Differential potentiation by depolarization of the effects of calcium antagonists on contraction and Ca2+ current in guinea-pig heart

Compartmentalized regulations of ion channels in the heart

The rate and force of contraction of the heart are precisely controlled by compartmentalized regulation of cardiac ion channels which determine electrical activities. It is known that modulation of cardiac ion channels, which is caused by drug administration, sympathetic nervous system stimulation and gender difference, can increase risks of lethal arrhythmias in carriers of inherited disease mutations. These modulations are thought to also be involved in common cardiac arrhythmias. Because many signaling molecules are localized within single cells, an understanding of the molecular basis of compartmentalized regulation of cardiac channels is a key for understanding and treating the lethal arrhythmias. In this review, I will discuss molecular mechanisms of compartmentalized regulation of cardiac ion channels via drugs, cAMP and sex hormones.

Selective modulation of L-type calcium current by magnesium lithospermate B in guinea-pig ventricular myocytes

Magnesium lithospermate B (MLB) is the main water-soluble principle of Salviae Miltiorrhizae Radix (also called as 'Danshen' in the traditional Chinese medicine) for the treatment of cardiovascular diseases. MLB was found to possess a variety of pharmacological actions. However, it is unclear whether and how MLB affects the cardiac ion channels. In the present study, the effects of MLB on the voltage-activated ionic currents were investigated in single ventricular myocytes of adult guinea pigs. MLB reversibly inhibited L-type Ca(2+) current (I(Ca,L)). The inhibition was use-dependent and voltage-dependent (the IC(50) value of MLB was 30 microM and 393 microM, respectively, at the holding potential of -50 mV and -100 mV). In the presence of 100 microM MLB, both the activation and steady-state inactivation curves of I(Ca,L) were markedly shifted to hyperpolarizing membrane potentials, whereas the time course of recovery of I(Ca,L) from inactivation was not altered. MLB up to 300 microM had no significant effect on the fast-inactivating Na(+) current (I(Na)), delayed rectifier K(+) current (I(K)) and inward rectifier K(+) current (I(K1)). The results suggest that the voltage-dependent Ca(2+) antagonistic effect of MLB work in concert with its antioxidant action for attenuating heart ischemic injury.

[Molecular and pharmacological bases for the gating regulation of L-type voltage-dependent Ca2+ channels]

The voltage-dependent L-type Ca(2+) channel plays a key role in the spacial and temporal regulation of Ca(2+). In cardiac excitation-contraction coupling, Ca(2+)-induced Ca(2+) release (CICR) from ryanodine receptors (RyRs), triggered by Ca(2+) entry through the nearby L-type Ca(2+) channel, induces the Ca(2+)-dependent inactivation (CDI) of the Ca(2+) channel. We demonstrated that the CICR-dependent CDI of L-type Ca(2+) channels, under control of the privileged cross-signaling between L-type Ca(2+) channels and RyRs, plays important roles for monitoring and tuning the SR Ca(2+) content via changes of AP waveform and the amount of Ca(2+)-influx during AP in ventricular myocytes. L-type Ca(2+) channels are modulated by the binding of Ca(2+) channel antagonists and agonists to the pore-forming alpha(1C) subunit. We identified Phe(1112) and Ser(1115) in the pore-forming IIIS5-S6 linker region of the alpha(1C) subunit as critical determinants of the binding of dihydropyridines (DHP). Interestingly, double mutant Ca(2+) channel (F1112A/S1115A) failed to discriminate between a DHP Ca(2+) channel agonist and antagonist stereoisomers. We proposed that Phe(1112) and Ser(1115) in the pore-forming IIIS5-S6 linker region is required for the stabilization of the Ca(2+) channel in the open state by Ca(2+) channel agonists and further proposed a novel model for the DHP-binding pocket of the alpha(1C) subunit. These integrative studies on the gating regulation of cardiac L-type Ca(2+) channels will provide the molecular basis for the pharmacology of Ca(2+) channel modulators.

