Uptake of bepridil into isolated ventricular myocytes. Retention by actin

Dose-dependent red blood cell volume increase induced by bepridil

1. Bepiridil, (beta-[(2-methylpropoxy)methyl]-N-phenyl-N-(phenyl-methyl)-1-py rro lidine-ethanamine), a calcium channel blocker, inhibits sickling of red blood cells (RBC) from patients with sickle cell disease (SCD) at micromolar concentrations in vitro. 2. Bepridil induces dose-dependent osmotic swelling of RBC and a concomitant decrease in mean corpuscular hemoglobin concentration (MCHC). 3. Modest decreases in MCHC greatly lengthen the delay time for polymerization of deoxygenated sickle hemoglobin (Hb S) and inhibit RBC sickling. 4. Equilibrium dialysis studies of bepridil and purified hemoglobin showed a low binding affinity (Kb = 10(3)/M). 5. The partition coefficient (Kp) determined for the interaction of RBC and bepridil was 1-3 x 10(3), which is similar to the Kp determined for other amphipathic molecules, such as chlorpromazine.

Calcium antagonists and Bay K8644 promote depolarization of the rat heart mitochondrial membrane potential. Further evidence for a role in alteration of oxidative metabolism

Studies were carried out using a tetraphenylphosphonium (TPP+)-selective electrode to monitor the effect of selected calcium (Ca2+) antagonists and the dihydropyridine Ca2+ agonist Bay K8644 on membrane potential (psi) associated with isolated rat heart mitochondria. Verapamil and diltiazem (10-500 microM), standard Ca2+ antagonists, produced a depolarization of both liver and heart mitochondria at concentrations > 150 microM. In contrast, nitrendipine (10-200 microM), a dihydropyridine compound, produced a concentration-related inhibition of psi in mitochondria from both sources, effects which were statistically significant at concentrations > 50 microM. Cinnarizine (10-100 microM) and bepridil (10-100 microM) also produced inhibition of heart psi, these effects being particularly noted in the presence of bepridil, where depolarization of the membrane was statistically significant with only 10 microM drug. The results indicate the complexity of action of these drugs at the mitochondrial level. In general, drug actions on psi appear to be correlated with previously reported effects on Ca2+ transportation rather than oxidative phosphorylation associated with rat heart mitochondria. The findings also illustrate that the mitochondrial actions of cardiovascular compounds may be of relevance in situ, particularly during ischaemia/reperfusion when mitochondria become loaded with Ca2+.

Inhibition of sodium pump by bepridil. An in vitro and microcalorimetric study

The effects of diltiazem, verapamil, bepridil, nicardipine and nifedipine were studied in vitro on Na+,K(+)-ATPase from dog kidney (EC 3.6.1.37). Except diltiazem, all the drugs tested showed an inhibitory effect on Na+,K(+)-ATPase activity in a dose-dependent manner. Among these drugs bepridil is far more effective than the others (IC50 approximately 10(-4) M). Competition studies showed that bepridil acted in a non-competitive manner with the ATP-Mg2+ complex and in a partially competitive manner with K+. Since ouabain acted similarly under the same experimental conditions, we tested the interaction of bepridil and ouabain on Na+,K(+)-ATPase. With low doses of ouabain, the enzyme inhibition corresponded to a potentiated synergy of the two drugs. We then studied the action of bepridil on the sodium pump activity of intact red blood cells by an ex vivo microcalorimetric technique. At 10(-5) M bepridil caused a significant decrease in sodium pump activity (33 +/- 8%).

Effects of bepridil compared to those of its quaternary analogue on femoral arteries from spontaneously hypertensive rats

The effects of bepridil and its quaternary analogue, methylated bepridil, were studied on femoral arterial strips from spontaneously hypertensive rats (SHR). Methylated bepridil relaxed precontracted strips more rapidly than did bepridil, but bepridil was a more potent vasorelaxant. Both drugs non-competitively inhibited arterial contraction evoked by voltage-dependent or alpha-adrenoceptor-operated Ca2+ influx with bepridil being more potent than methylated bepridil. In the absence of extracellular Ca2+, the contractile responses of Wistar-Kyoto rats (WKY) and SHR arteries to norepinephrine and caffeine were reduced similarly by methylated bepridil and bepridil. It is suggested that the effects of bepridil involve both extracellular and intracellular actions. The altered sensitivity of the contraction mechanism with norepinephrine-releasable Ca2+ to the Ca2+ antagonist may be due to an abnormality in the intracellular vasocontraction process distal to Ca2+ release from storage sites of the SHR blood vessels.

Studies on the activity of bepridil as a scavenger of free radicals

Bepridil, a calcium antagonist with anti-anginal, anti-ischemic, and anti-arrhythmic properties was assessed for its ability to scavenge free radicals. Bepridil reduced the stable free radical 1,1-diphenyl-2-picrylhydrazil (DPPH) in the molar ratio 2:1 and, in this respect, was as active as the reference anti-oxidants hydroquinone and alpha-tocopherol. Allopurinol and SOD inhibited cytochrome c reduction in a hypoxanthine-xanthine oxidase superoxide generating system, whereas bepridil was ineffective. Deoxyribose degradation induced by the .OH radical was prevented by bepridil (IC50 = 0.050 mM). This ability to scavenge .OH was similar to that of dimethyl sulfoxide (DMSO) (IC50 = 0.056 mM) and more potent than that observed with mannitol and allopurinol (IC50 values of 0.74 mM and 0.92 mM, respectively). The powerful .OH scavenging activity of bepridil was confirmed in vivo on alloxan induced diabetes in mice. Bepridil exerted a marked protective effect at 0.150 mmol/kg whilst, ethanol and DMSO were active at the doses of 90 and 94 mmol/kg, respectively. These results demonstrate that bepridil is a potent .OH radical scavenger. This property may contribute to the therapeutic activity of this drug in myocardial ischaemia.

