UK-1745 slightly inhibited the delayed rectifier K(+) current (I(K)). Although UK-1745 antagonized the isoproterenol-induced increase in I(Ca), it potentiated the histamine-induced increase in I(Ca). On the other hand, milrinone per se significantly increased I(Ca) and markedly enhanced the isoproterenol-induced increase in I(Ca). It can be concluded that UK-1745 is a unique cardiotonic drug possessing beta-adrenoceptor blocking and weak phosphodiesterase-inhibitory actions in addition to direct inhibitory actions on the Na(+), Ca(2+) and K(+) channels with its high concentrations.     

A novel Na+ channel agonist, dimethyl lithospermate B, slows Na+ current inactivation and increases action potential duration in isolated rat ventricular myocytes

Voltage-gated Na(+) channel blockers have been widely used as local anaesthetics and antiarrhythmic agents. It has recently been proposed that Na(+) channel agonists can be used as inotropic agents. Here, we report the identification of a natural substance that acts as a Na(+) channel agonist. Using the patch-clamp technique in isolated rat ventricular myocytes, we investigated the electrophysiological effects of the substances isolated from the root extract of Salvia miltiorrhiza, which is known as 'Danshen' in Asian traditional medicine. By the intensive activity-guided fractionation, we identified dimethyl lithospermate B (dmLSB) as the most active component, while LSB, which is the major component of the extract, showed negligible electrophysiological effect. Action potential duration (APD(90)) was increased by 20 microM dmLSB from 58.8 +/- 12.1 to 202.3 +/- 9.5 ms. In spite of the prolonged APD, no early after-depolarization (EAD) was observed. dmLSB had no noticeable effect on K(+) or Ca(2+) currents, but selectively affected Na(+) currents (I(Na)). dmLSB slowed the inactivation kinetics of I(Na) by increasing the proportion of slowly inactivating component without inducing any persistent I(Na). The relative amplitude of slow component compared to the peak fast I(Na) was increased dose dependently by dmLSB (EC(50) = 20 microM). Voltage dependence of inactivation was not affected by dmLSB, while voltage dependence of activation shifted by 5 mV to the depolarised direction. Since the APD prolongation by dmLSB did not provoke EAD, which is thought as a possible mechanism for the proarrhythmia seen in other Na(+) channel agonists, dmLSB might be an excellent candidate for a Na(+) channel agonist.     

Effect of dronedarone on Na+, Ca2+ and HCN channels

Previous studies showed that amiodarone causes state-dependent inhibition of Na(+) channels thereby mediating an atrial-selective drug effect. The aim of the present study was to investigate the impact of the new antiarrhythmic compound dronedarone on Na(+), Ca(2+) and hyperpolarization-activated cyclic nucleotide-gated ion channels. Monophasic action potentials (MAP) and effective refractory period (ERP) were studied in arterially perfused left atria and ventricular wedge preparations of the pig. Fast Na(+) and Ca(2+) currents in isolated guinea pig ventricular myocytes as well as human HCN4 channels expressed in Chinese hamster ovary (CHO) cells were investigated with the patch-clamp technique. In left atrial epicardial tissue, dronedarone (3?μM) had no effect on the MAP duration, but the drug caused a significant prolongation of the ERP from 145?±?9 to 184?±?17?ms (n?=?6; p?相似文献   

Characterization of SN-6, a novel Na+/Ca2+ exchange inhibitor in guinea pig cardiac ventricular myocytes

We examined the effect of SN-6, a new benzyloxyphenyl Na(+)/Ca(2+) exchange (NCX) inhibitor on the Na(+)/Ca(2+) exchange current (I(NCX)) and other membrane currents in isolated guinea pig ventricular myocytes using the whole-cell voltage-clamp technique. SN-6 suppressed I(NCX) in a concentration-dependent manner. The IC(50) values of SN-6 were 2.3 microM and 1.9 microM for the outward and inward components of the bi-directional I(NCX), respectively. On the other hand, SN-6 suppressed the outward uni-directional I(NCX) more potently (IC(50) value of 0.6 microM) than the inward uni-directional I(NCX). SN-6 at 10 microM inhibited the uni-directional inward I(NCX) by only 22.4+/-3.1%. SN-6 and KB-R7943 suppressed I(NCX) more potently when intracellular Na(+) concentration was higher. Thus, both drugs inhibit NCX in an intracellular Na(+) concentration-dependent manner. Intracellular application of trypsin via a pipette solution did not change the blocking effect of SN-6 on I(NCX). Therefore, SN-6 is categorized as an intracellular-trypsin-insensitive NCX inhibitor. SN-6 at 10 microM inhibited I(Na), I(Ca), I(K) and I(K1) by about 13%, 34%, 33% and 13%, respectively. SN-6 at 10 microM shortened the action potential duration at 50% repolarization (APD(50)) by about 34%, and that at 90% repolarization (APD(90)) by about 25%. These results indicate that SN-6 inhibits NCX in a similar manner to that of KB-R7943. However, SN-6 at 10 microM affected other membrane currents less potently than KB-R7943.     

