The effects of calcium antagonists on calcium overload contractures in embryonic chick myocytes induced by ouabain and veratrine

Effects of µ-Conotoxin GIIIB on the cellular activity of mouse skeletal musculoblast: combined transcriptome and proteome analysis

µ-Conotoxin GIIIB (µ-CTX GIIIB) is a polypeptide containing three disulfide bridges, produced by the sea snail Conus geographus. This study was aimed to explored the cytotoxic effects of µ-CTX GIIIB on mouse skeletal musculoblast (Sol8). Sol8 cells were exposed to ouabain and veratridine to establish the cell injury model, and then treated with µ-CTX GIIIB. CCK-8 was adopted to evaluate the cytotoxicity of µ-CTX GIIIB. Then, proteomics and transcriptome were conducted, and the explore the differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) affected by µ-CTX GIIIB were found. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis was used to investigate the affected signaling pathways. µ-CTX GIIIB increased the cell survival rate of injured Sol8 cells. We found and identified 1,663 DEGs and 444 DEPs influenced by µ-CTX GIIIB. 106 pairs of correlated DEGs and DEPs were selected by combining transcriptome and proteome data. The results of KEGG and GO analysis showed that µ-CTX GIIB affected the cell cycle, apoptosis, DNA damage and repair, lipid metabolism and other biological processes of Sol8 cells. µ-CTX GIIIB could affected cell cycle regulation, DNA damage repair, and activation of tumor factors, with potential carcinogenic effects. Our results provide an important basis for the study of in vitro toxicity, the mechanism of toxicity and injury prevention by µ-CTX GIIIB.

Robust anti-arrhythmic efficacy of verapamil and flunarizine against dofetilide-induced TdP arrhythmias is based upon a shared and a different mode of action

BACKGROUND AND PURPOSE

The high predisposition to Torsade de Pointes (TdP) in dogs with chronic AV-block (CAVB) is well documented. The anti-arrhythmic efficacy and mode of action of Ca(2+) channel antagonists, flunarizine and verapamil against TdP were investigated.

EXPERIMENTAL APPROACH

Mongrel dogs with CAVB were selected based on the inducibility of TdP with dofetilide. The effects of flunarizine and verapamil were assessed after TdP and in different experiments to prevent dofetilide-induced TdP. Electrocardiogram and ventricular monophasic action potentials were recorded. Electrophysiological parameters and short-term variability of repolarization (STV) were determined. In vitro, flunarizine and verapamil were added to determine their effect on (i) dofetilide-induced early after depolarizations (EADs) in canine ventricular myocytes (VM); (ii) diastolic Ca(2+) sparks in RyR2(R4496+/+) mouse myocytes; and (iii) peak and late I(Na) in SCN5A-HEK 293 cells.

KEY RESULTS

Dofetilide increased STV prior to TdP and in VM prior to EADs. Both flunarizine and verapamil completely suppressed TdP and reversed STV to baseline values. Complete prevention of TdP was achieved with both drugs, accompanied by the prevention of an increase in STV. Suppression of EADs was confirmed after flunarizine. Only flunarizine blocked late I(Na). Ca(2+) sparks were reduced with verapamil.

CONCLUSIONS AND IMPLICATIONS

Robust anti-arrhythmic efficacy was seen with both Ca(2+) channel antagonists. Their divergent electrophysiological actions may be related to different additional effects of the two drugs.

The L-Type Ca+ and KATP channels may contribute to pacing-induced protection against anoxia-reoxygenation in the embryonic heart model

L-Type Ca(2+) and K(ATP) Channels in Pacing-Induced Cardioprotection.

AIMS

The L-type Ca(2+) channel, the sarcolemmal (sarcK(ATP)), and mitochondrial K(ATP) (mitoK(ATP)) channels are involved in myocardial preconditioning. We aimed at determining to what extent these channels can also participate in pacing-induced cardioprotection.

METHODS

Hearts of 4-day-old chick embryos were paced in ovo during 12 hour using asynchronous intermittent ventricular stimulation at 110% of the intrinsic rate. Sham operated and paced hearts were then submitted in vitro to anoxia (30 minutes) and reoxygenation (60 minutes). These hearts were exposed to L-type Ca(2+) channel agonist Bay-K-8644 (BAY-K) or blocker verapamil, nonselective K(ATP) channel antagonist glibenclamide (GLIB), mitoK(ATP) channel agonist diazoxide (DIAZO), or antagonist 5-hydroxydecanoate. Electrocardiogram, electromechanical delay (EMD) reflecting excitation-contraction (E-C) coupling, and contractility were determined.