Effects of anipamil on myocardial sarcolemmal and mitochondrial calcium transport, comparison with verapamil and nifedipine

The calcium antagonists anipamil, verapamil and nifedipine inhibited, dose dependently, passive and ATP-driven 45Ca2(+)-uptake in purified rabbit ventricular sarcolemmal vesicles exposed to a wide range of free calcium concentration (from 0 to 200 microM). The IC50 values for passive binding were virtually identical for all calcium antagonists and the inhibition was relatively independent of the amount of free calcium employed. On the contrary, the order of potency for inhibition of the ATP-driven calcium uptake was: anipamil greater than verapamil greater than nifedipine. The inhibition of nifedipine, at free calcium concentrations lower than 80 microM, was preceded by a slight stimulation. The inhibitory effects of anipamil and verapamil, but not those of nifedipine, on the ATP-driven calcium uptake were more evident with increasing external calcium concentration. Verapamil and nifedipine failed to modify the initial rate of mitochondrial calcium transport either in the presence or in the absence of ADP; on the contrary, anipamil induced a dose-dependent inhibition of mitochondrial calcium transport. The inhibition occurred over the whole range of calcium concentrations tested, independent of the presence of ADP. The effects of anipamil, but not those of verapamil and nifedipine, on sarcolemmal and mitochondrial calcium transport were long lasting and survived membrane isolation.

Calcium channel effectors are potent non-competitive blockers of acetylcholine receptors

Nicardipine and other calcium channel effectors (CCEs) were studied for their effects on nicotinic acetylcholine receptor (nAChR) activity. While CCEs had no effect on frog (Rana pipiens) skeletal muscle contractions resulting from nerve stimulation or direct stimulation of the muscle, nicotinic agonist-induced contractures were blocked. Nicardipine did not affect nAChR single-channel open time or amplitude, corroborating data from endplate currents (EPCs); EPC amplitudes and decays were unaffected. All the CCEs tested, however, non-competitively blocked nAChRs. The block of nAChRs resulted in a shortened agonist-induced depolarization and thus a diminished contracture response. An increase in cultured mouse skeletal muscle (C-2) cell single-channel closed times was observed with the intracellular addition of nicardipine, verifying a direct block of nAChRs. Using single-channel analysis, nicardipine's site of action, or at least access to its site of action, was determined to be at the intracellular side of the receptor. A direct action of the CCEs on the nAChR was also shown by their ability to block phencyclidine (PCP) binding to Torpedo nobiliana membranes. All the CCEs blocked specific binding of [3H]-PCP to its binding site on the nAChR-channel complex, with bepridil and nicardipine being the most potent. These data are compatible with a model in which nicardipine and other CCEs, at concentrations which do not alter nAChR channel open time or conductance, block the effects of superfused nicotinic agonist on nAChRs either by stabilizing the formation of the nAChR desensitized state or by effecting a slow channel block.

Electrophysiological effects of bepridil and its quaternary derivative CERM 11888 in closed chest anaesthetized dogs: a comparison with verapamil and diltiazem

1. The electrophysiological effects of bepridil, its quaternary derivative, CERM 11888 (methylpyrrolidinium bromide) (both 2.5 mg kg-1 i.v.) and those of verapamil and diltiazem (0.2 mg kg-1 i.v.) were studied in closed chest anaesthetized dogs at doses used in clinical studies. 2. The four drugs caused a bradycardia with the following order of potency: bepridil greater than CERM 11888 greater than diltiazem greater than verapamil. 3. All the compounds slowed conduction in the AV node, increased the refractory period (RP) and decreased Wenckebach rates with the following order: verapamil much greater than diltiazem greater than bepridil greater than CERM 11888. 4. Verapamil and diltiazem did not affect conduction or the RP in atria while bepridil weakly slowed the former and markedly increased the latter. CERM 11888 caused a lengthening of RP but this was a delayed effect. 5. In the ventricle, bepridil and CERM 11888 caused a small increase in the QRS and a more pronounced increase in the RP. Both compounds increased QTc but did not modify HV. Verapamil and diltiazem had no significant effects at the ventricular level. 6. Our results confirm that the main sites of action of calcium antagonists are the SA and AV nodes. Bepridil has a broader spectrum of activity and also acts at the atrial and ventricular levels. A comparison of the effects of bepridil with those of its quaternary derivative suggests the involvement of an intracellular action in the electrophysiological effects of bepridil.

Binding of bepridil to isolated rat heart mitochondria

We investigated mass action of isolated rat heart mitochondria with the calcium antagonist bepridil. At pH 7.20 bepridil in basic form b associates rapidly with the mitochondrial membrane but the amount fixed is higher in non-energized mitochondria than in mitochondria energized by succinate or ATP Mg2+. This effect is related to the dissociation state of the drug since conditions favoring the acidic form bH+ suppress this difference. Tritiated bepridil bound to mitochondrial membrane is only partially displaced by high concentrations of unlabeled drug (greater than or equal to 510(-5) M). No membrane energization effect is noted on this displacement. Binding values of bepridil to mitochondrial membrane (KD 1.710(-5) M; Bmax 23.8 nmol.mg-1 protein) show only low affinity receptor sites. Bepridil binding to the lipid part of the inner membrane surface is postulated. This interaction is used to explain some of the in vitro effects of this calcium antagonist on membrane bound enzyme activities.

