Molecular localization of regions in the L-type calcium channel critical for dihydropyridine action

A Drosophila calcium channel alpha1 subunit gene maps to a genetic locus associated with behavioral and visual defects

We have cloned cDNAs that encode a complete open reading frame for a calcium channel alpha1 subunit from Drosophila melanogaster. The deduced 1851 amino acid protein belongs to the superfamily of voltage-gated sodium and calcium channels. Phylogenetic analysis shows that the sequence of this subunit is relatively distant from sodium channel alpha subunits and most similar to genes encoding the A, B, and E isoforms of calcium channel alpha1 subunits. To indicate its similarity to this subfamily of vertebrate isoforms, we name this protein Dmca1A, for Drosophila melanogaster calcium channel alpha1 subunit, type A. Northern blot analysis detected a single 10. 5 kb transcript class that is regulated developmentally, with expression peaks in the first larval instar, midpupal, and late pupal stages. In late-stage embryos, Dmca1A is expressed preferentially in the nervous system. Variant transcripts are generated by alternative splicing. In addition, single nucleotide variations between cDNAs and genomic sequence are consistent with RNA editing. Dmca1A maps to a chromosomal region implicated in, and is the likely candidate for, the gene involved in the generation of behavioral, physiological, and lethal phenotypes of the cacophony, nightblind-A, and lethal(1)L13 mutants.

Motif III S5 of L-type calcium channels is involved in the dihydropyridine binding site. A combined radioligand binding and electrophysiological study

The alpha1 subunit of L-type voltage-dependent Ca2+ channels (alpha1C) has been shown to harbor high affinity binding sites for the Ca2+ channel dihydropyridine (DHP) modulators. It has been suggested by a number of investigators that the binding site may be composed of III S6 and IV S6. Evidence with chimeric channels indicated the possible involvement of III S5 in DHP binding. Site-directed mutations were introduced in motif III S5 region of the alpha1C, changing the amino acids to their counterparts in the DHP-insensitive alpha1A channel. The mutant channels were expressed both in HEK 293 cells and in Xenopus oocytes. Equilibrium binding and electrophysiological studies showed that the Thr1006 to Tyr substitution produced a mutant channel with at least 1000-fold decreased affinity in [3H](+)isopropyl-4-(2,1, 3-benzoxadiazol-4-yl)-1,4-dihydro-(2, 6-dimethyl-5-methoxycarbonyl)pyridine-3-carboxylate (PN200-110, isradipine) binding and in sensitivity of R(-)-4(2,1, 3-benzoxadiazol-4-yl)-1,4-dihydro-2, 6-dimethyl-5-nitro-3-pyridincarboxylic acid isopropylester (R202-791) in terms of inhibition of current through the L-type voltage-dependent Ca2+ channels. Replacing Gln1010 with Met resulted in more than a 10-fold decrease in binding affinity for [3H](+)PN200-110 and in the potency of channel modulation by S202-791. Four additional mutations in this region also lead to a slight but statistically significant increase of KD values for [3H](+)PN200-110 binding. The binding and electrophysiological results show that certain residues of the transmembrane segment III S5 are important in contributing to the DHP binding "pocket" and are critical for DHP binding and for its calcium channel effect.

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.

Two amino acid residues in the IIIS5 segment of L-type calcium channels differentially contribute to 1,4-dihydropyridine sensitivity

The transmembrane segment IIIS5 of the L-type calcium channel alpha1 subunit participates in the formation of the 1,4-dihydropyridine (DHP) interaction domain (Grabner, M., Wang, Z., Hering, S., Striessnig, J., and Glossmann, H. (1996) Neuron 16, 207-218). We applied mutational analysis to identify amino acid residues within this segment that contribute to DHP sensitivity. DHP agonist and antagonist modulation of Ba2+ inward currents was assessed after coexpression of chimeric and mutant calcium channel alpha1 subunits with alpha2delta and beta1a subunits in Xenopus oocytes. Whereas DHP antagonists required Thr-1066, DHP agonist modulation crucially depended on the additional presence of Gln-1070 (numbering according to alpha1C-a), which also further increased the sensitivity to DHP antagonists. Asp-955, which is found at the corresponding position in the calcium channel alpha1S subunit from carp skeletal muscle, displayed functional similarity to Gln-1070 with respect to DHP interaction. We conclude that these residues (Thr-1066 plus Gln-1070 or Asp-955), which are located in close vicinity on the same side of the putative alpha-helix of transmembrane segment IIIS5, form a crucial DHP binding motif.

