The inhibition of Na-dependent Ca uptake by verapamil in synaptic plasma membrane vesicles

The Na+/Ca2+ exchanger inhibitor, KB-R7943, blocks a nonselective cation channel implicated in chemosensory transduction

The mechanism(s) of olfactory transduction in invertebrates remains to be fully understood. In lobster olfactory receptor neurons (ORNs), a nonselective sodium-gated cation (SGC) channel, a presumptive transient receptor potential (TRP)C channel homolog, plays a crucial role in olfactory transduction, at least in part by amplifying the primary transduction current. To better determine the functional role of the channel, it is important to selectively block the channel independently of other elements of the transduction cascade, causing us to search for specific pharmacological blockers of the SGC channel. Given evidence that the Na(+)/Ca(2+) exchange inhibitor, KB-R7943, blocks mammalian TRPC channels, we studied this probe as a potential blocker of the lobster SGC channel. KB-R7943 reversibly blocked the SGC current in both inside- and outside-out patch recordings in a dose- and voltage-dependent manner. KB-R7943 decreased the channel open probability without changing single channel amplitude. KB-R7943 also reversibly and in a dose-dependent manner inhibited both the odorant-evoked discharge of lobster ORNs and the odorant-evoked whole cell current. Our findings strongly imply that KB-R7943 potently blocks the lobster SGC channel and likely does so directly and not through its ability to block the Na(+)/Ca(2+) exchanger.

Mechanism of ouabain-induced contractions in guinea-pig tracheal rings

The aim of the present study was to analyse the mechanism that underlies the force development induced by ouabain (ED(100) = 100 micromol/L) in guinea-pig tracheal rings. The dose-response curve showed that concentrations of ouabain above 100 micromol/L evoked smaller contractions. Ouabain, at 100 micromol/L, produced two long-lasting consecutive transient contractions. The peak of the first contraction was 750 +/- 75 mg, whereas the peak of the second contraction was 280 +/- 46 mg. Both contractions induced by ouabain were dependent on extracellular Ca(2+). Consistent with this, verapamil (10 micromol/L) inhibited the first and second contractions by 77 and 59%, respectively. 3,4-Dichlorobenzamil (20 micromol/L) inhibited the first and second contractions by 68 and 97%, respectively. Simultaneous exposure to 15 mmol/L sodium solution and 100 micromol/L ouabain evoked only one transient contraction, larger (987 +/- 135 mg) than either of the ouabain-induced contractions. Inhibition of the sarcoendoplasmic reticulum Ca-ATPase with cyclopiazonic acid potentiated the first and second ouabain-induced contractions by 47 and 300%, respectively. Atropine (1 micromol/L) inhibited the first and second contractions by 44 and 76%, respectively. In conclusion, the results of the present study are relevant to the understanding of the mechanisms by which ouabain (100 micromol/L) contracts guinea-pig tracheal rings. At the muscular level, oubain induces Ca(2+) influx through L-type Ca(2+) channels and the reverse mode of the sodium-calcium exchanger. At the nerve terminals, ouabain promotes the release of acetylcholine secondary to the increase in Ca(2+) influx mediated by the reverse mode of the sodium-calcium exchanger.

The potential of Na+/Ca2+ exchange blockers in the treatment of cardiac disease

The Na(+)/Ca(2+) exchanger (NCX), a surface membrane antiporter, is the primary pathway for Ca(2+) efflux from the cardiac cell and a determinant of both the electrical and contractile state of the heart. Enhanced expression of NCX has recently been recognised as one of the molecular mechanisms that contributes to reduced Ca(2+) release, impaired contractility and an increased risk of arrhythmias during the development of cardiac hypertrophy and failure. The NCX has also been implicated in the mechanism of arrhythmias and cellular injury associated with ischaemia and reperfusion. Hence, NCX blockade represents a potential therapeutic strategy for treating cardiac disease, however, its reversibility and electrogenic properties must be taken into consideration when predicting the outcome. NCX inhibition has been demonstrated to be protective against ischaemic injury and to have a positive inotropic and antiarrhythmic effect in failing heart cells. However, progress has been impaired by the absence of clinically useful agents. Two drugs, KB-R7943 and SEA-0400, have been developed as NCX blockers but both lack specificity. Selective peptide inhibitors have been well characterised but are active only when delivered to the intracellular space. Gene therapy strategies may circumvent the latter problem in the future. This review discusses the effects of NCX blockade, supporting its potential as a new cardiovascular therapeutic strategy.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Evoked GABA release is not mediated by N-type VDCC in the frontal cortex of awake rats: effects of neomycin

The purpose of the present study was to analyze the Ca2+ channel involved in GABA release under resting and K(+)-evoked conditions in vivo. We used microdialysis to investigate the effects of the voltage-dependent calcium channel (VDCC) blockers neomycin, kanamycin, and omega-conotoxin GVIA, and the voltage-dependent Na+ channel blocker tetrodotoxin, in the frontal cortex of awake rats. The GABA content in frontal dialysates was analyzed by high performance liquid chromatography coupled to fluorescence detection. Basal GABA release was kanamycin, omega-conotoxin, and tetrodotoxin resistant, whereas neomycin induced a significant increase from the basal level. The K(+)-evoked release of GABA was kanamycin and omega-conotoxin resistant, but tetrodotoxin sensitive. The effects of neomycin were masked by the action of this drug on basal release. These results suggest that neomycin may affect GABA release in the frontal cortex through a mechanism independent of VDCC. In addition, the K(+)-evoked release of GABA in this cortical area was not mediated by the N-type voltage-dependent calcium channels, but was dependent on neural activity or TTX sensitive.