Bepridil: a pharmacological reappraisal of its potential beneficial effects in angina and tissue protection following ischemia

In this review the pharmacologic properties of the calcium antagonist bepridil have been reexamined, particularly the evidence for an intracellular locus of action for the drug. Physicochemical properties of bepridil show it to be highly lipophylic, rapidly and extensively taken up, and accumulated in certain tissues. Combined electrophysiologic and mechanical studies have provided convincing, but indirect, evidence for an intracellular action of bepridil in cardiac muscle. Bepridil also fulfills, to a greater or lesser extent, certain important pharmacologic criteria necessary for evoking an intracellular action of a drug in cardiac and vascular smooth muscle: 1. Responses to agonists known to utilize intracellular calcium in the response are inhibited to a similar extent to depolarization-induced K+ responses. 2. Phasic and tonic responses to noradrenaline in vascular tissues are not, or are only to a minor extent, differentially antagonized. 3. Responses to the calcium ionophore A 23187 are antagonized. 4. Activity is retained following removal of the cell membrane by surfactants. 5. Isolated enzyme systems (e.g., calmodulin, myosin light-chain kinase) are affected by the drug at similar concentrations to those that are effective in whole cells or tissues. Finally results obtained with bepridil in ischemic myocardium have been reviewed to ascertain whether its broader pharmacologic spectrum over the calcium-entry blockers is associated with enhanced tissue protective properties. Positive results with bepridil in hypoxic myocytes and ischemic myocardium distinguishes this drug from the classical antianginal agents verapamil, nifedipine, and diltiazem. It is suggested that bepridil, because of its paucity of hemodynamic effects, may be of special therapeutic interest in the management of silent ischemia where cellular mechanisms leading to cytoprotection are more desirable than strong hemodynamic activity.

Comparative effects of bepridil, its quaternary derivative CERM 11888 and verapamil on caffeine-induced contracture in ferret hearts

1. The effects of bepridil, its quaternary derivative: CERM 11888 (methyl-pyrrolidinium bromide) (10(-7)-10(-5) M), and verapamil (10(-7)-10(-6) M) were compared on caffeine-induced contracture of isolated ventricular trabeculae of the ferret. 2. Bepridil diminished the amplitude of contracture in a concentration-dependent fashion, and this effect was significantly different from that of CERM 11888 which, like verapamil, only reduced the amplitude at the highest concentration used. 3. Bepridil (10(-6) M) significantly shortened the time to peak tension and accelerated the relaxation phase of contracture. This latter effect was different from that of CERM 11888. Verapamil (10(-6) M) also tended to accelerate the relaxation phase. At 10(-5) M these actions of bepridil on the time to peak and relaxation tended to reverse. 4. At all concentrations bepridil and verapamil reduced the rate of repriming of contracture and this effect of bedpridil was significantly different from that of its quaternary derivative which only showed a significant effect at 10(-5) M. 5. These results demonstrate a clear intracellular effect of bepridil in the ferret heart. Verapamil and CERM 11888 had only weak intracellular effects even at high concentrations. 6. Analysis of the results suggests that the main sites of action of bepridil in this model are the sarcoplasmic reticulum and one or two calcium compartments in the sarcolemma.

Bepridil versus verapamil in stable angina pectoris--a controlled clinical trial

The calcium antagonist bepridil (bepridil monohydrochloride monohydrate, Org 5730, Cordium) was investigated in comparison with verapamil in a double-blind cross-over design in patients with angina pectoris. Exercise parameters, frequency of anginal attacks, consumption of nitroglycerin and subjective preference were analysed for 36 patients. The exercise capacity, estimated by exercise time and total work load during standard bicycle testing, was significantly more improved with bepridil treatment than with verapamil treatment. No significant differences were observed in frequencies of anginal attack, consumption of nitroglycerin and subjective preference. Side effects were mild and equally divided over both treatment groups.

Modulation of Na+-Ca2+ exchange in cardiac sarcolemmal vesicles by Ca2+ antagonists

The purpose of this study was to examine the effect of three classes of Ca2+ antagonists, diltiazem, verapamil and nifedipine on Na+-Ca2+ exchange mechanism in the sarcolemmal vesicles isolated from canine heart. Na+-Ca2+ exchange and Ca2+ pump (ATP-dependent Ca2+ uptake) activities were assessed using the Millipore filtration technique. Sarcolemmal vesicles used in this study are estimated to consist of several subpopulations wherein 23% are inside-out and 55% are right side-out sealed vesicles in orientation. The affect of each Ca2+ antagonist on the Na+ dependent Ca2+ uptake was studied in the total population of sarcolemmal vesicles, in which none of the agents depressed the initial rate of Ca2+ uptake until concentrations of 10 microM were incubated in the incubation medium. However, when sarcolemmal vesicles were preloaded with Ca2+ via ATP-dependent Ca2+ uptake, cellular Ca2+ influx was depressed only by verapamil (28%) at 1 microM in the efflux medium with 8 mM Na+. Furthermore, inhibition of Ca2+ efflux by verapamil was more pronounced in the presence of 16 mM Na+ in the efflux medium. The order of inhibition was verapamil greater than diltiazem greater than nifedipine. These results indicate that same forms of Ca2+-antagonist drugs may affect the Na+-Ca2+ exchange mechanism in the cardiac sarcolemmal vesicles and therefore we suggest this site of action may contribute to their effects on the myocardium.