RESULTS

Under normoxia, heart rate, QT duration, conduction, EMD, and ventricular shortening were similar in sham and paced hearts. During reoxygenation, arrhythmias ceased earlier and ventricular EMD recovered faster in paced hearts than in sham hearts. In sham hearts, BAY-K (but not verapamil), DIAZO (but not 5-hydroxydecanoate) or GLIB accelerated recovery of ventricular EMD, reproducing the pacing-induced protection. By contrast, none of these agents further ameliorated recovery of the paced hearts.

CONCLUSION

The protective effect of chronic asynchronous pacing at near physiological rate on ventricular E-C coupling appears to be associated with subtle activation of L-type Ca(2+) channel, inhibition of sarcK(ATP) channel, and/or opening of mitoK(ATP) channel.

Calcium overload enhances hydroxyl radical generation by 1-methyl-4 phenylpyridinium ion (MPP+) in rat striatum

We examined whether ouabain-induced Ca(2+) overload increases hydroxyl radical (*OH) generation by 1-methyl-4-phenylpyridinium ion (MPP(+)) in rat striatum. These elevations seem to induce lipid peroxidation of striatum of rats, as detected by increases in non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) levels. Ouabain enhanced MPP(+)-induced *OH formation trapped as DHBA. Moreover, when iron (II) was administered to MPP(+) then ouabain (100 micro M)-pretreated animals, a marked elevation in the level of DHBA was observed, as compared with the iron (II)-only-treated animals. These results suggests that Ca(2+) overload might enhance *OH generation by MPP(+) in rat striatum.

Effects of veratrine and paeoniflorin on isolated mouse vas deferens

In this study, we attempted to identify the interactions and mechanisms between veratrine and paeoniflorin on isolated mouse vas deferens. Paeoniflorin had no effect on isolated mouse vas deferens. Veratrine (1 x 10(-5) approximately 1 x 10(-3) g/ml) could directly induce contraction of isolated rat and mouse vas deferens. The concentration induced by veratrine (1 x 10(-5) g/ml) was completely inhibited by Ca2+-free solution and verapamil (1 x 10(-5) M), in both the epididymal and the prostatic portions of isolated mouse vas deferens. Naloxone (1 x 10(-5) M) did not alter the contraction induced by veratrine (1 x 10(-5) g/ml) in either the epididymal or the prostatic portions of isolated mouse vas deferens. Paeoniflorin (4.8 x 10(-5) g/ml) inhibited the contraction induced by veratrine (1 x 10(-5) g/ml) in both the epididymal and the prostatic portions of isolated mouse vas deferens. Paeoniflorin (4.8 x 10(-5) g/ml) potentiated norepinephrine (1 x 10(-5) M)-induced phasic contraction in the epididymal portion, but decreased contractions in the prostatic portion. Paeoniflorin (4.8 x 10(-5) g/ml) increased KCI (56 mM)-induced phasic contraction in the epididymal portion, but decreased the tonic contraction in either the epididymal or the prostatic portion. Veratrine (1 x 10(-5) g/ml)-induced contractions could be decreased by pretreatment with ryanodine (1 x 10(-5) M) in both the epididymal and the prostatic portions. Pretreatment with the combination of paeoniflorin (4.8 x 10(-5) g/ml) and ryanodine (1 x 10(-5) M) did not potentiate the inhibition of paeoniflorin in the veratrine-induced contraction in both the epididymal and the prostatic portions of isolated mouse vas deferens.

Comparison of the effects of nifedipine and nisoldipine on coronary vasoconstriction in the Langendorff-perfused rat heart