High-affinity binding of [3H]DTZ323 to the diltiazem-binding site of L-type Ca2+ channels

D-cis-[N-Methyl-3H]-3-(acetyloxy)-5-[2-[[2-(3,4-dimethoxyphenyl)ethyl]-methylamino]ethyl]-2,3-dihydro-2-(4-methoxyphenyl)-1,5-benzothiazepine-4(5H)-one ([3H]DTZ323), a novel 1,5-benzothiazepine radioligand, was characterized in a ligand-receptor binding study. Specific binding of [3H]DTZ323 to rabbit skeletal muscle T-tubule membranes was saturable and reversible. Scatchard analysis indicated a single binding site with a K(d) value of 1.4 and 1.8 nM at 25 and 37 degrees C, respectively. DTZ323 and diltiazem derivatives inhibited specific [3H]DTZ323 binding with a rank order of DTZ323>DTZ417 (quaternary ammonium derivative of DTZ323)>diltiazem>L-cis-DTZ323. The affinity of DTZ323 was 51 times higher than that of diltiazem. [3H]DTZ323 binding was also completely inhibited by verapamil and tetrandrine, thus revealing the unique nature of the diltiazem-binding site. Specific [3H]DTZ323 binding to crude guinea pig ventricular membranes was inhibited by diltiazem, DTZ323 and its derivatives with IC(50) values close to those previously reported for the blockade of L-type Ca(2+) channel currents. These results indicate that [3H]DTZ323 is a potent and selective radioligand for the diltiazem-binding site of L-type Ca(2+) channels.

Differences in protective profiles of diltiazem isomers in ischemic and reperfused guinea pig hearts

The effects of L-cis and D-cis diltiazem on the extracellular potassium concentration ([K(+)]e), pH and cardiac function were compared in ischemic guinea pig hearts. Before inducing ischemia, L-cis diltiazem (10 and 30 microM) reduced the left ventricular developed pressure (LVDP) with a marginal inhibition of heart rate (HR), whereas lower doses of the D-cis isomer decreased both LVDP and HR. L-cis Diltiazem only slightly inhibited the increase in [K(+)]e and the decrease in pH but significantly inhibited ischemic contractures in contrast to the marked inhibition of these parameters produced by even low doses of the D-cis isomer. Notably, at equipotent doses for the ischemic parameters, L-cis diltiazem restored the left ventricular end-diastolic pressure (LVEDP) and HR after reperfusion to a greater extent than the D-cis isomer. These results suggest that the L-cis isomer may specifically improve postischemic function, in addition to the modest action on [K(+)]e and pH, in guinea pig hearts.

Effects of inhibition of calcium and potassium currents in guinea-pig cardiac contraction: comparison of beta-caryophyllene oxide, eugenol, and nifedipine

1. To investigate the effects of the clove oil constituents beta-caryophyllene oxide and eugenol on the heart muscle, experiments were performed on isolated papillary muscles and on ventricular myocytes of the guinea-pig. The results were compared with those obtained with the dihydropyridine, nifedipine. 2. All three substances exerted negative inotropic effects in heart muscle although with different potencies and different influences on the time course of the contraction curve. 3. They all reduced rested-state contractions (RSCs) in the presence of isoprenaline which are thought to be due to the Ca(2+) current (I(Ca)). 4. beta-Caryophyllene oxide, eugenol and nifedipine inhibited the I(Ca) in single cells from the guinea-pig ventricle in a concentration-dependent, reversible way, but with different potencies. 5. In addition to the I(Ca)-inhibiting effect, beta-caryophyllene oxide strongly inhibited and eugenol slightly inhibited the potassium current. 6. The action potential of papillary muscles at a 1 Hz contraction frequency was greatly shortened by nifedipine, slightly shortened by eugenol, but not changed by beta-caryophyllene oxide. 7. The inhibition of the potassium current by beta-caryophyllene oxide obviously prevents a shortening of the action potential due to the diminution of the I(Ca). Accordingly, the negative inotropic effect of beta-caryophyllene oxide is closely related to the inhibition of I(Ca). In contrast, eugenol and nifedipine, which shorten the action potential, exert stronger negative inotropic effects than expected from their influence on I(Ca). 8. The results show that the negative inotropic effect of a calcium channel blocker can be attenuated by an additional inhibition of potassium channels.

JTV-519, a novel cardioprotective agent, improves the contractile recovery after ischaemia-reperfusion in coronary perfused guinea-pig ventricular muscles

A newly synthesized benzothiazepine derivative, JTV-519 (JT) has been reported to be cardioprotective. However, the precise mechanism underlying the cardioprotective effect of this drug is unknown. Coronary-perfused guinea-pig ventricular muscles were subjected to 20-min no-flow ischaemia followed by 60-min reperfusion (I/R). I/R significantly decreased the contraction in untreated preparations (control group, 34+/-4% of baseline value, n=6). Brief administration of JT (1.0 microM) prior to ischaemia significantly improved the postischaemic contractile recovery (63+/-5% of baseline value, n=4), as compared to the control group. JT (1.0 microM) slightly prolonged action potential duration before ischaemia and induced conduction disturbance (2 : 1 block) after the initiation of ischaemia. The cardioprotective effect of JT was antagonized by chelerythrine (CH, 5.0 microM), an inhibitor of protein kinase C (PKC) or by 5-hydroxydecanoic acid (5-HD, 400 microM), an inhibitor of mitochondrial ATP-sensitive K(+) (K(ATP)) channels. These results suggest that the protective effect of JT is due to the opening of mitochondrial K(ATP) channels, which, in turn, is linked to PKC activation.