Electromechanical characterization of cinnamophilin, a natural thromboxane A2 receptor antagonist with anti-arrhythmic activity, in guinea-pig heart

BACKGROUND AND PURPOSE:Cinnamophilin, a thromboxane A(2) receptor antagonist, has been identified as a prominent anti-arrhythmic agent in rat heart. This study aimed to determine its electromechanical and anti-arrhythmic effects in guinea-pig hearts. EXPERIMENTAL APPROACH: Microelectrodes were used to study action potentials in ventricular papillary muscles. Fluo-3 fluorimetric ratio and whole-cell voltage-clamp techniques were used to record calcium transients and membrane currents in single ventricular myocytes, respectively. Intracardiac electrocardiograms were obtained and the anti-arrhythmic efficacy was determined from isolated perfused hearts. KEY RESULTS: In papillary muscles, cinnamophilin decreased the maximal rate of upstroke (V(max)) and duration of action potential, and reduced the contractile force. In single ventricular myocytes, cinnamophilin reduced Ca(2+) transient amplitude. Cinnamophilin decreased the L-type Ca(2+) current (I(Ca,L))(IC(50)=7.5 microM) with use-dependency, induced a negative shift of the voltage-dependent inactivation and retarded recovery from inactivation. Cinnamophilin also decreased the Na(+) current (I(Na)) (IC(50)=2.7 microM) and to a lesser extent, the delayed outward (I(K)), inward rectifier (I(K1)), and ATP-sensitive (I(K,ATP)) K(+) currents. In isolated perfused hearts, cinnamophilin prolonged the AV nodal conduction interval and Wenckebach cycle length and the refractory periods of the AV node, His-Purkinje system and ventricle, while shortening the ventricular repolarization time. Additionally, cinnamophilin reduced the occurrence of reperfusion-induced ventricular fibrillation. CONCLUSIONS AND IMPLICATIONS: These results suggest that the promising anti-arrhythmic effect and the changes in the electromechanical function induced by cinnamophilin in guinea-pig heart can be chiefly accounted for by inhibition of I(Ca,L) and I(Na).     

Rotundifolone-induced relaxation is mediated by BK(Ca) channel activation and Ca(v) channel inactivation

Rotundifolone is the major constituent of the essential oil of Mentha x villosa Hudson. In preliminary studies, rotundifolone induced significant hypotensive, bradycardic and vasorelaxant effects in rats. Thus, to gain more insight into the pharmacology of rotundifolone, the aim of this study was to characterize the molecular mechanism of action involved in relaxation produced by rotundifolone. The relaxant effect was investigated in rat superior mesenteric arteries by using isometric tension measurements and whole-cell patch-clamp techniques. Rotundifolone relaxed phenylephrine-induced contractions in a concentration-dependent manner. Pre-treatment with KCl (20 mM), charybdotoxin (10(-7) M) or tetraethylammonium (TEA 10(-3) or 3 × 10(-3) M) significantly attenuated the relaxation effect induced by rotundifolone. Additionally, whole-cell patch-clamp recordings were made in mesenteric smooth muscle cells and showed that rotundifolone significantly increased K(+) currents, and this effect was abolished by TEA (10(-3) M), suggesting the participation of BK(Ca) channels. Furthermore, rotundifolone inhibited the vasoconstriction induced by CaCl(2) in depolarizing nominally Ca(2+) -free medium and antagonized the contractions elicited by an L-type Ca(2+) channel agonist, S(-)-Bay K 8644 (2 × 10(-7) M), indicating that the vasodilatation involved inhibition of Ca(2+) influx through L-type voltage-dependent calcium channels (Ca(v) type-L). Additionally, rotundifolone inhibited L-type Ca(2+) currents (I(Ca) L), affecting the voltage-dependent activation of I(Ca) L and steady-state inactivation. Our findings suggest that rotundifolone induces vasodilatation through two distinct but complementary mechanisms that clearly depend on the concentration range used. Rotundifolone elicits an increase in the current density of BK(Ca) channels and causes a shift in the steady-state inactivation relationship for Ca(v) type-L towards more hyperpolarized membrane potentials.     