By using Langendorff-perfused rat hearts, we compared the relaxant effects of two dihydropyridine calcium antagonists (nifedipine and nisoldipine) on the coronary vasculature. Hearts were perfused at a constant flow rate (13 ml/min), and the coronary perfusion pressure was monitored to assess vasoconstriction and relaxation. Administration of 5 nM endothelin significantly increased the perfusion pressure within 5 min, and it did not decrease during the subsequent 10-min washout period. Washout with 1 nM nisoldipine or 10 nM nifedipine almost completely normalized the perfusion pressure. Administration of a nitric oxide synthase inhibitor, N(G)-nitro-L-arginine (100 microM), elevated the perfusion pressure at 5 min. When 1 nM nifedipine was added to the perfusate, it failed to decrease the pressure, but 10 and 100 nM nifedipine partly decreased it (96.4+/-8.2 and 87.0+/-8.8 mm Hg for 10 and 100 nM, respectively). Nisoldipine alleviated the pressure elevation almost completely at 10 nM (60.0+/-5.2 mm Hg). The vasodilatory effect of nisoldipine was also approximately 10 times more potent than that of nifedipine when coronary vasoconstriction was induced by hypoxia-reoxygenation. These results suggest that nisoldipine has a stronger effect than nifedipine at the organ level, which does not depend on the cause of coronary vasoconstriction.

Effects of veratrine and paeoniflorin on the isolated rat aorta

The interactions and mechanisms between veratrine and paeoniflorin on the isolated rat aorta were studied. Veratrine (1x10(-6) to 1x10(-4) g/ml) could induce contraction on the isolated rat aorta in a concentration-related manner. Paeoniflorin had no effect on the isolated rat aorta. Pretreatment with prazosin (1x10(-6) M) and nifedipine (1x10(-6) M) but not yohimbine (1x10(-5) M) could decrease the tension of contraction induced by veratrine (1x10(-4) g/ml). Sodium nitroprusside (1x10(-4) M) could inhibit the contraction induced by veratrine (1x10(-4) g/ml) with or without endothelium, whereas methylene blue (5x10(-5) M) could increase the contraction induced by veratrine (1x10(-4) g/ml). Treatment with veratrine (1x10(-4) g/ml) could decrease the tension of contraction induced by norepinephrine (1x10(-6) M) or phenylephrine (1x10(-4) M). The inhibition of veratrine on norepinephrine-induced contraction was potentiated by L-arginine (1x10(-4) M) and reversed by L-NAME (1x10(-5) M). Paeoniflorin (1x10(-4) M) could decrease the tension of contraction induced by veratrine (1x10(-4) g/ml) and methylene blue (5x10(-5) M). The inhibition of paeoniflorin on veratrine was more potent on rat isolated aorta with endothelium than without endothelium. Ryanodine (1x10(-5) M) and Ca2+ -free medium could inhibit methylene blue-induced contraction. From the above results, the relaxation of veratrine on the norepinephrine-induced contraction might be related to the increase of NO and cGMP. The contraction of veratrine on the isolated rat aorta was via the increase of intracellular calcium which was inhibited by paeoniflorin.

Evidence of hydroxyl free radical generation by calcium overload in rat myocardium

Although calcium (Ca2+) is important in cardiac dysfunction and has also been reported as a source of oxidative toxicity, the connection between Ca2+ overload and oxygen free radicals in the myocardium is not clear. We have investigated whether Ca2+ overload generates hydroxyl free radicals in rat ventricle. HPLC with electrochemical detection was used to measure the levels of 2,3- and 2,5-dihydroxybenzoic acid (DHBA) formed when the hydroxyl free radical reacts with salicylate. Ringer's solution containing salicylic acid (0.5 nmol microL-1 min-1) was infused through a microdialysis probe in the region of the left anterior descending coronary artery of the rat ventricle. A positive linear correlation was obtained between Ca2+ and hydroxyl free radical formation trapped as 2,3-DHBA (r2 = 0.976) and 2,5-DHBA (r2 = 0.982) in the myocardial dialysate. The administration of ouabain (1 mg kg-1, i.v.), a Ca2+ elevator, into the femoral vein significantly increased the level of 2,3- and 2,5-DHBA. These results indicate that Ca2+ overload generates hydroxyl free radicals in rat heart.