Transfer of high sensitivity for benzothiazepines from L-type to class A (BI) calcium channels

To investigate the molecular basis of the calcium channel block by diltiazem, we transferred amino acids of the highly sensitive and stereoselective L-type (alpha1S or alpha1C) to a weakly sensitive, nonstereoselective class A (alpha1A) calcium channel. Transfer of three amino acids of transmembrane segment IVS6 of L-type alpha1 into the alpha1A subunit (I1804Y, S1808A, and M1811I) was sufficient to support a use-dependent block by diltiazem and by the phenylalkylamine (-)-gallopamil after expression in Xenopus oocytes. An additional mutation F1805M increased the sensitivity for (-)-gallopamil but not for diltiazem. Our data suggest that the receptor domains for diltiazem and gallopamil have common but not identical molecular determinants in transmembrane segment IVS6. These mutations also identified single amino acid residues in segment IVS6 that are important for class A channel inactivation.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Binding constants determined from Ca2+ current responses to rapid applications and washouts of nifedipine in frog cardiac myocytes

1. A fast perfusion system was used to analyse the kinetics of the response of L-type calcium current (ICa) to rapid applications and washouts of the dihydropyridine antagonist nifedipine in whole-cell patch-clamped frog ventricular myocytes. 2. Both the inhibition of ICa induced by nifedipine and the recovery from inhibition upon washout of the drug behaved as mono-exponential functions of time. 3. During application or washout of 100 nM nifedipine, only the peak amplitude of ICa varied but not its time course of activation or inactivation. 4. The rate constant of the onset of ICa inhibition increased with the concentration of nifedipine. However, the time course of the recovery from inhibition was independent of drug concentration. 5. Both rate constants were strongly sensitive to the holding potential but insensitive to the test potential. 6. Using simple rate equations and a one-binding-site analysis it was possible to determine the rate constants for association (k1) and dissociation (k-1) and the equilibrium dissociation constant (KD) of the reaction between nifedipine and Ca2+ channels. KD values for nifedipine were identical to IC50 values obtained from classical steady-state experiments. 7. With depolarized holding potentials, KD decreased strongly due to a large reduction in k-1 and a modest increase in k1. Assuming that these changes result from the distribution of Ca2+ channels between resting and inactivated states, a low-affinity binding to the resting state (R) and a high-affinity binding to the inactivated state (I) were obtained with the binding constants: k1R = 1.0 x 10(6) M-1 S-1, k-1R = 0.077 S-1, and KDR = 77 nM for the resting state; k1I = 4.47 x 10(6) M-1 S-1, k-1I = 7.7 x 10(-4) S-1, and KDI = 0.17 nM for the inactivated state. 8. Rapid application/washout experiments provide a unique way to determine, in an intact cell and in a relatively short period (2-4 min), the binding rate constants and the KD value of the reaction between a dihydropyridine antagonist and the Ca2+ channels.

Effects of calciseptine on unitary barium channel currents in guinea-pig portal vein