Potassium-dependent calcium influx in acutely isolated hippocampal astrocytes

Potassium depolarization can increase the intracellular ionized calcium concentration ([Ca2+]i) of cultured astrocytes, but it is not known if astrocytes that have matured in the intact CNS also exhibit voltage-dependent [Ca2+]i signalling. To address this issue, fluorometric measurements of [Ca2+]i were obtained from astrocytes acutely isolated from young adult rat hippocampus. In control artificial cerebrospinal fluid containing 5 mM [K+]o, average resting [Ca2+]i was 195 nM. Elevation of [K+]o to 50 mM caused [Ca2+]i to increase 150 nM to 1 microM above resting levels. The threshold [K+]o necessary to evoke an elevation in [Ca2+]i was 20-25 mM, and the magnitude of the [Ca2+]i signal grew progressively with increasing [K+]o (up to 50 mM). These [Ca2+]i increases were blocked completely by removal of external Ca2+, and markedly suppressed by the calcium channel blockers verapamil (30 microM and greater) and Co2+ (1 mM). Neither reversal of Na(+)-Ca2+ exchange, nor Ca(2+)-activated Ca2+ release, nor Ca2+ influx through stretch-activated channels contributed to the [Ca2+]i increase. These results suggest that [K+]o-evoked [Ca2+]i signals are mediated by influx through voltage-gated calcium channels. In contrast to results from cultured astrocytes and acutely isolated neurons, these [Ca2+]i increases were insensitive to dihydropyridine compounds. We conclude that increases in interstitial [K+], observed in situ during several pathological conditions, trigger voltage-dependent [Ca2+]i signals in astroglial cells. This may constitute an important form of neuron-to-glial communication.

Intracellular free Ca2+, Na+, and H+ concentrations in the isolated perfused rat heart during the Ca2+ paradox

Enzyme release from the normothermic perfused rat heart was determined to evaluate myocardial cell damage during the perfusion sequence of the Ca2+ Ca2+ paradox. In addition sarcosolic free Ca2+ and monovalent cation concentrations were measured using ion-selective microelectrodes (ISMEs). A reduction of the extracellular Ca2+ concentration, [Ca2+]e, from 1.0 mmol/l to 0.3 or 0.1 mmol/l, respectively, during the re-perfusion period markedly decreased the rate of enzyme release. Slow Ca2+ channel blockers were less (verapamil) or not at all (nifedipine) effective in providing protection. The sarcosolic free Ca2+ and Na+ concentrations, [Ca2+]i and [Na+]i, were significantly (P < 0.01) elevated during Ca(2+)-free perfusion, [H+]i was not significantly changed, while the membrane potential became continuously more positive. Addition of verapamil to the perfusion medium increased [Na+]i, but did not further increase [Ca2+]i. The critical [Ca2+]i for cell damage was between 12 and 18 mumol/l. It could be demonstrated, that Ca2+ entry during re-perfusion via Na/Ca exchange is thermodynamically unlikely to elevate [Ca2+]i to critical values. It is concluded therefore that the predisposition of the rat heart for the Ca2+ paradox is brought about by membrane leaks, which form during Ca(2+)-free perfusion and that Ca2+ influx into the sarcosol proceeds mainly through these leaks.

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

Ca2+ antagonists do not protect isolated perfused rat hepatocytes from anoxic injury

Ca2+ antagonists were studied during anoxia in perfused isolated rat hepatocytes. Cytosolic free calcium (Ca2+i) was measured with aequorin. Anoxia was induced for 2 h by saturating the perfusate with 95% N2/5+ CO2. Anoxia increased Ca2+i in two distinct phases reaching a maximum of 1.5 microM. The increase in Ca2+i was caused by Ca2+ influx from the extracellular fluids because the main Ca2+i surge was totally abolished in Ca(2+)-free media. LDH release increased 6-fold during the second hour of anoxia, but when Ca2+ was removed from the perfusate during the anoxic period, LDH rose only 2.7-fold. Ca2+ antagonists (10(-7) to 10(-5) M) did not prevent the increase in Ca2+i and the rise in LDH release. On the contrary, high concentrations (10(-6) and 10(-5) M) of the blockers nifedipine and diltiazem significantly increased anoxic cell injury. The observation that the increase in LDH and the rise in Ca2+i were not suppressed by Ca2+ antagonists suggests that (i) Ca2+ antagonists protect the whole liver from anoxic injury by acting on cells other than parenchymal cells; (ii) the influx of Ca2+ responsible for the massive increase in hepatocyte Ca2+i evoked by anoxia did not take place through voltage-sensitive Ca2+ channels but must have occurred via the Na(+)-Ca2+ antiporter operating in the reverse mode (Ca2+ influx vs. Na+ efflux), and (iii) high concentrations of Ca2+ antagonists may be deleterious to the parenchymal cells of the liver.

Perturbation effect of eight calcium channel blockers on liposomal membranes prepared from rat brain total lipids

EPR spectroscopy of phosphatidylcholine or stearic acid labeled at the doxyl group at the 16-carbon position was used to compare the perturbation effect of eight calcium channel blockers (CB) on overall dynamics/disorder of the hydrophobic part of liposome membranes prepared from rat brain total lipids at the drug/lipid molar ratio of 1/2. Nifedipine (NIF), nimodipine, niludipine and nitrendipine had a minor effect on the dynamics/disorder of the liposome membranes, whereas the disordering effect of verapamil (VER), mepamil, gallopamil and diltiazem was more pronounced. Concentration dependence of the overall disordering effect of VER on liposomal membranes was found at the VER/lipid ratio greater than 0.02 and for the tranquilizer thioridazine greater than 0.005. VER exerted a disordering effect at the hydrophobic part of synaptosomal membranes at concentrations greater than or equal to 0.32 mmol/l, whereas NIF did not exhibit a disordering effect even at concentrations of 10-20 mmol/l.