HOE 694 affords protection versus veratrine contractures in rat atria by Na+ channel blockade

We examined the effects of the benzoylguanidine derivative HOE 694, an inhibitor of Na(+)-H+ exchange, against veratrine-induced diastolic contractures and action potentials recorded in rat isolated left atria. Concentration-dependent protective effects against veratrine-contractures, in the absence of negative inotropic responses, were observed with HOE 694 (IC50 = 20.1(7.6-27.0) microM, n = 24) and with the chemically related amiloride derivatives DMA, EIPA, HMA and MIA, but not with amiloride itself. Concomitant Na(+)-H+ exchange blockade by a high concentration of amiloride (100 microM) failed to significantly modify the protective effects of HOE 694. HOE 694 decreased Vmax significantly at 10 microM (166.7 +/- 21 vs 154.7 +/- 20 V/s, P < 0.05, n = 6) without any effect on resting potential or action potential duration. High concentrations (100 microM) of HOE 694 further decreased Vmax and increased action potential duration. The protective effects of HOE 694 were compared with three of the class 1 antiarrhythmic agents, quinidine, lidocaine and flecainide against veratrine contracture. These Na+ channel blockers exerted protective effects in the same range of concentrations as HOE 694. Our findings demonstrate that HOE 694 prevents veratrine contractures at concentrations which presumably affect Na(+)-H+ exchange. However, the mechanism by which HOE 694 affords protection is apparently mediated by class 1-type Na+ channel blockade.

The effect of flunarizine and ryanodine on acquired torsades de pointes arrhythmias in the intact canine heart

INTRODUCTION

Ryanodine, a specific blocker of the Ca2+ release channel of the sarcoplasmic reticulum, and flunarizine, a [Ca2+]i overload blocker, possess antiarrhythmic effects against delayed afterdepolarizations (DADs) and DAD-dependent arrhythmias. In vitro controversy exists about their effect on early after-depolarizations (EADs): no effect was reported on cesium-induced EADs, while ryanodine did prevent EADs induced by isoproterenol. To study the possible role of intracellular Ca2+ overload in acquired EAD-dependent torsades de pointes (TdP) arrhythmias, we tested the effects of flunarizine and ryanodine in our animal model of TdP.

METHODS AND RESULTS

Anaesthetized dogs with chronic AV block received d-sotalol or almokalant followed by pacing. A subset of dogs with reproducible TdP (> or = 3 times) were selected to receive flunarizine (2 mg/kg per 2 min) or ryanodine (10 micrograms/kg per 10 min). After d-sotalol, TdP was induced at a mean cycle length of the idioventricular rhythm (CL-IVR) of 2070 +/- 635 msec and a QT(U) interval of 535 +/- 65 msec. Induction of TdP was prevented by flunarizine in all experiments (8/8): electrophysiologically this was associated with a decrease in CL-IVR, QT(U), and QTc interval (390 +/- 100 to 320 +/- 45, P < 0.05). Ryanodine prevented TdP induction in 4 of 5 experiments and decreased the CL-IVR, QT(U), and the QTc interval from 385 +/- 75 to 320 +/- 20 msec (P < 0.05). Both drugs also suppressed the almokalant-induced EADs and related ectopic activity. This antiarrhythmic action corresponded with the inability to reinduce TdP by pacing.

CONCLUSIONS

Blockade of the Ca2+ release channel of the sarcoplasmic reticulum by ryanodine or the reduction of [Ca2+]i overload by flunarizine prevents induction of EAD-dependent acquired TdP arrhythmias, suggesting a role for [Ca2+]i overload in acquired TdP.

Prevention by specific chemical classes of alpha 1-adrenoceptor antagonists of veratrine-contractures in rat left atria independently of alpha 1-adrenoceptor blockade

1. The putative direct protective effects of a series of chemically diverse alpha 1-adrenoceptor antagonists against veratrine alkaloid-induced tetanic contractures in rat isolated left atria have been investigated. 2. Atria were mounted in organ baths containing normal, oxygenated physiological salt solution (20 ml, pH 7.4), for isometric tension recording. Atria were electrically driven at 4 Hz and were maintained at 34 degrees C. Veratrine (100 micrograms ml-1) was applied to the atria to elicit tetanic (diastolic) contracture. 3. Concentration-dependent protective effects against veratrine-contractures, in the absence of negative inotropic responses, were observed with the quinazoline congeners, prazosin and doxazosin and with the benzodioxane-related compounds, WB 4101 and its thio analogue, benoxathian. IC50 concentrations and apparent Hill coefficients of all four drugs ranged from 0.27 to 0.93 microM, and from 0.86 to 1.09, respectively, and are consistent with interaction at a single site. 4. In contrast, no protective activity versus veratrine-contractures was observed with corynanthine, 5-methyl-urapidil, phenoxybenzamine, phentolamine or chloroethylclonidine (10 microM). 5. Contractures were prevented by prazosin at concentrations 2-3 log units higher than those which antagonized methoxamine-evoked inotropic responses. In addition, concomitant alpha 1-adrenoceptor occupancy by high concentrations of methoxamine (100 microM), phentolamine (10 microM, inactive per se in preventing contracture), or both drugs together, failed, in each case, to modify significantly the protective effects of prazosin or WB 4101 against veratrine-contractures. 6. Our findings demonstrate that alpha 1-adrenoceptor antagonists which prevent veratrine-contractures belong to specific chemical classes of the quinazoline- and benzodioxane-type. The mechanism by which these drugs afford protection is apparently independent of an interaction with defined alpha 1-adrenoceptors.