Effects of synthesized calciseptine (CaS), found naturally in the venom of the black mamba, on voltage-dependent Ca2+ channels in smooth muscle cells of the guinea-pig portal vein were investigated. In the whole-cell voltage-clamp configuration, extracellular application of CaS (>/= 10 nM) inhibited the inward current in a concentration- and voltage-dependent manner at a holding potential of -90 mV. The Ca2+ current recorded at a high holding potential (-50 mV) was approximately 8 times more sensitive to CaS than that at a more negative holding potential (-90 mV). CaS (50 nM) shifted to the left the steady-state inactivation curve obtained by using single 8-s conditioning pulses of various amplitudes. When CaS (>/= 200 nM) was present in the pipette, the Ca2+ current remained for the duration of the experiments (more than 60 min) in the whole-cell configuration. Two different Ca2+ channel conductances are present in this tissue (25-pS and 12-pS channels). Both channels are blocked by dihydropyridine (DHP) derivatives, but have different sensitivities. In the cell-attached condition, CaS hardly changed the activity of either unitary Ca2+ channel current. To prevent the "run down" of the Ca2+ channels in cell-free conditions, we added cardiac cytosol, a supernatant from homogenized cardiac cells and an endogenous Ca2+ channel activating factor, in the pipette. The unitary Ca2+ channel currents were then recorded using the outside-out membrane patch configuration. Application of CaS (1 microM) in the bath completely blocked the open events of the 25-pS Ca2+ channel. CaS (10 nM) in the bath reduced the mean open time and channel availability, resulting in a decrease in the open probability of the 25-pS channel currents without affecting the amplitude of the single-channel conductance. CaS also reduced the open probability (though less potently) and channel availability of the 12-pS Ca2+ channel without a change in its amplitude. From these results, we conclude that CaS has inhibitory effects on the voltage-dependent Ca2+ current that are similar to those of DHP derivatives and that it acts from the outside of the membrane.

Transfer of L-type calcium channel IVS6 segment increases phenylalkylamine sensitivity of alpha1A

Conditioned ("use-dependent") inhibition by phenylalkylamines (PAAs) is a characteristic property of L- type calcium (Ca2+) channels. To determine the structural elements of the PAA binding domain we transferred sequence stretches of the pore-forming regions of repeat III and/or IV from the skeletal muscle alpha1 subunit (alpha1S) to the class A alpha1 subunit (alpha1A and expressed these chimeras together with beta1a and alpha2/delta subunits in Xenopus oocytes. The corresponding barium currents (IBa) were tested for PAA sensitivity during trains of depolarizing test pulses (conditioned block). IBa of oocytes expressing the alpha1A subunit were only weakly inhibited by PAAs (less than 10% conditioned block of IBa during a 100-ms pulse train of 0.1 Hz). Transfer of the transmembrane segment IVS6 from alpha1S to alpha1A produced an enhancement of PAA sensitivity of the resulting alpha1A/alpha1S chimera comparable to L-type alpha1 subunits (about 35% conditioned block Of IBa during a 100-ms pulse train of 0.1 Hz). Our results demonstrate that substitution of 11 amino acids within the segment RVS6 of alpha1A with the corresponding residues of alpha1S is sufficient to transfer L-type PAA sensitivity into the low sensitive class A Ca2+ channel.

Molecular determinants of high affinity dihydropyridine binding in L-type calcium channels

The pore-forming alpha1 subunit of L-type voltage-gated Ca2+ channels is pharmacologically modulated by dihydropyridine (DHP) Ca2+ antagonists and agonists. Site-directed mutation of amino acids within transmembrane segments IIIS6 and IVS6 to those characteristic of DHP-insensitive channels revealed 2 mutations in IIIS6 (I1049F and I1052F) and 4 mutations in IVS6 (Y1365I, M1366F, I1372M, and I1373L) with increased KD values for (+)-[3H]PN200-110 binding. A tyrosine residue (Y1048) in IIIS6 that is conserved between DHP-sensitive and -insensitive Ca2+ channels was also altered by mutagenesis. Y1048F had a KD for (+)-[3H]PN200-110 binding that was increased 12-fold, and Y1048A had a KD at least 1000-fold higher than that of wild-type. These results support the hypothesis that transmembrane segments IIIS6 and IVS6 both contribute critical amino acid residues to the DHP receptor site and that Tyr-1048 within transmembrane segment IIIS6 is required for high affinity DHP binding, even though it is conserved between DHP-sensitive and -insensitive Ca2+ channels.