Na(+)-Ca2+ antiporter activity of rat hepatocytes. Effect of adrenalectomy on Ca2+ uptake and release from plasma membrane vesicles

The presence and mode of Na(+)-Ca2+ antiporter activity were studied in hepatocytes isolated from sham-operated or adrenalectomized rats and in inside-out plasma membrane vesicles isolated from rat liver. Decreasing extracellular Na+ (Na+o) immediately increased cytosolic free calcium (Ca2+i). The rise in Ca2+i was proportional to the reduction in Na+o and was caused by an increased calcium influx, presumably on the Na(+)-Ca2+ antiporter operating in the reverse mode. Perfusing the cells with Ca(2+)-free media stimulated Ca2+ efflux and decreased Ca2+i, an effect dependent on Na+o. This suggests an activation of the forward mode of Na(+)-Ca2+ exchange. There was little difference in these parameters between sham and adx groups. In contrast, steady-state calcium uptake by inside-out plasma membrane vesicles was inhibited 40% after adrenalectomy. The decreased calcium uptake was not caused by a deficiency in the ATP-dependent Ca2+ pump, whose Km and Vmax were unaffected by adrenalectomy, but by an Na(+)-dependent leak from the vesicles. Ca2+ efflux was proportional to the extravesicular Na+ concentration, suggesting that the calcium leak may take place on a Na(+)-Ca2+ antiporter. This Na(+)-dependent calcium efflux was significantly increased in vesicles prepared from adx rat livers. These results suggest that hepatocytes have functional Na(+)-Ca2+ antiporters that can operate in both forward and reverse modes. Under normal conditions, the Na(+)-Ca2+ antiporter apparently operates in the reverse mode as a Ca2+ influx pathway. The increase in Na(+)-dependent Ca2+ efflux evoked by adrenalectomy in plasma membrane vesicles could explain the recent results we obtained in hepatocytes isolated from adx rats, showing increased calcium influx, increased Ca2+i, increased intracellular calcium sequestration, and increased plasmalemmal calcium cycling.

Cromakalim (BRL 34915) counteracts the epileptiform activity elicited by diltiazem and verapamil in rats

1. The effects of BRL 34915 (cromakalim), a potassium channel opener, have been tested on the epileptiform activity elicited by high dose/concentrations of some calcium antagonists in in vivo (diltiazem) and in vitro (diltiazem and verapamil) experiments in rats. 2. Diltiazem (150-300 mg kg-1, i.p.) induced behavioural and electroencephalographic (EEG) seizures that were completely prevented by cromakalim (10 nmol/10 microliters, i.c.v.). Whereas, pentobarbitone (5-10 mg kg-1, i.p.) only prevented the behavioural component of the seizures. 3. In hippocampal slices, verapamil (1.5-2.0 mM) produced, within 30-60 min of perfusion, a CA1 epileptiform bursting in 80% of the experiments. This epileptiform activity was prevented by the cromakalim concentration (50 microM) that did not affect the control CA1 synaptic transmission per se. Pentobarbitone also prevented verapamil-induced epileptiform bursting only at the concentration (100 microM) that also reduced control CA1 synaptic transmission. 4. Diltiazem (1.5 mM) produced a biphasic excitatory-depressant effect within 60 min of perfusion. A CA1 epileptiform bursting appeared in 100% of the experiments within 30 min of perfusion. These excitatory effects were followed by a depression phase, characterized by a reduction of the magnitude of CA1 excitatory postsynaptic potentials (e.p.s.ps) and population spike. 5. The diltiazem-induced epileptiform bursting was prevented by cromakalim at a concentration (50 microM) that did not affect the control CA1 synaptic transmission per se. Pentobarbitone also prevented the diltiazem-induced epileptiform bursting only at a concentration (100 microM) that also reduced the control CA1 synaptic transmission. Both cromakalim (50 microM) and pentobarbitone (100 microM) failed to affect the depressant effects of diltiazem on CA1 hippocampal area. On the contrary, high (3.3mM) calcium solutions prevented both the excitatory and the depressant effects of 1.5 mm diltiazem within 60 min.6. These data indicate an involvement of potassium currents in the epileptiform activity elicited by high doses of diltiazem and verapamil.

Na+ influx-induced decrease of (Na+ + K+)-ATPase activity in rat brain slices: role of Ca2+

Treatment of rat brain slices with veratrine and monensin decreased (Na+ + K+)-ATPase activity in the membranes in a dose-dependent manner. The effect of monensin, like that of veratrine, was accompanied by a decrease of maximal binding sites for ouabain. The inhibitory effect of monensin on the enzyme activity was dependent on external Ca2+ at low concentrations, but not at a high concentration. The decreased enzyme activity induced by monensin was restored by subsequent incubation of the slices in a Ca(2+)-free medium containing 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. The effect of monensin at a low concentration on enzyme activity was antagonized by amiloride (1 mM), bepridil (5 microM), quinacrine (30 microM) or verapamil (30 microM), but not by nifedipine (1 microM) or omega-conotoxin (1 microM). Furthermore, the inhibitory effect of monensin at a high concentration under Ca(2+)-free conditions was blocked by BAPTA-AM (30 microM) and by bepridil (100 microM) or diazepam (500 microM), inhibitors of mitochondrial Na(+)-Ca2+ exchange. Inhibitors of calmodulin, protein kinase C, phospholipase A2 and calpain did not affect the monensin-induced decrease of enzyme activity. Dithiothreitol (10 mM) blocked the effect of monensin on enzyme activity but did not affect the ionophore-induced influx of Ca2+ in the slices.

The inhibitory effect of Mn2+ on the ATP-dependent Ca2+ pump in rat brain synaptic plasma membrane vesicles