Structure-activity relationships of new thienothiazine derivatives in isolated heart and smooth muscle preparations of guinea pigs

New thienothiazine derivatives that differ in their side chains on the nitrogen atom of the thienothiazine ring were investigated regarding structure-activity relationships and calcium antagonistic and/or potassium channel opening properties. Isometric contraction force was measured in guinea pig papillary muscles, aortic strips, and terminal ilea. Chronotropic activity was studied in right atria of guinea pigs. The derivatives with a dimethylaminoethylcarboxamide side chain (HO4) and with a dimethoxyphenylethyl-N-methylaminoethylcarboxamide side chain (HO7) had the most potent negative inotropic effects on papillary muscles and spontaneously beating right atria. The negative inotropic and chronotropic effects of the compounds with a methylpiperazinylcarbonyl side chain (HO5) or a diethylaminopropylcarboxamide side chain (HO6) were less pronounced. The negative inotropic action was reversed by increasing the extracellular calcium concentration. It was also reversed by glibenclamide, for concentrations of the compounds up to the EC50, but not at higher compound concentrations. Among all the compounds studied, HO 7 had the strongest relaxing effect on aortic strips and terminal ilea. The effects of the derivatives on the smooth muscles could not be reversed by glibenclamide. The calcium antagonistic effect of the thienothiazine derivatives is more pronounced than the potassium channel opening activity, at least in high drug concentrations. Compounds with an aromatic or heterocyclic ring in the side chain have the weakest negative inotropic and negative chronotropic effects on papillary muscles and right atria. However, HO7 showed a tissue specificity with the most potent relaxing effect on aortic strips and terminal ilea.

Functional interaction between benzothiazepine- and dihydropyridine binding sites of cardiac L-type Ca2+ channels

We have previously shown, in a radioligand binding study with single ventricular myocytes, that benzothiazepine and dihydropyridine binding sites interact with each other. To further examine whether this interaction between the two binding sites is reflected in the function of L-type Ca2+ channels, the blocking action of diltiazem, nitrendipine, and the combination of these two drugs on L-type Ca2+ channel currents was investigated using baby hamster kidney cells expressing the alpha 1C, alpha 2/delta, beta and gamma subunits of the Ca2+ channel. The effects of diltiazem and nitrendipine were additive at room temperature but synergistic at 33 degrees C. The use-dependent block by 3 microM of diltiazem was significantly enhanced from 28% to 68% by addition of 30 nM of nitrendipine, which by itself did not have a blocking effect. Thus, we conclude that benzothiazepine- and dihydropyridine binding sites interact and potentiate their blocking action on L-type Ca2+ channels in a temperature-dependent manner.

Diltiazem inhibits the late increase in extracellular potassium by maintaining glycolytic ATP synthesis during myocardial ischemia

During myocardial ischemia, the extracellular potassium concentration increases in a triphasic pattern, an initial early increase, a constant phase, and a late increasing phase. The aim of this study was to determine whether diltiazem inhibits the late increasing phase by maintaining glycolytic adenosine triphosphate (ATP) synthesis in ischemic myocardium. The extracellular potassium concentration and pH were measured simultaneously with ion-sensitive electrodes during 30 min of global ischemia in isolated guinea-pig hearts. In the control hearts, the late increasing phase occurred 13 min after the onset of ischemia when the change in extracellular pH had reached a plateau. There was a sharp increase in the myocardial lactate level in the control hearts, which was maintained for approximately 8 min after the onset of ischemia. Iodoacetate (1 mM) led to a ATP depletion and rapid accumulation in extracellular potassium shortly after the onset of ischemia without a decrease in extracellular pH. The preischemic treatment with diltiazem (3 microM) reduced cardiac function both before ischemia and during the early period of ischemia. Diltiazem almost completely abolished the late increasing phase with a continuous decrease in extracellular pH throughout the ischemic period. The myocardial lactate level in the diltiazem-treated group increased sharply between 2 and 15 min after the onset of ischemia. The myocardial ATP level was preserved throughout the ischemic period. This study shows that diltiazem inhibits the late increasing phase in extracellular potassium by maintaining glycolytic ATP synthesis during ischemia.