Structural model of a synthetic Ca2+ channel with bound Ca2+ ions and dihydropyridine ligand

Grove et al. have demonstrated L-type Ca2+ channel activity of a synthetic channel peptide (SCP) composed of four helices (sequence: DPWNVFDFLI10VIGSIIDVIL20SE) tethered by their C-termini to a nanopeptide template. We sought to obtain the optimal conformations of SCP and locate the binding sites for Ca2+ and for the dihydropyridine ligand nifedipine. Eight Ca2+ ions were added to neutralize the 16 acidic residues in the helices. Eight patterns of the salt bridges between Ca2+ ions and pairs of the acidic residues were calculated by the Monte Carlo-with-energy-minimization (MCM) protocol. In the energetically optimal conformation, two Ca2+ ions were bound to Asp-1 residues at the intracellular side of SCP, and six Ca2+ ions were arrayed in two files at the diametrically opposite sides of the pore, implying a Ca2+ relay mechanism. Nine modes of nifedipine binding to SCP were simulated by the MCM calculations. In the energetically optimal mode, the ligand fits snugly in the pore. The complex is stabilized by Ca2+ bound between two Asp-17 residues and hydrophilic groups of the ligand. The latter substitute water molecules adjacent to Ca2+ in the ligand-free pore and thus do not obstruct Ca2+ relay. The ligand-binding site is proximal to a hydrophobic bracelet of Ile-10 residues whose rotation is sterically hindered. In some conformations, the bracelet is narrow enough to block the permeation of the hydrated Ca2+ ions. The bracelet may thus act as a "gate" in SCP. Nifedipine and (R)-Bay K 8644, which act as blockers of the SCP, extend a side-chain hydrophobic moiety toward the Ile-10 residues. This would stabilize the pore-closing conformation of the gate. In contrast, the channel activator (S)-Bay K 8644 exposes a hydrophilic moiety toward the Ile-10 residues, thus destabilizing the pore-closing conformation of the gate.

Transfer of 1,4-dihydropyridine sensitivity from L-type to class A (BI) calcium channels

L-type Ca2+ channels are characterized by their unique sensitivity to organic Ca2+ channel modulators like the 1,4-dihydropyridines (DHPs). To identify molecular motifs mediating DHP sensitivity, we transferred this sensitivity from L-type Ca2+ channels to the DHP-insensitive class A brain Ca2+ channel, BI-2. Expression of chimeras revealed minimum sequence stretches conferring DHP sensitivity including segments IIIS5, IIIS6, and the connecting linker, as well as the IVS5-IVS6 linker plus segment IVS6. DHP agonist and antagonist effects are determined by different regions within the repeat IV motif. Sequence regions responsible for DHP sensitivity comprise only 9.4% of the overall primary structure of a DHP-sensitive alpha 1A/alpha 1S construct. This chimera fully exhibits the DHP sensitivity of channels formed by L-type alpha 1 subunits. In addition, it displays the electrophysiological properties of alpha 1A, as well as its sensitivity toward the peptide toxins omega-agatoxin IVA and omega-conotoxin MVIIC.

Altered pharmacologic responsiveness of reduced L-type calcium currents in myocytes surviving in the infarcted heart

The pharmacologic responses of macroscopic L-type calcium channel currents to the dihydropyridine agonist, Bay K 8644, and beta-adrenergic receptor stimulation by isoproterenol were studied in myocytes enzymatically dissociated from the epicardial border zone of the arrhythmic 5-day infarcted canine heart (IZs). Calcium currents were recorded at 36 degrees to 37 degrees C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second intervals. A Cs+ -rich and 10 mM EGTA-containing pipette solution and a Na+ -and K+ -free external solutions were used to isolate calcium currents from other contaminating currents. During control, peak ICa,L density was found to be significantly less in IZs (4.0 +/- 1.1 pA/pF) than in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 +/- 1.8 pA/pF). Bay K 8644 (1 micro M) significantly increased peak ICa,L density 3.5-fold above control levels in both NZs (to 22.5 +/- 6.2 pA/pF; n = 7) and IZs (to 12.8 +/- 3.0 pA/pF; n = 5), yet peak ICa,L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak ICa,L were similar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 micro M) was significantly diminished in IZs compared to NZs regardless of whether Ba2+ or Ca2+ ions carried the current. Thus, these results indicate an altered responsiveness to beta-adrenergic stimulation in cells that survive in the infarcted heart. Furthermore, application of forskolin (1 micro M and 10 micro M) or intracellular cAMP (200 micro M), agents known to act downstream of the beta-receptor, also produced a smaller increase in peak IBa density in IZs versus NZs, suggesting that multiple defects exist in the beta-adrenergic signaling pathway of IZs. In conclusion, these studies illustrate that reduced macroscopic calcium currents of cells in the infarcted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.