Synaptic plasma membrane (SPM) vesicles take up Ca2+ when both ATP and Mg2+ are added to the reaction medium. Maximal ATP-dependent Ca2+ uptake is obtained with between 3 and 5 mM Mg2+. Higher [Mg2+] results in a slight decrease in the ATP-dependent Ca2+ influx, which at 10 mM Mg2+ is equal to 70% of the maximal uptake. [Mn2+] up to 0.3 mM, can support ATP-dependent Ca2+ uptake as efficiently as equimolar [Mg2+]. Maximal ATP-dependent Ca2+ uptake in the presence of Mn2+ is obtained between 1 and 2 mM but its extent is only 70% of the maximal uptake obtained in the presence of Mg2+. As the [Mn2+] increases the ATP-dependent Ca2+ uptake decreases and at 10 mM Mn2+ it is only 40% of that obtained in the presence of Mg2+. The reduction in ATP-dependent Ca2+ uptake in the presence of Mn2+ is obtained also when Mg2+ is present in the reaction medium. The effect is independent of the [Mg2+] used and depends on the [Mn2+] alone. Studying the relationship between [Mn2+] and [Ca2+] revealed three patterns: at 0.3 mM Mn2+ this ion could replace Mg2+ over the entire [Ca2+] range tested, 5-200 microM, supporting ATP-dependent Ca2+ uptake. When the [Mn2+] was increased to 1.8 mM, it exhibited a competitive behaviour with Ca2+ which resulted in an increase in apparent Km to Ca2+ of the SPM Ca2+ pump from 10.69 microM (SD = 3.49) to 28.88 microM (SD = 21.08). Four millimolar Mn2+ inhibited ATP-dependent Ca2+ uptake to 50% of that obtained in equimolar Mg2+ over the entire [Ca2+], 5-200 microM, tested. No transport of 54Mn2+ by the SPM Ca2+ pump could be detected. Inclusion of Mn2+ in the reaction medium led to excessive phosphorylation of SPM proteins. The excessive levels of phosphorylation persisted also when Ca2+ and/or Mg2+ were present in the reaction media.

Inhibition of K(+)-evoked [3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K(+)-evoked [3H]D-aspartate release from rat hippocampal slices and the K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K+ caused a concentration-dependent release of [3H]D-aspartate that was approximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, omega-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K(+)-evoked release of [3H]D-aspartate. omega-Conotoxin GVIA and nitrendipine caused a relatively small (20-30%) inhibition of K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K(+)-evoked [3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K(+)-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K(+)-evoked release of [3H]D-aspartate with IC50 values of 30, 100 and 120 microM, respectively. The K(+)-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 microM, respectively). At 300 microM, neither verapamil, diltiazem nor dextromethorphan inhibited [3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison between the cardiac effects induced by muzolimine and furosemide in guinea-pig atria

Muzolimine (10-500 microM) induced a concentration-dependent reduction of both the contractile force and frequency in spontaneously beating atria and in electrically driven left atrium from reserpine-treated guinea pigs. This negative inotropic response was unaffected by the addition of atropine to the perfusion fluid, and it was highly sensitive to changes in external Ca2+ concentration. Both in spontaneously beating and in electrically driven atrium, muzolimine (50-400 microM) antagonized, in an apparently competitive manner, the increase in contractile force induced by cumulative addition of CaCl2 (0.68-9.59 mM) to the bathing fluid. Muzolimine (50-100 microM) reduced the inotropic response to low (5-30 nM), but not high (50-100 nM) concentrations of Bay K 8644, a calcium-channel agonist. The inotropic effects of 8-phenyltheophylline and of ouabain were antagonized by muzolimine (10-100 microM) in a noncompetitive manner, while the response to noradrenaline was not altered. Similar to muzolimine, verapamil at a concentration suitable to block calcium channels inhibited, in a noncompetitive way, the inotropic effect induced by 8-phenyltheophylline and by ouabain without altering the contractile response to noradrenaline. Furosemide (10 and 100 microM) did not influence the contractile force or the frequency of spontaneously beating atria, nor the inotropic effect induced by CaCl2, 8-phenyltheophylline, ouabain, or noradrenaline. These results indicate that the influence of muzolimine on guinea-pig atria originates from an inhibition of Ca2+ influx into cardiac cells and that furosemide does not mimic the effect of muzolimine at this level.

Endogenous release of gamma-aminobutyric acid from the medial preoptic area measured by microdialysis in the anaesthetised rat

The characteristics of gamma-aminobutyric acid (GABA) release as monitored by microdialysis have been investigated in the chloral hydrate anaesthetised rat. The high outflow of GABA following insertion of the microdialysis probe (membrane 2 mm in length, 0.5 mm in diameter) into the medial preoptic area was found to decline to a stable baseline level after 2 h. After this time, perfusion with a medium containing 100 mM potassium ions evoked a 56-fold increase in GABA outflow. The addition of the calcium channel blocker verapamil (100 microM) to the perfusion medium induced significant 25 and 50% reductions in basal and potassium-stimulated GABA outflow, respectively. In the same animals, verapamil caused an 80% decrease in potassium-stimulated noradrenaline outflow. The glutamic acid decarboxylase inhibitors 3-mercaptopropionic acid and L-allylglycine added to the perfusion medium at a concentration of 10 mM reduced basal GABA release by approximately 50% with different time-courses of action. Ethanolamine-O-sulfate, a GABA-transaminase inhibitor, induced significant increases in basal GABA outflow 90 min after inclusion in the perfusion medium. These results demonstrate that microdialysis is a suitable technique with which to monitor extracellular levels of GABA and provide in vivo data on GABA release and degradation mechanisms.

Action of antiestrogens on the (Ca2+ + Mg2+)-ATPase and Na+/Ca2+ exchange of brain cortex membranes

The effect of tamoxifen (TAM) and other antiestrogens on the Ca2+ transport activity of synaptic plasma membranes (SPM) and microsomal membranes isolated from sheep brain cortex was investigated. The maximal (Ca2+ + Mg2+)-ATPase activity of SPM, which is reached at a pCa of about 6.0-6.5, is decreased by about 30% in the presence of 50 microM TAM, whereas the (Ca2+ + Mg2+)-ATPase activity of microsomes, which is maximal at a pCa of about 5.0, is decreased by about 90% by 50 microM TAM. In parallel experiments, we observed that the ATP-dependent Ca2+ uptake is also affected differently by TAM in the two membrane preparations. We found that 50 microM TAM inhibits SPM Ca2+ uptake by about 25-30%, whereas the ATP-dependent Ca2+ uptake by the microsomal fraction is inhibited by about 60%. No significant effect of TAM was observed on the Na+/Ca2+ exchange of either membrane system. The results indicate that TAM is a more potent inhibitor of the active, calmodulin-independent Ca2+ transport system of the intracellular membranes than of that of the plasma membranes, which is calmodulin-dependent. It appears that TAM inhibits calmodulin-mediated reactions, probably through its binding to calmodulin, as we showed previously. However, the Ca2+ transport system of microsomes, which does not depend on calmodulin, is also particularly sensitive to TAM.