Dihydropyridine Ca2+ channel antagonists and agonists block Kv4.2, Kv4.3 and Kv1.4 K+ channels expressed in HEK293 cells

(1) We have determined the molecular basis of nicardipine-induced block of cardiac transient outward K(+) currents (I(to)). Inhibition of I(to) was studied using cloned voltage-dependent K(+) channels (Kv) channels, rat Kv4.3L, Kv4.2, and Kv1.4, expressed in human embryonic kidney cell line 293 (HEK293) cells. (2) Application of the dihydropyridine Ca(2+) channel antagonist, nicardipine, accelerated the inactivation rate and reduced the peak amplitude of Kv4.3L currents in a concentration-dependent manner (IC(50): 0.42 micro M). The dihydropyridine (DHP) Ca(2+) channel agonist, Bay K 8644, also blocked this K(+) current (IC(50): 1.74 micro M). (3) Nicardipine (1 micro M) slightly, but significantly, shifted the voltage dependence of activation and steady-state inactivation to more negative potentials, and also slowed markedly the recovery from inactivation of Kv4.3L currents. (4) Coexpression of K(+) channel-interacting protein 2 (KChIP2) significantly slowed the inactivation of Kv4.3L currents as expected. However, the features of DHP-induced block of K(+) current were not substantially altered. (5) Nicardipine exhibited similar block of Kv1.4 and Kv4.2 channels stably expressed in HEK293 cells; IC(50)'s were 0.80 and 0.62 micro M, respectively. (6) Thus, at submicromolar concentrations, DHP Ca(2+) antagonist and agonist inhibit Kv4.3L and have similar inhibiting effects on other components of cardiac I(to), Kv4.2 and Kv1.4.     

Negative modulation of L-type Ca2+ channels via beta-adrenoceptor stimulation in guinea-pig detrusor smooth muscle cells

beta-Adrenergic stimulation enhances the activity of L-type Ca(2+) channels through mechanisms mediated by adenosine 3'5'-cyclic monophosphate (cAMP) and protein kinase A in cardiac myocytes. However, in smooth muscle cells, the effect of beta-adrenoceptor stimulation on the L-type Ca(2+) channel activity has been controversial, and the exact mechanism is still unclear. The present study was aimed at elucidating the effect of beta-adrenergic stimulation upon the activity of L-type Ca(2+) channels in guinea-pig detrusor smooth muscle cells. Isoproterenol (0.1-1 microM) inhibited Ba(2+) currents through L-type Ca(2+) channels (I(Ba)). Isoproterenol (0.1 microM) shifted the steady-state inactivation curve to negative voltages by 11 mV without affecting activation curves. The stimulation of cAMP-mediated signal transduction pathway by forskolin, 8-bromoadenosine 3'5'-cyclic monophosphate (8-Br-cAMP), or the intracellular application of cAMP also mimicked the effects of isoproterenol on I(Ba), which was blocked by the inhibition of protein kinase A. These results indicate that, in detrusor smooth muscles, the stimulation of beta-adrenoceptors exerts negative modulation of L-type Ca(2+) channels via cAMP/protein kinase A-dependent mechanism.     

Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca(2+) currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K(+)-free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs(+) dialysate). However, the inhibitors (100 microM) also induced membrane currents that reversed between -40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein-induced current behaved like Cl(-) current, and was unaffected by either the addition of divalent cations (0.5 mM Cd(2+); 3 mM Ni(2+)) that block the Na(+)-Ca(2+) exchanger (NCX), or the removal of external Na(+) and Ca(2+). A23-induced current was independent of Cl(-) driving force, and strongly suppressed by addition of Cd(2+) and Ni(2+), and by removal of either external Na(+) or Ca(2+). These and other results suggested that A23 activated an NCX current driven by submembrane Na(+) and Ca(2+) concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na(+) and Ca(2+) concentrations was obtained by suppressing cation influx (10 microM verapamil) and raising dialysate Na(+) to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC(50) 68 +/- 4 microM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 microM A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein-insensitive TK.     