Dihydropyridines, phenylalkylamines and benzothiazepines block N-, P/Q- and R-type calcium currents

We compared the effects of representative members of three major classes of cardiac L-type channel antagonists, i.e. dihydropyridines (DHPs), phenylalkylamines (PAAs) and benzothiazepines (BTZs) on high-voltage-activated (HVA) Ca2+ channel currents recorded from a holding potential of -100 mV in rat ventricular cells, mouse sensory neurons and rat motoneurons. Nimodipine (DHP), verapamil (PAA) and diltiazem (BTZ) block the cardiac L-type Ca2+ channel current (EC50: 1 microM, 4 microM and 40 microM, respectively). At these concentrations, the drugs could also inhibit HVA Ca2+ channel currents in both sensory and motor neurons. Large blocking effects (> 50%) could be observed at 2-10 times these concentrations. The omega -conotoxin-GVIA-sensitive (omega -CTx-GVIA, N-type), omega -agatoxin-IVA-sensitive (omega -Aga-IVA, P- and Q-types) and non-L-type omega -CTx-GVIA-, omega -Aga-IVA-insensitive (R-types) currents accounted for more than 90% of the global current. Furthermore, our data showed that omega -CTx-GVIA and omega -Aga-IVA spare L-type currents and have only additive blocking effects on neuronal HVA currents. We conclude that DHPs, PAAs and BTZs have substantial inhibitory effects on neuronal non-L-type Ca2+ channels. Inhibitions occur at concentrations that are not maximally active on cardiac L-type Ca2+ channels.

Molecular biology of calcium channels

Pharmacological and electrophysiological studies have established that there are multiple types of voltage-gated Ca2+ channels. Molecular biology has uncovered an even greater number of channel molecules. Thus, the molecular diversity of Ca2+ channels has its basis in the expression of many alpha 1 and beta genes, and also in the splice variants produced from these genes. This ability to mix and match subunits provides the cell with yet another mechanism to control the influx of calcium. Future studies will describe new subunits, the subunit composition of each type of channel, and the cloning of new Ca2+ channel types.

Calcium binding in the pore of L-type calcium channels modulates high affinity dihydropyridine binding

The pore-forming alpha 1 subunit of L-type voltage-gated Ca2+ channels contains a Ca(2+)-binding site that is allosterically coupled to the receptor site for dihydropyridine (DHP) Ca2+ antagonists. Site-directed mutations of conserved Phe and Glu residues in the pore-lining SS1/SS2 segments greatly reduced Ca2+ enhancement of DHP binding. Substitution of Phe-1013 in the alpha 1 subunit from rabbit skeletal muscle (alpha 1S) with Gly (F1013G) as in DHP-insensitive Ca2+ channels caused a 4-fold decrease in sensitivity to Ca2+. Mutation of the Ca(2+)-binding residues Glu-1014 in domain III and Glu-1323 in domain IV to Gln (E1014Q and E1323Q) caused 11- and 35-fold decreases in sensitivity to Ca2+, respectively, as well as decreases in the maximal DHP binding affinities attained at optimal concentrations of Ca2+. DHP binding to the charge-reversal mutation, E1014K, had no sensitivity to Ca2+. Our results demonstrate that high affinity Ca2+ binding to the Glu residues in the SS1/SS2 segments of domains III and IV of alpha 1S stabilizes the DHP receptor site in its high affinity state. We propose a three-state model in which the affinity for DHPs is dependent on the presence of 0, 1, or 2 bound Ca2+ ions at sites in the pore.