Bepridil, myocardial accumulation kinetics and dynamic effects in the isolated rabbit heart

Both myocardial uptake and disposition of bepridil in the isolated rabbit heart showed two-compartment characteristics which possibly reflects the existence of superficial and deep binding sites. Terminal accumulation and disposition half-lives were 218 and 196 min., respectively. The half-times of the initial distributory processes were about 33 min. At a drug concentration in the perfusion liquid of 0.54 micrograms ml-1 (1.27 microM) the average concentration of bepridil in the myocardium at steady state was about 489 micrograms g-1 (1161 microM) with 43% referable to the deepest, presumably intracellular compartment. Increasing bepridil concentrations from 3 to 2333 ng ml-1 (7-5542 nM) in the perfusion liquid caused a terminal decrease in coronary flowrate to 58% of the mean control flowrate. Amplitude and velocity of myocardial contraction both decreased in a biphasic way to about 28.6 and 13.6%, respectively. Apparent dynamic steady states developed within about 20 min. Inhibitory Em-values related to the first phase were 39.8 and 53.2%, and to the second phase 97.7 and 98.5%, respectively. Heart beating frequency also decreased biphasically to 53.9% and showed inhibitory Em-values of 17.2 and 47.5% related to the two phases. Myocardial oxygen consumption decreased to 55.6%. The electrocardiographic PQ- and QRS-intervals increased to 147 and 133%, respectively. The frequency-corrected QT-interval also increased significantly from 100 to 123%. Our findings demonstrate a slow and very pronounced accumulation of bepridil in the rabbit heart. Biphasic and very marked negative inotropic and chronotropic effects and a less than proportional decrease in oxygen consumption developed much faster.(ABSTRACT TRUNCATED AT 250 WORDS)

The anti-ischaemic activity of the novel compound, CERM 11956, compared with that of bepridil and nifedipine in isolated guinea-pig hearts

A comparison between the protective activity of bepridil, its novel derivative, CERM 11956, and nifedipine in isolated electrically paced guinea-pig hearts after 60 min of global ischaemia followed by 30 min of reperfusion has been made. All three compounds exerted a significant anti-ischaemic effect, as indicated by an improved recovery of functional parameters (left ventricular pressure and coronary perfusion), a delayed onset of the ischaemic contracture, and an enhanced recovery of biochemical (CrP, ATP and adenylate energy charge) parameters. The most pronounced anti-ischaemic activity was shown by the compound CERM 11956 at concentrations that displayed only minor negative inotropic activity. From the results it may be concluded that the new bepridil derivative, CERM 11956, is a promising and potent anti-ischaemic compound, which has little influence on haemodynamic parameters.

Effects of bepridil on calcium release from rat heart mitochondria

Bepridil at concentrations above 10 microM, and at pH 7.2 stimulates calcium release from rat heart mitochondria. However this action is different from that of ClCCP, an uncoupler of oxidative phosphorylations, since it is ruthenium red insensitive. At lower concentrations bepridil may inhibit the Na-induced calcium release. The effects of bepridil depend on the pH and indicate that the protonated form of the drug is more efficient on calcium release than the basic form.

Differentiation of the drug-binding sites of calmodulin

Calmodulin contains several binding sites for hydrophobic compounds. The apparent specificity of various 'calmodulin antagonists' for these sites was investigated. The Ki values for the inhibition of calmodulin-activated cyclic-nucleotide phosphodiesterase and myosin light-chain kinase was determined. In addition, the Kd values of the same compounds for binding to calmodulin were measured. The compounds could be separated into four groups. Group I and II compounds inhibited competitively the activation of the phosphodiesterase and myosin light-chain kinase by calmodulin. Group I compounds inhibited the activation of the phosphodiesterase and myosin light-chain kinase at identical concentrations. In contrast, group II compounds inhibited the activation of the phosphodiesterase at 5-10-fold lower concentrations than that of myosin light-chain kinase. Group III compounds inhibited the activation of these enzymes by an uncompetitive mechanism. Group IV compounds inhibited the activation of the phosphodiesterase with Ki values above 10 microM and did not affect the activation of myosin light-chain kinase. Binding of [3H]bepridil to calmodulin under equilibrium conditions yielded one high-affinity site (apparent Kd 0.4 microM) and four low affinity sites (apparent Kd 44 microM). Group I compounds interfered with the binding of bepridil to the high and low-affinity sites in a competitive manner. Group II compounds interfered in a non-competitive manner with the high-affinity site and apparently competed only with one of the low-affinity sites. Group III compounds did not compete with any of the bepridil-binding sites. Nimodipine, a group III compound, bound to one site on calmodulin with a Kd value of 1.1 microM. Other dihydropyridines competed with [3H]nimodipine for this site. The group I and II compounds, trifluoperazine and prenylamine, did not affect the binding of [3H]nimodipine. These data show that 'calmodulin antagonists' can be differentiated into at least three distinct groups. Kinetic and binding data suggest that the three groups bind to at least three different sites on calmodulin. Selective occupation of these sites may inhibit specifically the activation of distinct enzymes.

Comparison of the effects of bepridil and diltiazem upon globally ischemic rat hearts

Effects of the Ca2+ antagonists, bepridil (20 microM) and diltiazem (2.5 microM), upon ischemia/reperfusion injury were assessed in perfused, working rat hearts. These treatments were equally cardiodepressant in non-ischemic hearts. A lower concentration (5 microM) of bepridil was also assessed. Hearts which were reperfused following 33 min of global ischemia recovered 12.8% of preischemic pressure-rate product and had markedly reduced ATP, total adenine nucleotides, ATP/ADP ratio, and mitochondrial function. Treatment with bepridil before and during ischemia did not improve recovery of cardiac function, tissue energy reserves, or mitochondrial function upon reperfusion with control buffer. Control hearts treated with bepridil had normal levels of high energy compounds. Treatment with diltiazem significantly improved contractile function, and metabolic parameters. Ischemia/reperfusion injury was associated with a doubling of tissue Ca2+ content. Pretreatment with diltiazem, but not bepridil, reduced Ca2+ overload. Bepridil did not directly protect the myocardium from ischemia/reperfusion injury perhaps due to its inability to prevent Ca2+ overload.