Veratrine-induced tetanic contracture of the rat isolated left atrium. Evidence for novel direct protective effects of prazosin and WB4101

An investigation has been made of the putative direct myocardial protective effects of the alpha 1-adrenoceptor antagonists, prazosin and WB4101, against tetanic contractures of rat isolated left atria following modified Na+ channel function and consequent Ca2+ loading elicited by veratrine. Veratrine evoked concentration-dependent, reversible, tetanic contractures which were critically dependent upon the external Ca2+ concentration. Tetrodotoxin (TTX), prazosin, WB 4101 and R 56865 (0.1-10 microM) prevented tetanic contracture elicited by veratrine (100 micrograms/ml) at concentrations which were significantly lower than those which decreased active tension development. The apparent Hill coefficients (nH) obtained for TTX, prazosin, WB 4101 and R 56865 were comparable (range 0.79-0.93), and are consistent with a single site of action. In contrast, the class 1 antiarrhythmic agents, quinidine and lidocaine, elicited no significant inhibition of veratrine-induced contracture at 30 microM, but almost completely abolished the contractures at 100 microM. The nH values for quinidine and lidocaine were found to be significantly greater than unity (3.1 and 2.6, respectively). The L-type Ca2+ channel blockers, diltiazem, nicardipine, nifedipine and verapamil only weakly prevented tetanic contracture, whilst markedly, and concentration-dependently, decreasing active tension development. Neither atropine (10 microM) nor propranolol (1 microM) significantly modified either veratrine-induced contractures or active tension development. In conclusion, evidence is presented of novel, direct protective effects of prazosin and WB 4101 against tetanic contracture following modified Na+ channel function and Ca2+ loading provoked by veratrine. The precise mechanisms involved are unclear at present, but appear to be distinct from blockade of atrial alpha 1-adrenoceptors or L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.

Drugs acting on calcium channels: potential treatment for ischaemic stroke

Calcium subserves a ubiquitous role in the organisation of cell function. Ca2+ channels which control influx may be modified in disease states. Animal models of cerebral ischaemia do present some problems when investigating potential therapies involving Ca2+ channels. However, it is important not to be too rigid in searching for models which exactly mimic the human disease state, when even the best experimental approaches fall short of such an ideal. There are differences between different classes of calcium entry blocking drugs with regard to their activity on Ca2+ channels and transmembrane Ca2+ movement. Some calcium antagonists may also affect ion channels other than Ca2+, and this potential is exemplified by the novel ion channel modulator RS-87476, which affords experimental neurocytoprotection. Limitation of intracellular Na+ influx during ischaemia-induced depolarization may be useful.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Protective effect of bepridil against veratrine-induced contracture in rat atria

In isolated stimulated rat atria, superfusion with veratrine caused a marked contracture (VIC) which was absent in calcium-free medium and which was inhibited by tetrodotoxin (IC50VIC of 1.38 microM). Lowering the extracellular calcium concentration from 2.5 to 0.5 or 0.1 mM reduced the veratrine-induced contracture and delayed its onset. Superfusion of bepridil (1-10 microM) for 60 min before and during veratrine exposure markedly slowed the onset of contracture, reduced the maximum response (IC50VIC = 2.11 microM) and facilitated recovery upon washout of the alkaloid. The direct negative inotropic effect (NIE) of bepridil (IC50NIE = 10.96 microM) resulted in an VIC/NIE ratio of 5.19 for this drug. The protective effects of bepridil were rate-independent and were not modified by the presence of atropine (1.4 microM) and propranolol (0.3 microM) in the medium. Diltiazem, verapamil and nifedipine only reduced veratrine-induced contracture at concentrations much higher than those producing a negative inotropic effect, giving them negative NIE/VIC ratios of 0.31, 0.08 and 0.08 respectively. Like bepridil, flunarizine had a positive NIE/VIC ratio (15.87, IC50VIC = 3.71 microM). The lack of effect of the quaternary derivative of bepridil CERM 11888 indicated that intracellular sites of action may be involved in the activity of bepridil on veratrine-induced contracture. Given that veratrine-induced changes may mimic some of the pathological changes occurring in ischaemia, the results suggest that bepridil and flunarizine may be more effective than L-type, slow calcium ion-channel blockers in protecting against calcium overload during ischaemia and reperfusion injury.