Involvement of dihydropyridine receptors in terminating Ca2+ release in rat skeletal myotubes

1. Combined patch-clamp and fura-2 measurements were performed in order to investigate the effect of dihydropyridine (DHP) antagonists on termination of sarcoplasmic reticulum (SR) Ca2+ release in cultured rat skeletal myoballs. 2. Ca2+ transients induced by 10 mM caffeine were curtailed by depolarization (e.g. +20 mV for 1 s) and subsequent repolarization (-70 mV). This phenomenon is termed RISC (repolarization-induced stop of caffeine-induced Ca2+ release). 3. At 0.5 to 1 microM, DHP antagonists (nifedipine or (+)PN200-110) strongly inhibited RISC and also slowed the decay of intracellular Ca2+ concentration ([Ca2+]i) following repolarization after depolarization-induced Ca2+ release (-20 or -10 mV for 5 s). 4. The activation time course of the Ca2+ channel associated with the DHP receptor (DHPR) was investigated by measuring DHP-sensitive Ca2+ channel tail currents, while varying the duration of depolarizing pulses. The tail currents increased with pulse duration and peaked around 0.7, 0.9 and 1.1 s for depolarizations to +70, +40 and +20 mV, respectively. These values are compatible with the activation time course of RISC (0.5-1 s to maximally activate RISC at +20 to +60 mV). 5. These results suggest that the DHPR in T-tubular membranes regulates closing of the ryanodine receptor (RyR)-Ca2+ release channel complex through membrane potential change.

Different voltage-dependent inhibition by dihydropyridines of human Ca2+ channel splice variants

Voltage-dependent inhibition by 1,4-dihydropyridines is a characteristic property of L-type Ca2+ channels. Six out of 50 exons of the channel alpha 1C subunit gene are subjected to alternative splicing, thus generating channel isoform diversity. Using Xenopus oocytes as an expression system, we have found that transmembrane segment IIIS2 of human alpha 1C subunit is involved in the control of voltage dependence of dihydropyridine action. This segment is genetically regulated through alternative splicing of exons 21/22. Site-directed mutagenesis points to two amino acids in IIIS2, which determine the difference of the splice variants in their sensitivities to dihydropyridines. This finding provides new insight into molecular mechanisms of Ca2+ channel inhibition by this important class of drugs.

Functional consequences of sulfhydryl modification in the pore-forming subunits of cardiovascular Ca2+ and Na+ channels

The structure and function of many cysteine-containing proteins critically depend on the oxidation state of the sulfhydryl groups. In such proteins, selective modification of sulfhydryl groups can be used to probe the relation between structure and function. We examined the effects of sulfhydryloxidizing and -reducing agents on the function of the heterologously expressed pore-forming subunits of the cloned rabbit smooth muscle L-type Ca2+ channel and the human cardiac tetrodotoxin-insensitive Na+ channel. The known sequences of the channels suggest the presence of three or four cysteine residues within the putative pores of Ca2+ or Na+ channels, respectively, as well as multiple other cysteines in regions of unknown function. We determined the effects of sulfhydryl modification on Ca2+ and Na+ channel gating and permeation by using the whole-cell and single-channel variants of the patch-clamp technique. Within 10 minutes of exposure to 2,2'-dithiodipyridine (DTDP, a specific lipophilic oxidizer of sulfhydryl groups), Ca2+ current was reduced compared with the control value, with no significant change in the kinetics and no shift in the current-voltage relations. The effect could be readily reversed by 1,4-dithiothreitol (an agent that reduces disulfide bonds). Similar results were obtained by using the hydrophilic sulfhydryl-oxidizing agent thimerosal. The effects were Ca(2+)-channel specific: DTDP induced no changes in expressed human cardiac Na+ current. Single-channel Ba2+ current recordings revealed a reduction in open probability and mean open time by DTDP but no change in single-channel conductance, implying that the reduction of macroscopic Ca2+ current reflects changes in gating and not permeation. In summary, the pore-forming (alpha 1) subunit of the L-type Ca2+ channel contains functionally important free sulfhydryl groups that modulate gating. These free sulfhydryl groups are accessible from the extracellular side by an aqueous pathway.