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-independent GABA release from striatal slices: the role of calcium channels

We have investigated the role of Ca2+ and Ca2+ channels in the modulation of GABA release. Brain slices prepared from rat striatum were preincubated with [3H]GABA, superfused with Krebs bicarbonate buffer, and exposed to electrical field stimulation (2 Hz for 3 min). Tritium efflux was measured as an index of GABA release. Both resting and evoked efflux were greatly accelerated by deleting Ca2+ from the medium and adding EGTA (1 mM). However, when the concentration of Mg2+ in the buffer was elevated to 10 mM, no effect of the Ca2(+)-deficiency was observed on resting release and its impact on evoked overflow was diminished. Moreover, addition of verapamil (10 microM), a Ca2+ channel blocking agent, reduced evoked overflow even in the absence of external Ca2+, while 4-aminopyridine (10 microM), a K+ channel inhibitor, enhanced GABA efflux in normal buffer but had no effect in the absence of Ca2+. Finally, we have shown previously that nipecotic acid, an inhibitor of high affinity GABA transport, increases GABA overflow in normal buffer, but blocks it in Ca2(+)-free buffer. Collectively, these results suggest that Ca2+ channels may play two roles in the regulation of depolarization-induced GABA release. Firstly, these channels permit a depolarization-induced influx of Ca2+ which then promotes GABA release. In addition, these channels influence GABA release through a mechanism that does not involve external Ca2+. Although the precise nature of this latter involvement is unclear, we propose that the Ca2+ channels serve to permit an influx of Na+, which in turn promotes Ca2(+)-independent release through an influence on the high affinity GABA transport system.

Effects of diltiazem on bioenergetics, K+ gradients, and free cytosolic Ca2+ levels in rat brain synaptosomes submitted to energy metabolism inhibition and depolarization

Diltiazem was able to decrease the oxygen consumption rate and lactate production in synaptosomes isolated from rat forebrains, both under control and depolarized (40 microM veratridine) conditions, starting from a concentration of 250 microM. This effect was particularly evident when synaptosomes were depolarized by veratridine. This depolarization-counteracting action was evident also when transplasma membrane K+ diffusion potentials were measured after depolarization induced by veratridine and by rotenone with a glucose shortage. The concentrations of ATP, phosphocreatine, and creatine were less sensitive to diltiazem action. The concentration/response relationships were the same as those found for the oxygen consumption were the same as those found for the oxygen consumption rate, lactate production, and K+ diffusion potentials. The effects of 0.5 mM diltiazem in counteracting inhibition of energy metabolism induced by rotenone without glucose were no longer detectable when either Ca2+ or Na+ was absent from the incubation medium of synaptosomes. Diltiazem at the same concentrations (starting from 250 microM) was able to inhibit both the veratridine-induced and the rotenone-without-glucose-induced increase in intrasynaptosomal free Ca2+ levels evaluated with the fluorescent probe quin2. The results are discussed in view of a possible effect of diltiazem on voltage-dependent Na+ channels and the possibility of utilizing this approach for counteracting neuronal failure due to derangement of energy metabolism or hyperexcitation.

Effects of verapamil on ischaemia-induced impairment of ATP-dependent calcium extrusion in rat heart sarcolemma

1. The effects of ischaemia and reperfusion were studied on adenosine 5'-triphosphate (ATP)-dependent 45Ca2+-transport in rat heart sarcolemma vesicles. 2. The effect of verapamil, 1 mumol l-1, was studied by pretreatment of the hearts during Langendorff-perfusion and in vitro by adding the drug after isolation of the vesicles. 3. Without drug pretreatment the Ca2+-uptake appeared to be strongly reduced after 30 and after 60 min of global ischaemia, whereas after 30 min of reperfusion it was restored to slightly above the control level. 4. Verapamil pretreatment during the Langendorff perfusion significantly increased Ca2+-uptake in sarcolemma vesicles both before the onset of ischaemia and after 30 min of reperfusion, whereas no beneficial effect was found on the impaired uptake activity during the ischaemic period. 5. When tested in vitro after the isolation of the sarcolemma vesicles, verapamil only inhibited the Ca2+-uptake activity with an IC50 of 112 mumol l-1, which was increased to 250 mumol l-1 after ischaemia and reperfusion. 6. The present study indicates that pretreatment with verapamil, 1 mumol l-1, of the intact rat heart activates an ATP-dependent Ca2+ extrusion process that may contribute to decrease cellular calcium levels in control and, more importantly, in a reperfusion situation. In contrast, in vitro only a less potent inhibition of the extrusion process was found, indicating that physiological regulatory mechanisms may be altered in the vesicles.

Stimulation of D2 dopamine receptors decreases intracellular calcium levels in rat anterior pituitary cells but not striatal synaptosomes: a flow cytometric study using indo-1

An important question is whether all D2 dopamine (DA) receptors employ the same signal transduction mechanisms. Anterior pituitary cells and striatal synaptosomes, which possess pharmacologically similar D2 DA receptors, were compared with respect to the effect of D2 DA receptor stimulation on free intracellular Ca2+ levels [( Ca2+]i). Flow cytometry, in combination with either the fluorescent calcium indicator indo-1 or fluorescent voltage-sensitive dyes, was used to measure [Ca2+]i and to detect changes in membrane potential. In subpopulations of anterior pituitary cells, increases in [Ca2+]i were produced by elevated K+, veratridine, thyrotropin-releasing hormone, and BAY K 8644. These increases were blocked by nifedipine, suggesting the involvement of L-type voltage-sensitive calcium channels (VSCC's). In 10-15% of the cells, D2 agonists decreased resting [Ca2+]i, reversed stimulus-induced increases in [Ca2+]i, and caused a hyperpolarization. In striatal synaptosomes, elevated K+ and veratridine also increased [Ca2+]i. However, the K+-induced increase was eliminated if choline was substituted for Na+ in the medium, suggesting that Ca2+ entry in response to sustained K+ depolarization resulted from reversal of Na+/Ca2+ exchange. Nifedipine and verapamil inhibited K+-induced increases in [Ca2+]i only at concentrations greater than 10 microM, while omega-conotoxin had no effect. D2 agonists had no effect on resting or stimulated [Ca2+]i but did hyperpolarize 10-20% of the synaptosomes, indicating that D2 DA receptors are functional in this preparation. The ability of pituitary but not striatal D2 DA receptors to modulate [Ca2+]i may reflect the fact that the two systems differ with respect to pathways for Ca2+ influx.