The effects of bepridil, compared with calcium-channel inhibitors and calmodulin antagonists on both spontaneous activity and contractions induced by potassium or phenylephrine in rat portal vein

Bepridil is known to block calcium channels in some vascular tissues. Recent work has shown that bepridil also antagonises calmodulin. The present study attempted to more fully characterize the vasodilator actions of bepridil by comparing it with the known calcium channel blocking drugs, nifedipine, diltiazem, verapamil and flunarizine, the calmodulin inhibitors, trifluoperazine and W7 and propylmethylenedioxyindene, which is thought to act intracellularly, on rat portal vein. The relative activities of the test drugs were compared on spontaneous activity and on all components of the contractile responses to potassium and phenylephrine. Bepridil inhibited all components of the potassium and phenylephrine responses equally, actions similar to those of the intracellular acting drugs. The exception to this was trifluoperazine which also exerted alpha-adrenoceptor blocking actions. In contrast the calcium channel blocking drugs, with the exception of verapamil, inhibited the tonic component of both spasmogen responses more than the phasic component. Bepridil like the intracellular acting drugs, but unlike the calcium channel blockers, markedly increased the frequency of spontaneous contractions whilst reducing amplitude. It is concluded that the profile of bepridil on rat portal vein more closely resembles that of intracellularly acting drugs than that of classical calcium channel inhibitors.

Comparative antiarrhythmic and electrophysiological effects of drugs known to inhibit calmodulin (TFP, W7 and bepridil)

The potential antiarrhythmic and electrophysiological actions of drugs known to inhibit calmodulin, i.e. trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W7) have been compared with bepridil, whose antiarrhythmic actions have previously been ascribed to blockade of the fast inward sodium current in cardiac tissue. Like bepridil, both TFP and W7 reduced the severity of arrhythmias evoked by 30 min of coronary artery occlusion in the anaesthetized rat. TFP (2.5-10 mg kg-1, i.v.), W7 (2.5-10 mg kg-1, i.v.) and bepridil (1-5 mg kg-1, i.v.) also antagonized the development of ventricular fibrillation induced by 5 min of occlusion followed by reperfusion. All three drugs also reduced mortality. TFP and bepridil also reduced the incidence of reperfusion-induced ventricular tachycardia whilst all 3 drugs reduced its duration. Although TFP was shown to possess alpha-adrenoceptor blocking properties, the classical alpha-blocker, phentolamine, failed to reduce significantly the incidence or severity of reperfusion arrhythmias. In contrast to bepridil (2-20 microM), which markedly reduced the maximum rate of depolarization (Vmax) of guinea-pig isolated papillary muscle, W7(5-50 microM) showed only weak effects on Vmax and was at least 10 times less potent than bepridil whilst TFP only reduced Vmax in high concentrations (40-100 microM) which lowered resting membrane potential. Unlike bepridil, neither TFP (4-40 microM) nor W7 prolonged the absolute refractory period. The results suggest that drugs which inhibit calmodulin confer protection against both ischaemia-and reperfusion-induced arrhythmias in the rat. Although the electrophysiological actions of bepridil would adequately account for its antiarrhythmic activity, the same cannot be said of W7 and especially TFP. In conclusion, calmodulin antagonism may constitute a mechanism of antiarrhythmic activity.

Effect of methylated bepridil on slow action potentials in cardiac muscle and vascular smooth muscle

The anti-anginal agent bepridil blocks slow Ca2+ channels in a variety of tissues. Since bepridil accumulates inside cells, the possibility exists that bepridil acts, at least partially, from inside the cell. To test this possibility, we examined the effects of a quaternary ammonium analog of bepridil, methylated bepridil, which presumably would enter the cells less readily, on the Ca2+-dependent slow action potentials of guinea pig papillary muscles (in 25 mM [K+]0) and rabbit pulmonary arteries (in tetraethylammonium chloride). In cardiac muscle, methylated bepridil had little effect on the slow action potentials at low stimulation frequencies (0.5 Hz), but at higher frequencies (1.0 and 2.0 Hz) the slow action potentials were depressed and/or the muscle was unable to follow each stimulation. These effects are similar to those obtained with bepridil, but bepridil was more potent than methylated bepridil. In vascular smooth muscle cells, methylated bepridil inhibited the slow action potentials at a somewhat lower dose than bepridil. We conclude that, in cardiac muscle, bepridil probably has two sites of action, one outside the cell (presumably on or associated with the slow Ca2+ channel) and a second site inside the cell. On the other hand, in vascular smooth muscle cells, bepridil may act only on an external site.

Comparative effects of bepridil and diltiazem on systemic blood pressure and isolated peripheral arteries in spontaneously hypertensive rats

Bepridil and diltiazem were studied for their effects on blood pressure (BP) and heart rate (HR) of spontaneously hypertensive rats (SHR) and on vascular tone of femoral and mesenteric arterial strips from SHR. The drugs (i.v.) reduced BP and HR more markedly in SHR than in normotensive Wistar Kyoto rats (WKY). The effects of bepridil were less pronounced and less prolonged than those of diltiazem. Bepridil relaxed arterial strips precontracted by KCl or prostaglandin F2 alpha to almost the same extent as diltiazem in both SHR and WKY. Bepridil was almost as potent as diltiazem in inhibiting non-competitively the Ca2+-evoked contractions of arteries depolarized in a Ca2+-free, high K+ buffer. alpha-Adrenoceptor agonist-induced contractions accompanied and not accompanied by Ca2+ influx were inhibited more markedly by bepridil than diltiazem under certain conditions. The inhibitions were more marked in SHR than in WKY. Thus, it was suggested that both drugs acted as Ca2+ influx inhibitor to reduce vascular tone. Bepridil inhibited intracellular vasoconstriction mechanisms linked with alpha-adrenoceptors more potently than did diltiazem in SHR. Taken together, these actions can explain the antihypertensive properties of both drugs in SHR.