Differential effects of Ca2+ channel blockers on Ca2+ overload induced by lysophosphatidylcholine in cardiomyocytes

The effects of Ca2+ channel blockers (verapamil, diltiazem, nicardipine, bepridil and flunarizine) on Ca2+ overload induced by lysophosphatidylcholine were examined in rat isolated cardiomyocytes. Addition of lysophosphatidylcholine (15 microM) produced Ca2+ overload as evidenced by a marked increase in the concentration of intracellular Ca2+ and hypercontracture of cells. Verapamil, flunarizine and bepridil concentration dependently inhibited the lysophosphatidylcholine-induced Ca2+ overload, whereas diltiazem and nicardipine did not. Lysophosphatidylcholine increased the release of creatine kinase, which was significantly attenuated by verapamil, flunarizine or bepridil (5 microM for each), but not by diltiazem or nicardipine (20 microM for each). Verapamil, flunarizine, bepridil (which attenuated the lysophosphatidylcholine-induced Ca2+ overload) and nicardipine (which did not) inhibited the veratridine-induced increase in the concentration of intracellular Na+ (indicated by the increase in fluorescence ratio of Na(+)-binding benzofuran isophthalate) and cell contracture, whereas diltiazem did not. These results suggest that verapamil, bepridil and flunarizine attenuate the Ca2+ overload induced by lysophosphatidylcholine, and that the Ca2+ channel blocking action of these drugs does not contribute substantially to this effect. The Na+ channel inhibition together with high lipophilicity of these drugs may be important for the attenuation of the lysophosphatidylcholine-induced Ca2+ overload.

The effects of amrinone and glucagon on verapamil-induced myocardial depression in a rat isolated heart model

1. We measured the ability of glucagon and amrinone, used alone and in combination, to improve the myocardial function in a rat isolated heart model of calcium channel blocker (CCB) cardiotoxicity. 2. Verapamil 10(-4) mol consistently decreased heart rate and cardiac contractile force in our Langendorff rat isolated heart preparations. Glucagon increased the heart rate in a dose-dependent fashion. Amrinone increased the heart rate only at the 1 x 10(-1) mol concentration, and had no significant effect on cardiac contractility. 3. A positive linear correlation was found between the glucagon concentration and the percent recovery of baseline contractile force. 4. Although complete reversal of verapamil-induced myocardial depression occurred at glucagon concentrations of > 3 x 10(-6) mol, amrinone produced only 23.8 +/- 3.6% recovery from baseline at its highest concentration (4 x 10(-3) mol). 5. When glucagon and amrinone were administered together, there was no additional increase over glucagon alone in the increase in contractile force. 6. Glucagon, and not amrinone, is an appropriate agent, capable of reversing verapamil-induced myocardial toxicity in this rat isolated heart model. In vivo studies should be performed to assess whether this may be a reliable therapy in clinical cases.

Effects of a novel, potent benzothiazepine Ca2+ channel antagonist, DTZ323, on guinea-pig ventricular myocytes

The effects of a 1,5-benzothiazepine derivative, (+)-cis-3-(acetyloxy)-5-[2-[[2-(3,4-dimethoxyphenyl)ethyl]-methyla mino]ethyl]-2,3-dihydro-2-(4-methyoxyphenyl)-1,5-benzothiazepine-4 (5H)-one (DTZ323), on membrane currents were investigated in guinea-pig ventricular myocytes using the whole-cell patch-clamp technique. DTZ323 suppressed the L-type Ca2+ channel currents (I[Ca(L)]) more selectively than the T-type Ca2+ channel and the Na+ channel currents. DTZ323 inhibited I[Ca(L)] in a use- and a voltage-dependent manner with 24 times higher potency than that of diltiazem. Rate of recovery of I[Ca(L)] from the conditioned block by DTZ323 was faster compared with diltiazem and verapamil, and was steeply dependent on the holding potential at resting membrane potential range in ventricular myocytes (-90 to -60 mV). Our results suggest that DTZ323 is a selective Ca2+ channel antagonist, the most potent among the 1,5-benzothiazepine Ca2+ channel antagonists, and that the voltage- and use-dependent effect of DTZ323 on I[Ca(L)] is due to the steep voltage dependence of the rate of dissociation from the cardiac L-type Ca2+ channels.