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.

Voltage-sensitive calcium channels in differentiated neuroblastoma X glioma hybrid (NG108-15) cells: characterization by quin 2 fluorescence

Depolarization of differentiated neuroblastoma X glioma (NG108-15) cells with KCl (50 mM) or veratridine (50 microM) stimulated Ca2+ accumulation, was detected by quin 2 fluorescence. Intracellular Ca2+ concentrations ([Ca2+]i) were elevated about threefold from 159 +/- 7 to 595 +/- 52 nM (n = 12). Ca2+ entry evoked by high extracellular K+ concentration ([K+]o) was voltage-dependent and enhanced by the dihydropyridine agonists, BAY K 8644 and CGP 28 392, in a dose-dependent manner. CGP 28 392 was less potent and less efficacious than BAY K 8644. The (+) and (-) stereoisomers of 202-791 showed agonist and antagonist properties, respectively. (+)-202-791 was less potent, but as efficacious as BAY K 8644. In the absence of KCl, BAY K 8644 had no effect on Ca2+ entry. Voltage-sensitive calcium channel (VSCC) activity was blocked by organic Ca2+ channel antagonists (nanomolar range) both before and after KCl treatment and also by divalent metal cations (micromolar range). High [K+]o-induced Ca2+ accumulation was dependent on external Ca2+, but not on external Na+ ions ([Na]o), and was insensitive to both tetrodotoxin (3 microM) and tetraethylammonium (10 microM). In contrast, veratridine-induced Ca2+ accumulation required [Na+]o, and was blocked by tetrodotoxin, but not by nimodipine (1 microM). Veratridine-induced Ca2+ accumulation was slower (approximately 45 s), smaller in magnitude (approximately 30% of [K+]o-induced Ca2+ entry), and also enhanced by BAY K 8644 (approximately 50%). VSCC were identified in neuronal hybrid (NG108-15 and NCB-20) cells, but not in glial (C6BU-1), renal epithelial (MDCK), and human astrocytoma (1321N1) cells. NG108-15 cells differentiated with 1.0 mM dibutyryl cyclic AMP showed greater VSCC activity than undifferentiated cultures. These results suggest that cultured neural cells provide a useful system to study Ca2+ regulation via ion channels.

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

Multiple voltage-sensitive calcium channels are probably involved in endogenous GABA release from striatal neurones differentiated in primary culture

Calcium-dependent release of neurotransmitters is thought to be due to Ca2+ entry into nerve terminals, but the identities of the various voltage-sensitive Ca2+ channels (VSCC) involved in this process remain obscure. To elucidate the types of VSCCs involved in the release process, we studied the effects of various organic Ca2+ channel antagonists and agonists on the release of endogenous gamma-aminobutyric acid (GABA) from mouse striatal neurones differentiated in primary culture. Diltiazem, verapamil and methoxyverapamil (D 600) inhibited K+-evoked (30 mM) GABA release at very high concentrations (greater than 1 microM). The dihydropyridine (DHP) nifedipine, at low concentrations (0.01-1.00 microM), was able to inhibit part of the K+-evoked GABA release (25.6 +/- 7.3% inhibition at 1 microM). This is in agreement with the high affinity of nifedipine for DHP binding sites. The DHPs, BAY K 8644 (EC50 = 41 +/- 15 nM) and CGP 28.392, which possess agonist properties at VSCCs, increased the 15 mM K+-evoked GABA release. The release evoked by the combination of K+ (15 mM) and BAY K 8644 (up to 10 microM) remained smaller than the release elicited by 30 mM K+. The effect of BAY K 8644 (1 microM) was inhibited by nifedipine (IC50 0.55 +/- 0.05 microM). When Na+ ions were replaced by choline, basal and K+-evoked GABA release was significantly increased. Even in the absence of external Na+, nifedipine (1 microM) was not able to totally block the K+ effect. Moreover amiloride, a drug known to inhibit Na+/Ca2+ exchange, and tetrodotoxin (TTX), did not modify the 30 mM K+ response.(ABSTRACT TRUNCATED AT 250 WORDS)

The possible involvement of the phospholipid phase of membranes in mediating the effects of verapamil on Ca2+ transport

The effect of verapamil in a model system of A23187-induced Ca2+-uptake into liposomes was studied. This was done in order to separate the effects of verapamil on the lipid phase of membranes from its effects on membraneous proteins. In the absence of A23187, the liposomes exhibited a very low Ca2+ permeability, which did not change with addition of verapamil. Creation of a valinomycin-induced negative inside membrane potential combined with increased membrane permeability to Ca2+ (A23187), increased Ca2+-entry fivefold and more. Addition of verapamil under these conditions led to a further increase in Ca2+ entry. The negative inside polarization of the liposomes' membrane (as estimated from [3H]TPP+ uptake) was not affected by verapamil. [3H] Verapamil bound specifically to native synaptic plasma membranes with a Kd = 87.4 nM +/- 21.5 (SD) and Bmax = 2.19 pmol/mg protein +/- 0.92 (SD). Specific binding to the liposomes could not be demonstrated. High nonspecific binding of up to about 20% of the total verapamil in the external solution was observed (3.8 pmoles [3H]verapamil/mg phospholipid when 30 nM verapamil was used and 50 nmoles/mg phospholipid when 200 microM [3H] verapamil was used). The high nonspecific binding of verapamil to the liposomes had no detectable effect on the fluidity of their membrane, as seen in fluorescence-anisotropy studies with the fluorescent probe DPH.

