Characterization of the Ca2+ coordination site regulating binding of Ca2+ channel inhibitors d-cis-diltiazem, (+/-)bepridil and (-)desmethoxyverapamil to their receptor site in skeletal muscle transverse tubule membranes

Molecular modeling of benzothiazepine binding in the L-type calcium channel

Benz(othi)azepine (BTZ) derivatives constitute one of three major classes of L-type Ca(2+) channel ligands. Despite intensive experimental studies, no three-dimensional model of BTZ binding is available. Here we have built KvAP- and KcsA-based models of the Ca(v)1.2 pore domain in the open and closed states and used multiple Monte Carlo minimizations to dock representative ligands. In our open channel model, key functional groups of BTZs interact with BTZ-sensing residues, which were identified in previous mutational experiments. The bulky tricyclic moiety occupies interface between domains III and IV, while the ammonium group protrudes into the inner pore, where it is stabilized by nucleophilic C-ends of the pore helices. In the closed channel model, contacts with several ligand-sensing residues in the inner helices are lost, which weakens ligand-channel interactions. An important feature of the ligand-binding mode in both open and closed channels is an interaction between the BTZ carbonyl group and a Ca(2+) ion chelated by the selectivity filter glutamates in domains III and IV. In the absence of Ca(2+), the tricyclic BTZ moiety remains in the domain interface, while the ammonium group directly interacts with a glutamate residue in the selectivity filter. Our model suggests that the Ca(2+) potentiation involves a direct electrostatic interaction between aCa(2+) ion and the ligand rather than an allosteric mechanism. Energy profiles indicate that BTZs can reach the binding site from the domain interface, whereas access through the open activation gate is unlikely, because reorientation of the bulky molecule in the pore is hindered.

Effects of diltiazem or verapamil on calcium uptake and release from chicken skeletal muscle sarcoplasmic reticulum

The purpose of this study was to determine the effects of 2 Ca2+ channel blockers, verapamil and diltiazem, on calcium loading (active Ca2+ uptake) and the following Ca2+ release induced by silver ion (Ag+) and Ca2+ from the membrane of heavy sarcoplasmic reticulum (SR) of chicken skeletal muscle. A fluorescent probe technique was employed to determine the calcium movement through the SR. Pretreatment of the medium with diltiazem and verapamil resulted in a significant decrease in the active Ca2+ uptake, with IC50 of about 290 micromol/L for verapamil and 260 micromol/L for diltiazem. Inhibition of Ca2+ uptake was not due to the development of a substantial drug-dependent leak of Ca2+ from the SR. It might, in part, have been mediated by a direct inhibitory effect of these drugs on the Ca2+ ATPase activity of the SR Ca2+ pump. We confirmed that Ca2+ channel blockers, administered after SR Ca2+ loading and before induction of Ca2+ release, caused a dose-dependent inhibition of both Ca2+- and Ag+-induced Ca2+ release rate. Moreover, if Ca2+ channel blockers were administered prior to SR Ca2+ loading, in spite of Ca2+ uptake inhibition the same reduction in Ca2+- and Ag+-induced Ca2+ release rate was seen. We showed that the inhibition of Ag+-induced Ca2+ release by L-channel blockers is more sensitive than Ca2+-induced Ca2+ release inhibition, so the IC50 for Ag+- and Ca2+-induced Ca2+ release was about 100 and 310 micromol/L for verapamil and 79 and 330 micromol/L for diltiazem, respectively. Our results support the evidence that Ca2+ channel blockers affect muscle microsome of chicken skeletal muscle by 2 independent mechanisms: first, reduction of Ca2+ uptake rate and Ca2+-ATPase activity inhibition, and second, inhibition of both Ag+- and Ca2+-induced Ca2+ release by Ca2+ release channels. These findings confirm the direct effect of Ca2+ channel blockers on calcium release channels. Our results suggest that even if the SR is incompletely preloaded with Ca2+ because of inhibition of Ca2+ uptake by verapamil and diltiazem, no impairment in Ca2+ release occurs.

Pharmacophoric features and Ca2+ ion holding capacity of verapamil

Ab initio Hartree-Fock calculations have been performed at the 6-31G level to study the pharmacophoric features of verapamil. Both the unprotonated and the protonated forms of verapamil have been studied. The study predicts that the drug enters the body in protonated form and is anchored to the receptor via H-bond formation involving protonated amine. Huge conformational change as well as deprotonation is required before the drug is capable of holding Ca(2+) ions. Folded form of drug is capable of holding Ca(2+) ion and the chiral center also seems to be involved to certain extent.