Modulation of ATP-dependent calcium extrusion and Na+/Ca2+ exchange across rat cardiac sarcolemma by calcium antagonists

The calcium antagonists verapamil, bepridil, nifedipine and nimodipine inhibited ATP-dependent 45Ca2+ uptake in purified rat ventricular sarcolemma vesicles dose dependently. This inhibition was preceded by a slight stimulation in the case of the two dihydropyridines, but not with bepridil and verapamil. In contrast, Na+/Ca2+ exchange was only inhibited by verapamil and bepridil and not affected by the dihydropyridines. The steepness of the inhibition curves was significantly different for the two processes. No stereoselectivity was found with either process for inhibition by the verapamil enantiomers. Inhibition of the exchange was not due to a decrease of the exchange velocity but to a decrease in exchange capacity. Variation of the antagonist preincubation time did not modify the inhibition of the uptake. The results indicate that two different sites, located at the inner surface of the sarcolemma are involved in the modulation of the ATP-dependent uptake and the Na+/Ca2+ exchange. However, the possibility cannot be ruled out that inhibition of the exchange process is also mediated by an extracellularly located site.

Selective lysosomal uptake and accumulation of the beta-adrenergic antagonist propranolol in cultured and isolated cell systems

The beta adrenoreceptor antagonist propranolol is rapidly taken up and accumulated in various cultured cell lines. When incubated in the presence of low concentrations of propranolol (10(-9) M), Hela (non-differentiated epithelia), BC3H1 (smooth muscle) and MDCK (differentiated kidney epithelia) cell cultures take up (t1/2 = 4-10 min) and accumulate the drug such that the intracellular concentration is over 1000 times that in the incubation medium. The release of propranolol from the cells was slower (t1/2 = 22 min) than the rate of uptake but the dissociation was stimulated by the addition of 1 microM propranolol to the external medium (t1/2 = 9 min). Uptake, which is non-stereoselective, is dependent on pH and is inhibited by the lysosomotropic agents, NH4Cl, methylamine and chloroquine. At higher concentrations (greater than 10(6) M), uptake is accompanied by a visual swelling of intracellular acidic vesicles staining with acridine orange. These results suggest that propranolol, a basic amphiphilic amine, is accumulated within the lysosomes of these cells. Uptake was confined to these cultured cell systems with no chloroquine-sensitive propranolol uptake, being found in isolated rabbit ventricular myocytes, red blood cells or blood platelets. Although alprenolol and cyanopindolol competed with propranolol for uptake, isoprenaline, adrenaline, noradrenaline, phenylephrine, atenolol, practolol and salbutamol were not effective inhibitors. The possible consequences of this uptake and accumulation of propranolol by certain tissues is discussed in relation to the known actions of the drug, particularly during or after abrupt withdrawal from chronic applications.

Effects of bepridil and of its quaternary derivative on rat tail artery

The effects of bepridil and its quaternary ammonium derivative (BN+) were compared, showing that: (i) both drugs inhibited K+-induced contractions with similar time courses and potencies, (ii) bepridil blocked the tonic but not the phasic component of contractions elicited by noradrenaline, whereas BN+ had no effect on noradrenaline-elicited contractions. These results, and the relative insensitivity of skinned taenia caeci to bepridil, suggest that this drug and BN+ do not act directly on contractile proteins but affect K+- and noradrenaline-induced calcium channel activities differentially.

Adenine nucleotide synthesis de novo in mature rat cardiac myocytes

Inadequate oxygenation of cardiac muscle leads to rapid loss of high energy compounds essential for contractile function. ATP can be regenerated by synthesis de novo, a route operating at a relatively slow rate in the heart. Myocytes isolated from mature rat heart have been used to measure the rate of ATP synthesis de novo from both [14C]glycine and [14C]ribose. Incorporation of glycine into ATP is accelerated 10-fold in the presence of 1 mM ribose. Myocytes also accumulate both precursors into IMP and four other metabolites on the de novo synthesis pathway. These metabolites represent 80% of the glycine entering the pathway. The potential of de novo synthesis for restoration of adenine nucleotides appears to be limited by the rates of early reactions, adenylosuccinate synthetase being only one of the enzymes operating at a sufficiently slow rate to make this pathway an inherently weak route for the restoration of normal energy status in post-ischemic myocardium. Interventions are being sought to alleviate these apparent metabolic delays.