Effects of diltiazem and nifedipine on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

1. It is unknown whether the widely used L-type Ca(2+) channel antagonists diltiazem and nifedipine would block the repolarization K(+) currents, transient outward current (I(to1)) and ultra-rapid delayed rectifier K(+) current (I(Kur)), in human atrium. The present study was to determine the effects of diltiazem and nifedipine on I(to1) and I(Kur) in human atrial myocytes with whole-cell patch-clamp technique. 2. It was found that diltiazem substantially inhibited I(to1) in a concentration-dependent manner, with an IC(50) of 29.2+/-2.4 microM, and nifedipine showed a similar effect (IC(50)=26.8+/-2.1 muM). The two drugs had no effect on voltage-dependent kinetics of the current; however, they accelerated I(to1) inactivation significantly, suggesting an open channel block. 3. In addition, diltiazem and nifedipine suppressed I(Kur) in a concentration-dependent manner (at +50 mV, IC(50)=11.2+/-0.9 and 8.2+/-0.8 microM, respectively). These results indicate that the Ca(2+) channel blockers diltiazem and nifedipine substantially inhibit I(to1) and I(Kur) in human atrial myocytes.     

Diazepam, gamma-aminobutyric acid, and progesterone open K(+) channels in myocytes from coronary arteries

Benzodiazepines enhance coronary blood flow and lower blood pressure, but the cellular basis of this action remains unclear. The present study now demonstrates a direct effect of diazepam, gamma-aminobutyric acid (GABA), and progesterone on the large conductance, Ca(2+)- and voltage-activated K(+) channel (BK(Ca)) in single myocytes isolated from porcine coronary arteries. These GABA receptor agonists significantly increased whole-cell (perforated patch) K(+) currents and stimulated the activity of single BK(Ca) channels in cell-attached patches dramatically. This effect is not mediated via cyclic AMP or cyclic GMP, but involves stimulation of Ca(2+) influx in response to activation of a bicuculline-sensitive GABA(A)-like receptor. We propose that localized, subsarcolemmal increases in Ca(2+) levels open BK(Ca) channels, thereby promoting K(+) efflux, membrane repolarization, and coronary relaxation. This transduction pathway can now account, at least in part, for the direct vasodilatory effects of diazepam, progesterone, and GABA.     

Allitridi inhibits transient outward potassium currents in human atrial myocytes

1. It has been reported that allitridi, an active compound extracted from garlic, has many cardiovascular effects. However, it remains unknown whether allitridi affects major repolarization currents, such as the transient outward K(+) current (I(to) ), ultrarapid delayed rectifier K(+) current (I(Kur)) and the L-type Ca(2+) current (I(Ca)), in human atrial myocytes. 2. In the present study, we investigated the effects of allitridi on I(to), I(Kur), I(Ca) and the action potential in human isolated atrial myocytes using the whole-cell patch recording technique. 3. Allitridi reversibly inhibited I(to), but not I(Kur) and I(Ca), in human atrial myocytes. These effects of allitridi on I(to) were concentration dependent (IC(50) = 44.9 μmol/L). Inactivation of I(to) was accelerated and the voltage-dependent inactivation potential was shifted towards the negative direction. Allitridi (30 μmol/L) significantly prolonged action potential duration in human atrial myocytes. 4. The results of the present study indicate that allitridi inhibits I(to), but not I(Kur) and I(Ca), and prolongs the action potential duration in human atrial myocytes.     

Oscillatory transient inward currents in ventricular myocytes of healthy versus myopathic Syrian hamster

The present experiments were performed in order to study abnormal action potential configuration and ion channel activity in ventricular myocytes obtained from 23 male myopathic Syrian hamsters (Biobreeders strain 14.6, 32-52 weeks old) compared with 10 age-matched healthy control hamsters (Biobreeders F1B) by means of whole-cell patch-clamp techniques. The results show that the myopathic myocytes had a longer action potential duration, a reduced transient outward K(+) current on depolarization and a smaller transient inward current on repolarization after prolonged depolarizing pulses (> 500 msec). However, the L-type Ca(2+) current and the inwardly rectifing K(+) current were not significantly different from those of healthy myocytes. The oscillatory transient inward currents could be diminished by treatment with ryanodine (0.01-1 micromol/L), a sarcoplasmic reticulum (SR) Ca(2+) release channel blocker, or with Na(+)-free superfusate. We conclude that the hereditary myopathic hamsters are less likely to develop delayed after depolarization-related transient inward currents and triggered arrhythmias owing to a smaller SR Ca(2+) content.     