Calcium release by diltiazem from isolated sarcoplasmic reticulum of rabbit skeletal muscle

1. The effect of diltiazem on isolated sarcoplasmic reticulum (SR) from rabbit skeletal muscle was studied. To observe calcium movement into and out of the SR, a fluorescent chelate probe technique with chlortetracycline (CTC) as a reagent was employed. 2. Tris-ATP-induced calcium accumulation by the isolated SR was associated with a rise in the CTC fluorescence. The effect of ATP was dose dependent. 3. Diltiazem (6 x 10(-4)M, 2 x 10(-3)M) prevented ATP-induced calcium accumulation by the SR. 4. Addition of EGTA to the media chelates external calcium and caused calcium release that can be reversed by further addition of calcium chloride. Similarly diltiazem caused a rapid release of accumulated calcium from the SR, which is not reversed by the addition of calcium chloride. 5. It seems that the effect of diltiazem may be related to SR membrane-bound calcium being available for release.

Molecular determinants of drug binding and action on L-type calcium channels

The crucial role of L-type Ca2+ channels in the initiation of cardiac and smooth muscle contraction has made them major therapeutic targets for the treatment of cardiovascular disease. L-type channels share a common pharmacological profile, including high-affinity voltage- and frequency-dependent block by the phenylalkylamines, the benz(othi)azepines, and the dihydropyridines. These drugs are thought to bind to three separate receptor sites on L-type Ca2+ channels that are allosterically linked. Results from different experimental approaches implicate the IIIS5, IIIS6, and IVS6 transmembrane segments of the alpha 1 subunits of L-type Ca2+ channels in binding of all three classes of drugs. Site-directed mutagenesis has identified single amino acid residues within the IIIS5, IIIS6, and IVS6 transmembrane segments that are required for high-affinity binding of phenylalkylamines and/or dihydropyridines, providing further support for identification of these transmembrane segments as critical elements of the receptor sites for these two classes of drugs. The close proximity of the receptor sites for phenylalkylamines, benz(othi)azepines, and dihydropyridines raises the possibility that individual amino acid residues may be required for high-affinity binding of more than one of these ligands. Therefore, we suggest that phenylalkylamines and dihydropyridines bind to different faces of the IIIS6 and IVS6 transmembrane segments and, in some cases, bind to opposite sides of the side chains of the same amino acid residues. The results support the domain interface model for binding and channel modulation by these three classes of drugs.

Calcium antagonists and vasodilatation

Calcium antagonists comprise a diverse group of chemically unrelated agents that interact with voltage-operated calcium channels (L-type) and thereby inhibit smooth muscle contractility. They are used to treat several major cardiovascular disorders, including hypertension and angina pectoris; they are also studied in congestive heart failure and in atherosclerosis. The current view is that their therapeutic action is related to vasodilatation. This view is an oversimplification, as will be shown in this review. It will also be illustrated that all calcium antagonists are not identical pharmacological agents.

Characterization of the binding of [3H]SR 33805 to the slow Ca2+ channel in rat heart sarcolemmal membrane

SR 33805 is a representative of a new class of compounds (indole sulfone) that inhibits L-type Ca2+ channels. [3H]SR 33805 has been shown to bind with a high affinity (Kd approximately 20 pM calculated from saturation isotherms and association/dissociation kinetics) to a single site in a purified preparation of rat cardiac sarcolemmal membranes. The binding was found to be saturable and reversible. The maximal binding capacity was in approximately 1:1 stoichiometry with that of other Ca2+ channel antagonists. Various cations (Na+, Ca2+, Cd2+, and La3+) were shown to inhibit specific [3H]SR 33805 binding, with La3+ being the most potent. Using a range of receptor or channel ligands (including omega-conotoxin and Na+ and K+ channel modulators), only the L-type Ca2+ channel antagonists were found to displace [3H]SR 33805. However, dihydropyridines, phenylalkylamines, benzothiazepines, and diphenyl-butylpiperidines were found to inhibit [3H]SR 33805 in a non-competitive manner as demonstrated by displacement experiments in addition to dissociation kinetics. In contrast, the interaction of SR 33805 with fantofarone has been found to be competitive. Binding of [3H]SR 33805 (and [3H]fantofarone) is entropy driven as opposed to that of the [3H]nitrendipine which is enthalpy driven. From these results we suggest that SR 33805 binds with a high affinity to a unique site on the L-type Ca2+ channel found in rat cardiac sarcolemmal membranes. This site is equivalent to that of fantofarone and in allosteric interaction with that of the dihydropyridines, phenylalkylamines and benzothiazepines.