Slow calcium channel blockers and calmodulin. Effect of felodipine, nifedipine, prenylamine and bepridil on cardiac sarcolemmal calcium pumping ATPase

The effect of four slow Ca2+ channel blockers (felodipine, nifedipine, prenylamine and bepridil) that possess the ability to bind to calmodulin (CaM) section and to inhibit myosin light chain kinase (MLCK) on CaM-regulated Ca2+ pumping ATPase of cardiac sarcolemma (SL) and brain cyclic AMP phosphodiesterase (PDE) was studied. The ability of these drugs to inhibit Ca2+ pumping ATPase correlated with their inhibitory effect on CaM-activated Ca2+-dependent PDE. Nifedipine was unable to inhibit markedly both enzymes. Prenylamine also was a weak inhibitor, which was unexpected because of its CaM binding potency. Felodipine (10-50 microM) and bepridil (50 microM) markedly reduced activities of SL Ca2+ pumping ATPase and PDE. Striking differences were, however, demonstrated when Ca2+ and CaM concentrations, respectively, were increased. Previously it was reported that inhibition of the SL Ca2+ pumping ATPase by the CaM antagonist calmidazolium could be overcome by increasing Ca2+ concentrations (J. M. J. Lamers and J. T. Stinis, Cell Calcium 4, 281-294, 1983). Felodipine (10-50 microM) in the present study, appeared to be equipotent with calmidazolium in reducing Ca2+ pumping ATPase, but increasing Ca2+ up to 12.2 microM could not counteract this effect. Felodipine (2-10 microM) also inhibited brain PDE noncompetitively with respect to CaM contrary to the competitive effectors calmidazolium and bepridil. On the other hand, bepridil (10-20 microM) decreased or increased Ca2+ pumping ATPase activity depending on the Ca2+ concentration (0.29 and 12.2 microM, respectively) used. These findings suggest at least two types of CaM antagonists, which can be discriminated on basis of their inhibition patterns of PDE and heart SL Ca2+ pumping ATPase.

Diphenylalkylamine calcium antagonists interact with alpha-adrenoceptor binding sites in aortic membranes

Some interactions of calcium antagonists with [3H]prazosin and [3H]yohimbine binding sites were investigated in bovine aorta membranes. Diphenylalkylamines (flunarizine, cinnarizine and bepridil) acted as competitors of the two ligands with Ki values in the microM range. With the exception of verapamil, reference compounds (nifedipine, Bay-K 8644, diltiazem) and the peripheral benzodiazepine receptor antagonist PK 11195 did not displace the ligands. The apparent affinity of the diphenylalkylamines for alpha-adrenoceptor was consistent with the concentrations producing vasodilatation.

Effects of selective channel blocking agents on contractions and action potentials in K+-depolarized guinea-pig atria

Contractions and transmembrane action potentials were induced by 1 microM isoprenaline in K+-depolarized guinea-pig left atria driven at 0.5 Hz. The stability of these responses was significantly increased by doubling the extracellular glucose concentration to 22 mM. Action potential overshoot increased by 28 mV per ten fold increase in extracellular calcium concentration suggesting that the inward current in this preparation is carried by Ca2+. In depolarized driven preparations, nanomolar concentrations of nifedipine and nisoldipine reduced contractility, maximum rate of depolarization (dV/dt max) and action potential height, whereas the fast channel blocking agents tetrodotoxin and mexiletine (in micromolar concentrations) produced little change. Nifedipine also rendered spontaneously beating depolarized right atrial preparations quiescent. In concentrations which reduced dV/dt of normal action potentials, the sodium channel blocking agents quinidine and Org 6001 reduced the amplitude of contractions and reduced the maximum rate of phase 0 depolarization (dV/dt) of action potentials in depolarized tissue. These actions were reversed by Ca2+ and suggest calcium antagonistic activity. However action potential height was not reduced. Like bepridil, both drugs also reduced the frequency of spontaneous contractions in depolarized right atrial preparations. Unlike Org 6001, quinidine failed to produce a shift in calcium log dose-response curves in driven depolarized preparations and induced positive inotropy in the presence of functional sodium channels. Bepridil inhibited contractions in depolarized atria in the absence of a reduction in dV/dt suggesting that any calcium antagonistic action in atrial tissue is mainly located at an intracellular site. In conclusion, action potentials elicited by isoprenaline in potassium-depolarized atria bathed in high glucose appear to be Ca2+ mediated. In concentrations which inhibit the inward Na+ current, both quinidine and Org 6001 exhibit calcium channel blocking properties.

Characterization of digoxin binding and daunorubicin uptake by isolated mature rat cardiac myocytes

Myocytes isolated from ventricular muscle of mature rat heart have been used for characterization of digoxin binding and to establish whether a relationship exists between digoxin binding and uptake of daunorubicin. High- and low-affinity digoxin binding sites have been identified; respectively, 0.9 +/- 0.1 X 10(7) sites/myocyte, Kd 70-77 nM and 7 +/- 2 X 10(7) sites/myocyte, Kd 1.4-1.7 microM. Myocytes accumulate daunorubicin to an intracellular concentration 30-40 times that in the medium. We find no evidence that saturation of digoxin binding sites alters daunorubicin uptake or that daunorubicin influences binding of digoxin. Alteration of sarcolemmal membrane properties is demonstrated by inhibition of amino acid transport reflected in protein synthesis rates. Calmodulin activation of phosphodiesterase appears insensitive to daunorubicin.

Accumulation and salvage of adenosine and inosine by isolated mature cardiac myocytes

Reoxygenation of ischaemic, energy-depleted heart does not result in sufficiently rapid regeneration of normal adenine nucleotide concentrations for preservation of cardiac function and structure. Salvage of nucleoside as a mechanism for restoration of ATP in the post-ischaemic myocardium is limited by efflux of adenosine during ischaemia. Isolated cardiac myocytes have been used to establish the kinetics of uptake and salvage of adenosine and inosine, measuring the distribution of radioactive nucleoside incorporated into ATP, ADP and AMP. Maximum rates of catalysis of reactions on the salvage pathway, and of enzymes competing for substrates on the pathway, have been established in myocyte extracts. Myocytes have little capacity to salvage or catabolise inosine. Enzyme measurements indicate that salvage of adenosine should proceed at 7-8-times the rate exhibited by intact myocytes dependent upon extracellular adenosine as substrate. The data indicate that the rate of transport of adenosine is not determined by its metabolic utilization, but is the rate-limiting step in the salvage of adenosine.