Inhibitory effect of amiodarone on Na(+)/Ca(2+) exchange current in guinea-pig cardiac myocytes

The effect of amiodarone on Na(+)/Ca(2+) exchange current (I(NCX)) was examined in single guinea-pig ventricular myocytes using the whole-cell voltage clamp technique. I(NCX) was recorded by ramp pulses from the holding potential of -60 mV in the presence of 140 mM Na(+) and 2 mM Ca(2+) in the external solution, and 20 mM Na(+) and 398 nM free Ca(2+) (19 mM Ca(2+) and 30 mM BAPTA) in the internal solution. External application of amiodarone suppressed I(NCX) in a concentration-dependent manner. The IC(50) value was 3.3 microM with a Hill coefficient of 1. Intracellular application of trypsin via the micropipette attenuated the blocking effect of amiodarone, suggesting that amiodarone affects the cytoplasmic side of the molecule. This inhibitory effect of amiodarone on the Na(+)/Ca(2+) exchanger may contribute to the cardioprotective action of the drug.     

Multiple actions of U-37883A, an ATP-sensitive K+ channel blocker, on membrane currents in pig urethra

The effects of U-37883A, a vascular ATP-sensitive K(+) channel (K(ATP) channel) blocker, on membrane currents were investigated in pig urethral myocytes by use of patch-clamp techniques (conventional whole-cell recordings, nystatin-perforated patches and cell-attached configuration). Tension measurement was also performed to study the effects of U-37883A on the levcromakalim-induced urethral relaxation and the urethral resting tone in the absence and presence of Bay K 8644. Although cumulative application of U-37883A produced a concentration-dependent inhibitory effect on the levcromakalim-induced urethral relaxation, U-37883A did not abolish the relaxation. In nystatin-perforated patch recording, K(ATP) currents activated by levcromakalim were inhibited by U-37883A in a concentration-dependent manner (K(i), 4.7 microM). Approximately 10% of the K(ATP) currents still remained even in the presence of 300 microM U-37883A. In cell-attached mode, extracellular application of U-37883A (100 microM) irreversibly inhibited the activity of the levcromakalim-induced K(ATP) channels. In whole-cell configuration, U-37883A suppressed the peak amplitude of voltage-dependent Ba(2+) currents in a concentration- and voltage-dependent manner, and at 30 microM, shifted the steady-state inactivation curve of the Ba(2+) currents to the left at -90 mV. These results demonstrate that U-37883A reduces not only the activities of K(ATP) channels but also voltage-dependent Ca(2+) channels. Therefore, it is not appropriate to define U-37883A as solely a vascular K(ATP) channel blocker.     

Cardiac myocytes Ca2+ and Na+ regulation in normal and failing hearts

Ca(2+) is a central player in the excitation-contraction coupling of cardiac myocytes, the process that enables the heart to contract and relax. Mishandling of Ca(2+) is a central cause of both contractile dysfunction and arrhythmias in pathophysiological conditions such as heart failure (HF). Upon electrical excitation, Ca(2+) enters the myocytes via voltage-gated Ca(2+) channels and induces further Ca(2+) release from the sarcoplasmic reticulum (SR). This raises the free intracellular Ca(2+) concentration ([Ca(2+)](i)), activating contraction. Relaxation is driven by [Ca(2+)](i) decline, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion via the sarcolemmal Na(+)/Ca(2+) exchange, NCX. Intracellular Na(+) concentration ([Na(+)](i)) is a main regulator of NCX, and thus [Na(+)](i) plays an important role in controlling the cytosolic and SR [Ca(2+)]. [Na(+)](i) may have an even more important role in HF because NCX is generally upregulated. There are several pathways for Na(+) entry into the cells, whereas the Na(+)/K(+) pump (NKA) is the main Na(+) extrusion pathway and therefore is essential in maintaining the transmembrane Na(+) gradient. Phospholemman is an important regulator of NKA function (decreasing [Na(+)](i) affinity unless it is phosphorylated). Here we discuss the interplay between Ca(2+) and Na(+) in myocytes from normal and failing hearts.