Identification of a novel calcium antagonist binding site in rat brain by SR 33557

1. In K(+)-depolarized taenia preparations from guinea-pig caecum SR 33557 was a potent antagonist of Ca(2+)-induced contractions and antagonized the effect of the calcium channel activator Bay K 8644. 2. SR 33557 displayed high affinity (pKi 9.54 +/- 0.04, nH 1.01) for the [3H]-(+/-)-PN 200-110 binding site in rat cerebral cortex membranes. In the presence of 5 mM Ca2+ this affinity was reduced (pKi 8.82 +/- 0.01, nH 1.05) whilst the affinity of nitrendipine was unaffected by this concentration of Ca2+. 3. Saturation binding experiments in rat cerebral cortex carried out in the absence and presence of SR 33557 (0.1-1.0 nM) indicated an apparently competitive interaction at the dihydropyridine site, in that SR 33557 reduced the KD of [3H]-(+/-)-PN 200-110 binding without any effect on Bmax. In kinetic experiments, the rate of dissociation of [3H]-(+/-)-PN 200-110 from rat cerebral cortex was unchanged in the presence of SR 33557 (5 nM). 4. D-cis-diltiazem fully reversed the inhibition [3H]-nitrendipine binding to rat cerebral cortex produced by SR 33557 indicating the site of action of SR 33557 to be distinct from the dihydropyridine (DHP) binding site. 5. Saturation analysis indicated that [3H]-SR 33557 (0.01-0.8 nM) labelled a single class of binding sites in rat cerebral cortex membranes with high affinity (KD 0.12 +/- 0.01, Bmax 222 +/- 20 fmol mg-1 protein), although kinetic data indicated the existence of negative cooperativity between the binding sites. 6.In competition studies, a variety of different calcium antagonists displayed similar affinity for [3H]-SR 33557 and [3H]-(+/-)-PN 200-110 sites. The [3H]-SR 33557 site was sensitive to the inhibitory effect of divalent cations. The affinity of Cd2+ was 0.026 +/- 0.015 mM and the rank order of affinity was Cd2+ >Ca2+ >Mn2+ >Mg2+ >Na+.7. We propose that SR 33557 labels a distinct site in rat cerebral cortex. The coupling between the SR 33557 and DHP site appears to be very close, resulting in apparently competitive interactions in some experimental protocols but can be revealed as negatively allosteric in other circumstances.

The devapamil-binding site of the purified skeletal muscle receptor for organic-calcium channel blockers is modulated by micromolar and millimolar concentrations of Ca2+

The interaction of 2,7-dimethyl-3-(3,4-dimethoxyphenyl)-3-cyan-7-aza-9-(3- methoxyphenyl) nonahydrochloride (devapamil), a stereospecific analog of (3-[2-(3,4-dimethoxyphenyl)ethyl]- methylaminopropyl-3,4-dimethoxy-(1-methylethyl)benzeneacetonitr ile (verapamil), with the purified skeletal muscle receptor for calcium channel blockers (CaCB) was studied at 4 degrees C and 30 degrees C in the absence and presence of calcium. The purified CaCB receptor bound 0.9 mol devapamil/mol calcium-channel alpha 1 subunit, with an apparent Kd of 13 +/- 2.6 nM at 4 degrees C in the presence of 0.4 microM Ca2+. The affinity, and not the density, of the devapamil-binding site was decreased by lowering the pH from 8.5-6.5, or by increasing the Ca2+ concentration from 0.4 microM to 100 mM. The same results were obtained at 30 degrees C, although the ligand-receptor complex was not stable at Ca2+ concentrations below 10 microM. These binding data were confirmed by kinetic experiments. The rate constants calculated for a pseudo-first-order and a second-order reactions were identical and yielded fourfold lower k-1/k+1 (KD) values than the equilibrium experiments performed using 1 nM and 0.4 microM Ca2+, but the same values using 1 mM Ca2+. 1 mM Ca2+ increased the k-1/k+1 (KD) by decreasing 10-fold the association rate at 4 degrees C. The dissociation rate was increased about 10-fold by 5 microM devapamil or 100 microM D-cis-diltiazem, suggesting that the high affinity site is negatively regulated allosterically by millimolar Ca2+ concentrations and by the occupation of a second low-affinity site. Incubation of the CaCB receptors in the absence of Ca2+ and devapamil at 30 degrees C, but not at 4 degrees C, resulted in an apparent loss of devapamil-binding sites. The decrease in binding sites was caused by a reduced affinity. This apparent loss of binding sites was prevented by the addition of Ca2+ with an apparent median effective concentration of 0.4 microM. The apparent half-maximal inactivation times of the devapamil-binding site were 90 s and 12 min in the presence of 1 nM and 0.4 microM Ca2+, respectively. These results show that micromolar Ca2+ concentrations stabilize the CaCB receptor in a conformation which allows high-affinity binding of phenylalkylamines. Millimolar Ca2+ concentrations induce a low-affinity state of the devapamil-binding site on a stable CaCB receptor.