Changing surface charge with salicylate differentiates between subgroups of calcium-antagonists

Sodium salicylate (5-10 mM) has been used to distinguish the effects of the three calcium-antagonist subgroups which had been previously differentiated in functional studies. Sodium salicylate (10 mM) reduced the antagonistic effects of verapamil and diltiazem on Ca2+-induced contractions of K+ (40 mM)-depolarized taenia preparations from the guinea-pig caecum. In contrast, salicylate had no effect on the potency of nifedipine and increased the inhibitory effects of cinnarizine and flunarizine. Sodium salicylate (10 mM) had little effect on Ca2+-induced contractions per se. In preparations pretreated with calcium-antagonists and recontracted with high concentrations of Ca2+, salicylate (5 mM) caused an additional contraction when the preparations had been pretreated with verapamil or diltiazem but had no effect in control or nifedipine-treated preparations. In contrast, salicylate relaxed Ca2+-induced contractions in tissues which had been pretreated with cinnarizine, flunarizine, pimozide, bepridil, fendiline, perhexiline and with the calmodulin antagonist W-7. The mechanism of action of salicylate was investigated. Inhibition of prostaglandin biosynthesis or of oxidative phosphorylation by salicylate was not responsible for these effects because indomethacin (28 microM) and 2,4-dinitrophenol (20 microM) did not differentiate between calcium antagonists. The effects of salicylate are ascribed to an increase in negative surface charge on the membrane because other agents changing surface charge (3,5-dichlorosalicylate, 0.3 mM; benzoate, 20 mM) have similar effects and their potency is dependent on their affinity for lipid membranes. Furthermore, salicylate increased the effectiveness of the cationic local anaesthetic, (+)-propranolol (100 microM), but did not change the effects of the neutral local anesthetic, benzocaine (1 mM). It is argued that salicylate increases the effectiveness of cinnarizine by increasing accumulation of this drug in the cell membrane or at intracellular sites whereas the reduced effectiveness of verapamil and diltiazem is secondary to a change in the state of the Ca2+ channel.

Effects of bepridil on Ca2+ uptake by cardiac mitochondria

Isolated rat heart mitochondria accumulate large amounts of Ca2+ at the expense of respiration-linked energy or of that provided by the hydrolysis of ATP by the mitochondrial ATPase. At concentrations below 10 microM bepridil has no effect on the first mechanism but inhibits the second. At higher concentrations bepridil depresses both. At low concentrations bepridil decreases proton influx into mitochondria in ADP-stimulated respiration while it has no effect on proton ejection in Ca2+-stimulated respiration. A preliminary study shows that bepridil inhibits ATP hydrolysis linked to Ca2+ absorption by mitochondria. The calcium antagonists verapamil, nifedipine and diltiazem exhibit none of these effects.

Uptake of calcium antagonistic drugs into muscles as related to their lipid solubilities

Calcium antagonists, e.g. bepridil and verapamil, block the Ca2+-dependent slow action potentials in frog skeletal muscle [L.M. Kerr and N. Sperelakis, J. Pharmac. exp. Ther. 222, 80 (1982)]. To determine whether the calcium antagonistic drugs may enter the fibers and exert an internal action as well, uptake of tritiated bepridil, verapamil, nitredipine, nifedipine, and diltiazem into rat extensor digitorum longus (EDL) muscles was examined. It was found that the uptake values of verapamil, nitrendipine, and bepridil were much higher than those of nifedipine and diltiazem. The order of uptake was: bepridil greater than nitrendipine greater than verapamil much greater than nifedipine greater than diltiazem. The small uptake values of nifedipine and diltiazem may represent primarily binding to the surface membrane. In frog skeletal muscle (sartorius) also, the uptake of bepridil was greater than that of verapamil, and disruption of the T-tubules by the glycerol method did not change them. The same order of drug uptake values was found for monolayer cultures of vascular smooth muscle cells (rat aorta). The order of uptake in isolated sarcoplasmic reticulum (SR) from rat skeletal muscles was: verapamil greater than nitrendipine greater than bepridil greater than nifedipine greater than diltiazem. The lipid solubility values of the calcium antagonists were measured by their partition coefficients in oil/Ringer, octanol/Ringer, and chloroform/Ringer systems. The order of lipid solubility was: bepridil greater than verapamil greater than nitrendipine greater than nifedipine much greater than diltiazem. Thus, the calcium antagonists with the highest lipid solubilities were taken up more by the muscle cells and SR. It is concluded that verapamil, bepridil, and nitrendipine enter and accumulate inside the muscle cells, whereas nifedipine and diltiazem do not permeate readily.

Two site binding of bepridil and modulation of adenylate cyclase in cardiac sarcolemmal membranes

A preparation of cardiac sarcolemmal membranes is described. These membranes exhibit 9-24-fold purification of (Na+ + K+)-ATPase, potassium-stimulated nitrophenolphosphatase, 5'-nucleotidase, adenylate cyclase, sialic acid content, and beta-receptor number. Sarcolemmal membranes have two classes of binding sites for the calcium entry blocker, bepridil, 70 X 10(12) high-affinity sites/mg, Kd 25-40 nM; and 30 X 10(15) low-affinity sites/mg, Kd 54-70 microM. Binding of bepridil to these sites appears responsible for inhibition of isoprenaline-stimulated and activation of fluoride-stimulated adenylate cyclase. Since basal adenylate cyclase activity is not influenced, bepridil must act not at the catalytic site, but by altering the interactions between beta-receptor and catalytic and regulatory components of adenylate cyclase.