Veratridine-induced intoxication in the isolated left atrium of the rat: effects of some anti-ischemic compounds

Veratridine-induced Na+ and Ca2+ uptake was used as a simulation of ischemia-induced Na+ and Ca2+ uptake. Therefore, electrically driven (1 Hz) isolated left atria of the rat were intoxicated with veratridine and the 45Ca2+ uptake was determined. Veratridine (10(-4) mol/l) increased the 45Ca2+ uptake from 575 +/- 13 to 2320 +/- 86 dpm/mg ww (n = 20). The total tissue content of 45Ca was elevated from 4328 +/- 132 to 5136 +/- 303 dpm/mg ww (n = 13). The veratridine-induced 45Ca2+ uptake was completely suppressed by tetrodotoxin (10(-7) and 10(-6) mol/l), whereas amiloride (6.10(-6) mol/l) and phentolamine (10(-6) and 10(-5) mol/l) exhibited no effect on the veratridine-induced 45Ca2+ uptake. Nifedipine (10(-7) and 10(-6) mol/l) was ineffective on veratridine-induced 45Ca2+ uptake. Verapamil (10(-5) mol/l) suppressed the veratridine-induced 45Ca2+ uptake, but the 45Ca2+ uptake in the absence of veratridine was also suppressed by verapamil (10(-6) and 10(-5) mol/l). The novel anti-ischemic compounds R 56865 (10(-8)-10(-5) mol/l) and R 59494 (10(-8)-10(-5) mol/l) totally abolished veratridine-induced 45Ca2+ uptake. It is speculated that Ca2+ enters the cell via a Na+ channel which changes its selectivity upon veratridine treatment. Consequently, R 56865 and R 59494 could display their protective effect by either inhibiting the modified Na+ channel or preventing the transition of the normal Na+ channel to its altered state.(ABSTRACT TRUNCATED AT 250 WORDS)

RS 30026: a potent and effective calcium channel agonist

1. A series of dihydropyridine derivatives has been evaluated for calcium channel agonist activity using reversal of nisoldipine-induced inhibition of beating of aggregates of embryonic chick myocytes. This test appears to be specific for calcium channel agonists since isoprenaline and cardiac glycosides are inactive. 2. RS 30026 was the most potent of the series, was significantly more potent than CGP 28392 and of similar potency to Bay K 8644 (pEC50 = 7.45, 6.16 and 7.20, respectively). RS 30026 increased edge movement of individual aggregates, in the absence of nisoldipine, by 50% at 2 nM. 3. Compounds were also evaluated for their effects on guinea-pig papillary muscle and porcine coronary artery rings. RS 30026 displayed positive inotropism at concentrations between 10(-9) and 10(-6) M (pEC200 = 8.21), but was a much more powerful inotrope than Bay K 8644, increasing contractility to 1300% of control at 10(-6) M (compared to 350% of control for Bay K 8644). RS 30026 caused vasoconstriction at concentrations between 10(-10) and 10(-7) M. 4. Calcium channel currents in single embryonic chick myocytes were recorded by whole-cell voltage clamp techniques. RS 30026 (100 nM-500 nM) produced large increases in peak current amplitude and shifted the voltage for threshold and maximal currents to more negative values. RS 30026 (500 nM) also produced large increases in the inward tail currents evoked upon repolarization. The effects of Bay K 8644 (50 and 500 nM) were much less marked. 5. Analysis of the activation characteristics of currents showed parallel shifts in the activation curve to more negative potentials in the presence of 50 nm Bay K 8644, with a much smaller shift in the presence of 500nm Bay K 8644. RS 30026 (100 and 500nM) caused concentration-dependent shifts in the activation of the calcium channel currents with an increase of the slope of the curve. 6. RS 30026 appears to be the most potent and effective calcium channel agonist described to date.