The mechanisms of action of progesterone and the anti-progestin ZK 98734 on PGF2 alpha synthesis by early human decidua

The inhibition of prostaglandin (PG) synthesis found in early human decidua is antagonized by the anti-progestin, ZK 98734. This action of ZK 98734 is abolished by actinomycin, an inhibitor of protein synthesis and by the calcium channel blocker, verapamil. Calmidazolium, an antagonist of the intracellular calcium binding protein calmodulin was less effective in inhibiting the stimulation of PG synthesis induced by the anti-progestin. Chronic stimulation of protein kinase C activity by 1-oleoyl-2-acetyl-sn-glycerol (OAG) induced a slight reduction of PG release and was antagonized by polymixin. These findings suggest that inhibition of PG synthesis in early pregnancy is caused by progesterone and that increased release of PGs induced by anti-progestins is dependent on new protein formation and requires extracellular calcium.

Ca2+ channel blockers inhibit secretory C1-channels in intestinal epithelial cells

Outwardly rectifying Cl- channels are present in the human colonic cell line (HT29D4). The classical Cl- channel blocker 5-nitro-2(3-phenylpropylamino)benzoate inhibits Cl- channel activity with a K0.5 value of 20 microM. Epithelial Cl- channel activity is inhibited by Ca2+ channel blockers. Phenylalkylamines are the most effective inhibitors. (+/-)Verapamil and (-)desmethoxyverapamil induce flickering and then the complete blockade of Cl- channels recorded from outside-out patches. K0.5 values are 60 microM and 100 microM for (-)desmethoxyverapamil and (+/-)verapamil, respectively. Other classes of L-type Ca2+ channel blockers have also been studied but they are less active.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Desmethoxyverapamil-sensitive calcium channels in rat portal vein smooth muscle

The whole-cell patch clamp technique was used to analyze the properties of the phenylalkylamine-sensitive calcium channels in smooth muscle cells isolated from the portal vein. (-)-D888 dose dependently inhibited the calcium current elicited from a holding potential of -40 mV (IC50 = 1.3 nM) in a frequency-dependent manner. No voltage dependence of the inhibition was noted. Independent high- and low-affinity binding sites for (-)-[3H]D888 were identified. Calcium entry blockers such as (-)-D888, d-cis-diltiazem and nicardipine completely or partially antagonized the (-)-[3H]D888 binding at both types of sites. The properties of this cross-inhibition suggest that phenylalkylamines and d-cis-diltiazem bind at common sites in vascular smooth muscles whereas dihydropyridines bind at distinct sites which are allosterically coupled to the phenylalkylamine sites. As the IC50 for (-)-D888 found from electrophysiological experiments is not identical to the equilibrium dissociation constants for the high- and low-affinity sites found from binding data (0.47 and 50 nM, respectively), it is suggested that binding of (-)-D888 to both high- and low-affinity sites may be involved in the inhibitory effect of (-)-D888 on calcium channels. Furthermore, these two different binding sites may correspond to two different subtypes of phenylalkylamine-sensitive calcium channels in smooth muscle cells.