Inhibition of the Na+/Ca2+ exchange in cardiac sarcolemmal vesicles by amiloride

The pyrazine diuretic amiloride inhibits the Na+/Ca2+ exchange activity of cardiac sarcolemmal vesicles in a concentration-dependent way. A good relationship between the uptake of amiloride by the vesicles and the inhibition of the exchanger has been found. Kinetic analyses indicate that the inhibition of Na+/Ca2+ exchange activity by amiloride is non-competitively removed by Ca2+ and competitively overcome by an outwardly directed Na+ gradient.

Effects of Ca2+ channel blockers on Ca2+ translocation across synaptosomal membranes

The binding of [3H]nimodipine to purified synaptic plasma membranes (SPM) isolated from sheep brain cortex was characterized, and the effects of nimodipine, nifedipine, and (+)-verapamil on the [3H]nimodipine binding were compared to the effects on 45Ca2+ translocation under conditions that separate 45Ca2+ fluxes through Ca2+ channels from 45Ca2+ uptake via Na+/Ca2+ exchange. [3H]Nimodipine labels a single class of sites in SPM, with a KD of 0.64 +/- 0.1 nM, a Bmax of 161 +/- 27 fmol X mg-1 protein, and a Hill slope of 1.07, at 25 degrees C. Competition of [3H]nimodipine binding to purified SPM with unlabelled Ca2+ channel blockers shows that: nifedipine and nimodipine are potent competitors, with IC50 values of 4.7 nM and 5.9 nM, respectively; verapamil and (-)-D 600 are partial competitors, with biphasic competition behavior. Thus, (+)-verapamil shows an IC50 of 708 nM for the higher affinity component and the maximal inhibition is 50% of the specific binding, whereas for (-)-verapamil the IC50 is 120 nM, and the maximal inhibition is 30%; (-)-D 600 is even less potent than verapamil in inhibiting [3H]nimodipine binding (IC50 = 430 nM). However, (+)-verapamil, nifedipine, and nimodipine are less potent in inhibiting depolarization-induced 45Ca2+ influx into synaptosomes in the absence of Na+/Ca2+ exchange than in competing for [3H]nimodipine binding. Thus, (+)-verapamil inhibits Ca2+ influx by 50% at about 500 microM, whereas it inhibits 50% of the binding at concentrations 200-fold lower, and the discrepancy is even larger for the dihydropyridines. The Na+/Ca2+ exchange and the ATP-dependent Ca2+ uptake by SPM vesicles are also inhibited by the Ca2+ channel blockers verapamil, nifedipine, and d-cis-diltiazem, with similar IC50 values and in the same concentration range (10(-5)-10(-3) M) at which they inhibit Ca2+ influx through Ca2+ channels. We conclude that high-affinity binding of the Ca2+ blockers by SPM is not correlated with inhibition of the Ca2+ fluxes through channels in synaptosomes under conditions of minimal Na+/Ca2+ exchange. Furthermore, the relatively high concentrations of blockers required to block the channels also inhibit Ca2+ translocation through the Ca2+-ATPase and the Na+/Ca2+ exchanger. In this study, clear differentiation is made of the effects of the Ca2+ channel blockers on these three mechanisms of moving Ca2+ across the synaptosomal membrane, and particular care is taken to separate the contribution of the Na+/Ca2+ exchange from that of the Ca2+ channels under conditions of K+ depolarization.

gamma-Aminobutyric acid release from synaptosomes as influenced by Ca2+ and Ca2+ channel blockers

We studied the correlation between the high affinity binding of Ca2+ channel blockers to purified synaptic plasma membranes (SPM) and the effect of these drugs in blocking the 45Ca2+ uptake and the release of [3H]gamma-aminobutyric acid [( 3H]GABA) by preloaded synaptosomes. The Ca2+ channel blocker binding sites were characterized by studying the binding of the dihydropyridine, [3H]nimodipine, and of the phenylalkylamine, (-)-[3H]desmethoxyverapamil, to purified SPM isolated from sheep brain cortex synaptosomes. The purified SPM had high affinity binding sites for both Ca2+ channel blockers. The binding parameters were similar to those previously reported for whole brain homogenates: KD = 0.64 nM and Bmax = 160 fmol/mg of protein for [3H]nimodipine, and KD = 7.9 nM and Bmax = 1,500 fmol/mg of protein for (-)-[3H]desmethoxyverapamil. The Ca2+ channel blockers inhibited the release of [3H]GABA induced by K+ depolarization in the presence or in the absence of Ca2+. The Ca2+-dependent component of [3H]GABA release was inhibited by verapamil, (-)-D 600, d-cis-diltiazem, nifedipine and PY 108-86 with IC50 values of 2.2 X 10(-5) M, 6.3 X 10(-5) M, 3 X 10(-4) M, greater than 10(-4) M and 3 X 10(-5) M, respectively. Furthermore, the Ca2+ channel blockers also inhibited the Ca2+-independent [3H]GABA release which occurred in the presence, but not in the absence, of external Na+. The Ca2+ channel blockers at concentrations which inhibited [3H]GABA release inhibited the entry of Ca2+ through the Ca2+ channels and also the entry of Ca2+ by Na+/Ca2+ exchange. We conclude that the concentrations of Ca2+ blockers necessary to block Ca2+ uptake through the Ca2+ channels and by Na+/Ca2+ exchange coincide with the concentrations at which they inhibit [3H]GABA release, but that their effect on the relationship between Ca2+ uptake and [3H]GABA release is different for the various blockers. The effects of the drugs on Ca2+ movements and [3H]GABA release are not specifically mediated through the high affinity binding of the drugs since relatively high concentrations were necessary (greater than 10(-5) M) for the effects reported here.

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.

