Chronic administration of amiodarone does not affect Na+/Ca2+ exchange current in guinea pig cardiac ventricular myocytes

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.

Topics on the Na+/Ca2+ Exchanger: Pharmacological Characterization of Na+/Ca2+ Exchanger Inhibitors

Using the whole-cell voltage clamp, we examined acute effects of various agents on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac ventricular cells and transfected cells. Among the antiarrhythmic drugs, amiodarone, bepridil, dronedarone, cibenzoline, azimilide, and aprindine inhibited I(NCX) in a concentration-dependent manner. We also investigated the effects on NCX of 2,3-buanedione monoxim (BDM) and selective NCX inhibitors such as KB-R7943, SEA0400, and SN-6. The presence of trypsin in the pipette solution attenuated the inhibitory effects on NCX of amiodarone, bepridil, and BDM, suggesting that these drugs inhibit NCX from the cytosolic side. In contrast, the trypsin-insensitive NCX inhibitors were aprindine, azimilide, dronedarone, cibenzoline, KB-R7943, SEA0400, and SN-6. KB-R7943, SEA0400, and SN-6 suppressed the uni-directional outward I(NCX) more potently than the uni-directional inward I(NCX). The mechanism of this mode-dependency is unknown, but is suggested to be related to intracellular Na(+) concentration.

Down-Regulation of Na+/Ca2+Exchanger by Fluvastatin in Rat Cardiomyoblast H9c2 Cells: Involvement of RhoB in Na+/Ca2+Exchanger mRNA Stability

We investigated the effect of fluvastatin (Flv), an HMG-CoA reductase inhibitor, on Na(+)/Ca(2+) exchanger 1 (NCX1) expression in H9c2 cardiomyoblasts. Reverse transcriptase-polymerase chain reaction analyses revealed that Flv decreased NCX1 mRNA in a concentration- and time-dependent manner and NCX1 protein. This effect of Flv was caused by the inhibition of HMG-CoA reductase, because Flv did not affect the NCX1 mRNA in the presence of mevalonate. Flv-induced down-regulation of NCX1 mRNA was also cancelled by farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP), suggesting an involvement of small G-proteins. However, overexpression of neither constitutive active RhoA nor Ras affected NCX1 mRNA. In contrast, intracellular expression of C3 toxin, a specific inhibitor of Rho family proteins, decreased NCX1 mRNA, suggesting that Flv decreases NCX1 mRNA by inhibiting a signaling pathway of Rho family proteins other than RhoA. On the other hand, lysophosphatidylcholine (LPC), an activator of Rho signaling, increased both NCX1 mRNA and protein in a C3 toxin-sensitive manner. Western blot analyses revealed that membrane-associated RhoB, which is isoprenylated by either FPP or GGPP, was decreased by Flv but was increased by LPC. Selective inhibition of gene expression by short interfering RNA duplex showed that RhoB but not RhoA is involved in the regulation of NCX1 mRNA and protein. When transcription was blocked by 5,6-dichlorobenzimidazole riboside, the NCX1 mRNA stability was decreased by Flv. Long-term treatment of rat with Flv in vivo also down-regulated the cardiac NCX1 mRNA. These results suggest that a RhoB-mediated signaling pathway regulates cardiac NCX1 levels by controlling the NCX1 mRNA stability.

Effects of amiodarone on mutant Na+/Ca2+ exchangers expressed in CCL 39 cells

Using the whole cell voltage clamp, we reported previously that amiodarone acutely inhibits Na+/Ca2+ exchange current (INCX) in guinea pig cardiac ventricular myocytes. Intracellular application of trypsin via the patch pipette attenuated the blocking effect of amiodarone, suggesting that amiodarone affects the Na+/Ca2+ exchanger (NCX) from the cytoplasmic side. Here, we attempted to detect the site of amiodarone inhibition using wild type NCX1, mutants, and NCX3 expressed in CCL39 fibroblasts. INCX was recorded by ramp pulses. Amiodarone at 30 microM inhibited INCX by 80% in cells expressing wild type NCX1. However, 30 microM amiodarone inhibited INCX by about 55% in cells expressing mutant NCX1 with amino acids 217-671 (DeltaXIP) or 247-671 (Delta247-671) deleted in the long intracellular loop between the transmembrane segments (TM) 5 and 6. INCXs from NCX mutants deleted of cytoplasmic TM1-2, TM3-4 or the C-terminus were inhibited by amiodarone to a similar extent as the wild type. Amiodarone also inhibited INCX of NCX3 by 76%. These results suggest that a long intracellular loop may be involved in the inhibition of NCX1 by amiodarone, but that other intracellular loops, XIP region or C terminus are not involved in the amiodarone inhibition of NCX1.

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.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.