Interaction of palmitoyl carnitine with calcium antagonists in myocytes

1. Beating of aggregates of embryonic chick myocytes, in primary culture, was quantified by use of a motion-detector and video-recorder technique. Interactions of palmitoyl carnitine, a putative endogenous ligand at Ca2+ channels, with calcium antagonists were investigated. 2. Bay K 8644 (1-100 nM) and palmitoyl carnitine (0.2-30 microM) increased edge movement of the aggregates; beats fused so that there was an increase in baseline 'tone'. The concentrations required to produce a 50% increase in edge movement were 2.5 nM for Bay K 8644 and 2 microM for palmitoyl carnitine. Higher concentrations (20-30 microM) of palmitoyl carnitine caused tachycardia of abrupt onset but resulted in cessation of beating. The effects of palmitoyl carnitine were not stereo-selective in that the (+)- and (-)-isomers were equieffective. Lysophosphatidyl choline (LPC) had no effect in concentrations up to 10 microM but higher concentrations caused tachycardia followed by cessation of beating. High concentrations of both palmitoyl carnitine and LPC (100 microM) caused break-up of the aggregates, presumably as a result of detergent effects. 3. Palmitoyl carnitine (1-100 microM) reversed the inhibitory effects of nisoldipine (0.3 microM), diltiazem (10 microM) and verapamil (1 microM). Ouabain was ineffective in reversing the effects of nisoldipine, differentiating the effects of palmitoyl carnitine from those of Na+/K+ ATPase inhibition. In contrast, palmitoyl carnitine did not reverse the inhibitory effects of pimozide (2 microM) or lidoflazine (7 microM); palmitoyl carnitine showed a similar profile to Bay K 8644 in this respect. 4. These findings indicate that the effects of palmitoyl carnitine closely resemble those of Bay K 8644 and can be differentiated from those of lysophospholipids. As palmitoyl carnitine accumulates in the sarcolemma during myocardial ischaemia, the mode of action in the Ca2 + channel may have clinical relevance for the use of calcium antagonists in ischaemia.

Binding Ca2+ to intracellular or to extracellular sites of dihydropyridine receptor of rabbit skeletal muscle discriminates between in vitro binding of Ca2+-channel agonist and antagonist

Transverse tubule membrane vesicles contain dihydropyridine receptor of rabbit skeletal muscle in an insideout orientation. Digitonin-solubilized, purified dihydropyridine receptor is embedded in digitonin vesicles in an outside-out orientation. Ca2+ selectively stimulates binding of the Ca2+-channel antagonist [3H]PN200-110 to dihydropyridine receptor in the outside-out but not the inside-out orientation. The dissociation constant for binding Ca2+ to the extracellular Ca2+-specific binding site of dihydropyridine receptor is 2-3 microM. The data demonstrate that binding Ca2+ to the extracellular high-affinity Ca2+-binding site is required for binding dihydropyridines to dihydropyridine receptor. This binding is inhibited, however, by 1-10 mM concentrations of any divalent cation tested (Ba2+, Mn2+, Mg2+). Also, Ca2+ selectively stimulates binding of the Ca2+-channel agonist [3H]BayK8644 to dihydropyridine receptor in the inside-out orientation. The titration of this Ca2+ dependence indicates that the dissociation constant for binding Ca2+ to the intracellular Ca2+-specific binding site of dihydropyridine receptor is in the millimolar range. Thus, binding Ca2+-channel agonist or antagonist to dihydropyridine receptor is modulated by binding Ca2+ to different sites of the receptor. Measurements of dissociation rate constants for binding [3H]PN200-110 to dihydropyridine receptor in the presence of diltiazem, verapamil and/or Ca2+ indicate that Ca2+, like diltiazem or verapamil, is an allosteric effector of this receptor.

Characteristics of [3H]nimodipine binding to sarcolemmal membranes from rat vas deferens and its regulation by guanine nucleotide

The binding properties of a 1,4-dihydropyridine (DHP) calcium entry blocker, [3H]nimodipine, to a microsomal fraction from rat vas deferens was characterized. The specific binding was saturable, rapid and reversible. Scatchard analysis of the binding revealed a single binding site, and the dissociation constant and the maximum number of binding sites were 0.31 +/- 0.02 nM and 97.0 +/- 7.19 fmol/mg protein, respectively. Both the Kd value obtained from the kinetic study and the IC50 value from relaxation of the K+-depolarized organ were approximately 0.4 nM, indicating that the binding site is closely related to the functional Ca2+ channel. The specific [3H]nimodipine binding was displaced by DHP derivatives at low concentration and by verapamil at high concentration, but diltiazem had no effect on the binding. Calcium chelating agents decreased the [3H]nimodipine binding which was restored by adding Ca2+. 5'-Guanylylimidodiphosphate caused a rightward shift of the displacement curve for Bay K 8644 but not for nimodipine, suggesting the involvement of guanine nucleotide binding protein in the signal transduction between the DHP binding site and the Ca2+ channel.