The effects of some slow channel blocking drugs on high affinity serotonin uptake by rat brain synaptosomes

The effects of some slow channel blocking drugs were investigated on high affinity serotonin uptake into crude rat brain synaptosomes. Serotonin uptake was sodium-dependent and competitively inhibited by imipramine (IC50 0.6 microM, Ki 0.26 microM). Bepridil, verapamil and diltiazem produced an apparent competitive inhibition of serotonin uptake with respective IC50 of 4.8, 5.2 and 308 microM. Nitrendipine and the sodium channel blocker, lidocaine, were without effect, even at 100 microM. The mechanism of the inhibitory effect is unknown but may involve an allosteric interaction with the sodium-dependent transporter.

Verapamil is a competitive inhibitor of gamma-aminobutyric acid and calcium uptakes by mouse brain subcellular particles

We found that verapamil and its methoxy analogue, D600, were relatively potent (micromolar) inhibitors of Na+-dependent GABA uptake by a mouse brain microsomal subfraction (P3). Verapamil was competitive with GABA and uncompetitive with Na+ in the uptake assay with the P3 fraction. These substances were much less effective in inhibiting GABA binding in a receptor-related assay system with synaptosomal membranes. Inhibition by verapamil of Na+-dependent 45Ca++ uptake by the P3 particles was competitive with Ca++. A consideration of our results with those in the literature led to the suggestion that the interaction of verapamil and related substances with GABA and 45Ca uptake processes by the P3 fraction, as well as with many other membrane activities, may be allosteric in nature rather than directly competitive with specific ligands at their binding sites.

Restoration of doxorubicin responsiveness in doxorubicin-resistant P388 murine leukaemia cells

The effects of certain compounds on the in vitro growth rate and the sensitivity to doxorubicin of P388 murine leukaemia cell line and of a doxorubicin-resistant subline (P388/ADR) were studied. The calcium channel blocking activity of these compounds was evaluated by measuring their effects on the sodium-dependent and membrane potential-dependent calcium uptake in synaptic plasma membrane vesicles. At non-inhibitory concentrations, verapamil, dipyridamole, meclizine and nicardipine were highly active in restoring the sensitivity to doxorubicin of P388/ADR cells. Moderately active were propranolol, N-(beta-diethylaminoethyl)-N-(beta-hydroxy-beta-phenylethyl)-2,5-dich loranaline (MDL-6792), thioridazine and chlorocyclizine, while nifedipine, guanethidine, phentolamine, chloroquine and papaverine had zero or only minimal synergistic activity to doxorubicin in this cell line. Doxorubicin synergistic activity could not be demonstrated in the parent drug-sensitive cell line. No sodium-dependent or membrane potential-dependent calcium uptake could be demonstrated in vesicles prepared from plasma membranes of either cell line. There is no correlation between the ability of these compounds to inhibit calcium uptake in synaptic vesicles and their potency in restoring the sensitivity of P388/ADR cells to doxorubicin.

The inhibition of Ca uptake in cardiac membrane vesicles by verapamil

Cardiac membrane vesicles take up Ca2+ in response to Na+ gradient (high inside) and negative inside membrane potential. Both components of the Ca2+ uptake, the Na+ gradient dependent uptake and the membrane potential dependent uptake are inhibited by verapamil; the action is dose-dependent and the concentrations of verapamil required to inhibit the Ca2+ uptake to 50% of its maximal value are 50 and 60 microM respectively. In the concentration ranges tested (50-750 microM Ca2+), the inhibitory effect of verapamil could not be antagonized by increasing the Ca2+ concentration of the medium. Introducing verapamil into the vesicles by rapid freezing and slow thawing of the vesicles had the same inhibitory effect as adding the same concentration of verapamil on the outside of the vesicles. Adding verapamil to both sides of the vesicle membrane led to higher inhibition of Ca2+ uptake. It is proposed that addition of verapamil can cause a change in cardiac membranes which is manifested by a decrease in the driving membrane potential and Ca2+ transport.

The asymmetric effect of lanthanides on Na+-gradient-dependent Ca2+ transport in synaptic plasma membrane vesicles

Lanthanides (La3+, Pr3+ and Tb3+) inhibit Na+-gradient-dependent Ca2+ influx into synaptic plasma membrane vesicles. 50% inhibition is obtained by 7 microM lanthanide concentration. The inhibition of the Na+-gradient-dependent Ca2+ uptake exhibits competitive kinetic behaviour. The apparent Km of the Ca2+ influx is increased from 50 microM in the absence of lanthanides to 118 microM in the presence of La3+, 170 microM in the presence of Pr3+ and 130 microM in the presence of Tb3+. The maximal reaction velocity is not altered (8.35 nmol Ca2+ transported per mg protein per min in the absence of lanthanides and 8.16 nmol/mg per min in the presence of lanthanides). Lanthanides also inhibited Na+-gradient-dependent Ca2+ efflux from synaptic plasma membrane vesicles that were preloaded with Ca2+ in a Na+-gradient-dependent manner. Introduction of La3+ into the interior of the synaptic plasma membrane vesicles by rapid freezing of the vesicles in liquid N2 and slow thawing had no effect on either Na+-gradient-dependent Ca2+ influx or efflux. Synaptic plasma membrane vesicles can be preloaded with Ca2+ also in an ATP-dependent manner. This form of Ca2+ uptake is also inhibited by La3+ though at higher concentrations than the Na+-gradient-dependent Ca2+ uptake. Na+-gradient-dependent efflux from synaptic plasma membrane vesicles preloaded in an ATP-dependent fashion ('inside-out' vesicles) unlike efflux from synaptic plasma membrane vesicles preloaded in a Na+-gradient-dependent manner was not inhibited by La3+. These findings suggest that the inhibition by La3+ is manifested asymmetrically on both sides of the synaptic plasma membrane. Lanthanides are probably not transported via the Na+-Ca2+ exchanger since Tb3+ entry measured by fluorescence of Tb3+-dipicolinic acid complex formation occurred at high Tb3+ concentrations only (1.5 mM or above) and was not Na+-gradient dependent.