Molecular determinants of Ca2+ channel function and drug action

Molecular cloning has revealed the existence of six high-voltage activated Ca2+ channel types. Expression studies have shown that basic high-voltage activated channel function, which is typical for the L-(skeletal muscle, cardiac muscle and neuroendocrine tissue), N-, P-, Q- and R-type channels is carried by the corresponding alpha 1 subunits. Auxiliary subunits, such as alpha 2/delta and beta, modulate the kinetics of activation, inactivation, current density and drug binding, thereby creating considerable potential for multiple Ca2+ channel functions. Glutamic acid residues in the pore (P) loops are molecular components that impart high selectivity for Ca+. Binding or pharmacologically active sites for Ca2+ channel drugs have been localized on various segments of the alpha 1 subunit in close proximity to the pore lining. In this article, Gyula Varadi and colleagues review the roles of the different subunits in Ca2+ channel function and suggest that Ca2+ channel drugs act by blocking or, in some cases, activating channel function via binding directly or indirectly to the pore structure of the channel.

Expression of the L-type calcium channel with two different beta subunits and its modulation by Ro 40-5967

The smooth muscle alpha 1Cb subunit of the L-type calcium channel was expressed alone (CHO alpha 1 cell) or together with the skeletal beta 1 (CHO alpha 1 beta 1 cell) subunit or smooth muscle beta 3 (CHO alpha 1 beta 3 cell) subunit in Chinese hamster ovary (CHO) cells. The interaction of the expressed calcium channel with the non-dihydropyridine calcium channel blocker Ro 40-5967 was studied. Ro 40-5967 decreased isradipine binding by an apparent allosteric interaction and blocked the barium inward currents (IBa) in a voltage- and use-dependent manner in all cells. The steady-state inactivation curves were shifted to hyperpolarizing potentials in the presence of Ro 40-5967. The rate of channel inactivation was increased in CHO alpha 1 and CHO alpha 1 beta 3 cells. The shift in the steady-state inactivation curve and the increase in channel inactivation were less pronounced in CHO alpha 1 beta 1 cells than in the other cell lines. Low concentrations of Ro 40-5967 increased IBa by up to 198% in 33% of the CHO alpha 1 beta 1 cells. In addition, higher concentrations of Ro 40-5967 were required to inhibit IBa in 60% of the CHO alpha 1 beta 3 cells. These results suggest that the beta subunits modify the interaction of the non-dihydropyridine Ro 40-5967 with the expressed calcium channel alpha 1 subunit.

Alteration of channel characteristics by exchange of pore-forming regions between two structurally related Ca2+ channels

Several types of structurally homologous high voltage-gated Ca2+ channels (L-, P- and N-type) have been identified via biochemical, pharmacological and electrophysiological techniques. Among these channels, the cardiac L-type and the brain BI-2 Ca2+ channel display significantly different biophysical properties. The BI-2 channel exhibits more rapid voltage-dependent current activation and inactivation and smaller single-channel conductance compared to the L-type Ca2+ channel. To examine the molecular basis for the functional differences between the two structurally related Ca2+ channels, we measured macroscopic and single-channel currents from oocytes injected with wild-type and various chimeric channel alpha 1 subunit cRNAs. The results show that a chimeric channel in which the segment between S5-SS2 in repeat IV of the cardiac L-type Ca2+ channel, was replaced by the corresponding region of the BI-2 channel, exhibited macroscopic current activation and inactivation time-courses and single-channel conductance, characteristic of the BI-2 Ca2+ channel. The voltage-dependence of steady-state inactivation was not affected by the replacement. Chimeras, in which the SS2-S6 segment in repeat III or IV of the cardiac channel was replaced by the corresponding BI-2 sequence, exhibited altered macroscopic current kinetics without changes in single-channel conductance. These results suggest that part of the S5-SS2 segment plays a critical role in determining voltage-dependent current activation and inactivation and single-channel conductance and that the SS2-S6 segment may control voltage-dependent kinetics of the Ca2+ channel.