Characteristics of the phenylalkylamine binding site in canine cardiac sarcolemmal membranes

We have investigated the phenylalkylamine binding site in canine cardiac sarcolemmal preparations using (-)-[3H]-desmethoxyverapamil as the labeled ligand. Radioligand binding experiments were carried out in 10 mM Hepes (Na+) buffer and 1 mM EGTA, at pH 7.4 and 20 degrees C. A single high affinity binding site for (-)-[3H]-desmethoxyverapamil was identified both by saturation and competition binding experiments. Several phenylalkylamine derivatives such as (-)-D600, (+)-D600, verapamil and (+)-desmethoxyverapamil completely inhibited (-)-[3H]-desmethoxyverapamil binding with the following order of potency: (-)-desmethoxyverapamil greater than (-)-D600 greater than verapamil greater than (+)-desmethoxyverapamil = (+)-D600. In contrast to this, ronipamil, a new long acting phenylalkylamine derivative, produced only a 70% inhibition. Diltiazem also completely inhibited (-)-[3H]-desmethoxyverapamil binding to canine cardiac sarcolemma while nifedipine displaced only 70% of binding. (-)-[3H]-desmethoxyverapamil binding was also inhibited by Ca++ and Mg++. These data suggest the presence of a saturable, reversible and stereoselective phenylalkylamine binding site in canine cardiac sarcolemmal preparations which may be a receptor for the phenylalkylamine Ca++ channel inhibitors.

Direct activation of Ca2+ channels by palmitoyl carnitine, a putative endogenous ligand

1 Palmitoyl carnitine, a lipid metabolite which accumulates in cytoplasmic membranes during ischaemia, has been shown to resemble the Ca2+ channel activator, Bay K 8644, in K+-depolarized smooth muscle. Palmitoyl carnitine caused concentration-dependent (1-1000 mumol l-1) augmentations in the sensitivity to Ca2+ of K+-depolarized taenia preparations from the guinea-pig caecum. The (+/-)-isomer was equieffective with the (-)-isomer, whereas carnitine was ineffective and palmitic acid relaxed the tissues. The shift to the left of Ca2+ concentration-response curves induced by palmitoyl carnitine (100 mumol l-1) was additive with that of Bay K 8644 (1 mumol l-1). 2 The interactions of palmitoyl carnitine with the different classes of calcium-antagonist were similar to those seen with Bay K 8644. Schild plots of the calcium-antagonist effects of nifedipine were shifted to the right following preincubation of the taenia with palmitoyl carnitine (30-300 mumol l-1). The inhibitory effects of verapamil were especially sensitive to palmitoyl carnitine (100 mumol l-1). Whereas the potency of diltiazem as a calcium-antagonist was reduced by palmitoyl carnitine (100 mumol l-1), the inhibitory effects of the lipophilic class III calcium-antagonists, cinnarizine and flunarizine, were entirely resistant to palmitoyl carnitine (100 mumol l-1). 3 Although palmitoyl carnitine has detergent properties in high concentrations and lyses red blood cells, these effects were not Ca2+-dependent, nor were they modified by calcium-antagonists. Other detergents did not have selective interactions with Ca2+ channels. 4 Palmitoyl carnitine inhibited [3H]-nitrendipine, [3H]-verapamil and [3H]-diltiazem binding to rat cortical membranes with IC50 values (mumol l-1) of 120 +/- 1, 95 +/- 17 and 120 +/- 15 mumol l-1 respectively. The inhibition showed little temperature-dependence, in contrast to that of Bay K 8644, except for a small reduction in the IC50 value for [3H]-verapamil binding at 37 degrees C (42 +/- 5 mumol l-1). Palmitoyl carnitine interacted selectively with the Ca2+ channel, in that effects on ligand binding to alpha-adrenoceptors, beta-adrenoceptors and 5-HT1A receptors occurred only at 5-10 fold higher concentrations. 5 It is concluded that palmitoyl carnitine, at concentrations which have previously been shown to occur in the cytoplasm during myocardial ischaemia, may interact directly with Ca2+ channels and may therefore be considered as an endogenous modulator of channel function. The site of action differs from that of other agents.

Receptors for diphenylbutylpiperidine neuroleptics in brain, cardiac, and smooth muscle membranes. Relationship with receptors for 1,4-dihydropyridines and phenylalkylamines and with Ca2+ channel blockade

Neuroleptic molecules of the diphenylbutylpiperidine series (DPBP), such as fluspirilene, penfluridol, pimozide and clopimozide, antagonize binding of (-)[3H]desmethoxyverapamil ((-)[3H]D888) and (+)[3H]PN 200-110 to rabbit brain, heart and smooth muscle membranes. The diphenylbutylpiperidine binding site in all these tissues is distinct but is allosterically related to the 1,4-dihydropyridine binding site and to the phenylalkylamine binding site. High and low affinity binding sites for (-)D888 were identified. (-)[3H]D888 binding at both types of sites was inhibited following the saturation of a single type of diphenylbutylpiperidine binding site. Half-maximal inhibition (K0.5) of brain, heart and smooth muscle membranes binding by different diphenylbutylpiperidines was in the range of 10-100 nM. These K0.5 values were one to two orders of magnitude higher than those found for the high affinity diphenylbutylpiperidine receptor in skeletal muscle membranes. The K0.5 values found in binding experiments in smooth muscle were similar to the (IC50) values for half-maximal inhibition by diphenylbutylpiperidine of voltage-dependent 45Ca2+ influx through the slow Ca2+ channel.

Stereospecific action of diltiazem on the mitochondrial Na-Ca exchange system and on sarcolemmal Ca-channels

The benzothiazepine diltiazem is a potent Ca-channel blocker, which also inhibits the Na-dependent Ca-efflux from heart mitochondria. In this study, the action of the 4 stereoisomers of diltiazem has been investigated using guinea-pig heart and liver mitochondria. The rate of the Na-dependent Ca-efflux from liver mitochondria has been found to be 10 times smaller than in heart mitochondria. Otherwise, the exchange systems from the two tissues have been found to be pharmacologically indistinguishable. Both the (+)-optical isomers of the cis- and trans-forms of diltiazem inhibit Na-Ca exchange activity with comparable potency (IC50 of 10-20 microM), while the (-)-optical isomers are ineffective (IC50 greater than 200 microM). Radioligand binding experiments have revealed that only one stereoisomer of diltiazem, the (+)-cis form, interacts with high affinity with the Ca-channel receptors of guinea-pig heart sarcolemma preparations (KD = 120 nM). The results have shown that the Ca-channel of plasma membranes and the mitochondrial Na-Ca exchanger have different stereospecific requirements for the binding of diltiazem.

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.

Photoaffinity labelling of the phenylalkylamine receptor of the skeletal muscle transverse-tubule calcium channel

The tritiated arylazido phenylalkylamine (-)-5-[(3-azidophenethyl)[N-methyl-3H]methylamino]-2-(3,4, 5-trimethoxyphenyl)-2-isopropylvaleronitrile was synthesized and used to photoaffinity label the phenylalkylamine receptor of the membrane-bound and purified calcium channel from guinea-pig skeletal muscle transverse-tubule membranes. The photoaffinity ligand binds reversibly to partially purified membranes with a Kd of 2.0 +/- 0.5 nM and a Bmax of 17.0 +/- 0.9 pmol/mg protein. Binding is stereospecifically regulated by all three classes of organic calcium channel drugs. A 155 kDa band was specifically photolabelled in transverse-tubule particulate and purified calcium channel preparations after ultraviolet irradiation. Additional minor labelled polypeptides (92, 60 and 33 kDa) were only observed in membranes. The heterogeneous 155 kDa region of the purified channel was resolved into two distinct silver-stained polypeptides after reduction (i.e. 155 and 135 kDa). Only the 155 kDa polypeptide carries the photoaffinity label and it is concluded that the 135 kDa polypeptide (which migrates as a 165 kDa band under alkylating conditions) is not a high-affinity drug receptor carrying subunit of the skeletal muscle transverse-tubule L-type calcium channel.