A unique fluorescent phenylalkylamine probe for L-type Ca2+ channels. Coupling of phenylalkylamine receptors to Ca2+ and dihydropyridine binding sites

Nifedipine Inhibition of High-Voltage Activated Calcium Channel Currents in Cerebral Artery Myocytes Is Influenced by Extracellular Divalent Cations

Voltage-dependent calcium channels (VDCCs) play an essential role in regulating cerebral artery diameter and it is widely appreciated that the L-type VDCC, Ca_V1.2, encoded by the CACNA1C gene, is a principal Ca²⁺ entry pathway in vascular myocytes. However, electrophysiological studies using 10 mM extracellular barium ([Ba²⁺]_o) as a charge carrier have shown that ~20% of VDCC currents in cerebral artery myocytes are insensitive to 1,4-dihydropyridine (1,4-DHP) L-type VDDC inhibitors such as nifedipine. Here, we investigated the hypothesis that the concentration of extracellular divalent cations can influence nifedipine inhibition of VDCC currents. Whole-cell VDCC membrane currents were obtained from freshly isolated rat cerebral artery myocytes in extracellular solutions containing Ba²⁺ and/or Ca²⁺. In the absence of [Ca²⁺]_o, both nifedipine-sensitive and -insensitive calcium currents were observed in 10 mM [Ba²⁺]_o. However, VDCC currents were abolished by nifedipine when using a combination of 10 mM [Ba²⁺]_o and 100 μM [Ca²⁺]_o. VDCC currents were also completely inhibited by nifedipine in either 2 mM [Ba²⁺]_o or 2 mM [Ca²⁺]_o. The biophysical characteristics of all recorded VDCC currents were consistent with properties of a high-voltage activated VDCC, such as Ca_V1.2. Further, VDCC currents recorded in 10 mM [Ba²⁺]_o ± 100 μM [Ca²⁺]_o or 2 mM [Ba²⁺]_o exhibited similar sensitivity to the benzothiazepine L-type VDCC blocker, diltiazem, with complete current inhibition at 100 μM. These data suggest that nifedipine inhibition is influenced by both Ca²⁺ binding to an external site(s) on these channels and surface charge effects related to extracellular divalent cations. In sum, this work demonstrates that the extracellular environment can profoundly impact VDCC current measurements.

Co-localization of putative calcium channels (phenylalkylamine-binding sites) on oil bodies in protoplasts from dark-grown sunflower seedling cotyledons

Oil bodies are spherical entities containing a triacylglycerol (TAG) matrix encased by a phospholipid monolayer, which is stabilized by oil body-specific proteins, principally oleosins. Biochemical investigations in the recent past have also demonstrated the expression of calcium-binding proteins, called caleosins, as a component of oil body membranes during seed germination. Using DM-Bodipy-phenylalkylamine (PAA; a fluorescent derivative of phenylalkylamine)-a fluorescent probe known to bind L-type calcium channel proteins, present investigations provide the first report on the localization and preferential accumulation of putative calcium channel proteins on/around oil bodies during peak lipolytic phase in protoplasts derived from dark-grown sunflower (Helianthus annuus L. cv Morden) seedling cotyledons. Specificity of DM-Bodipy-PAA labeling was confirmed by using bepridil, a non-fluorescent competitor of PAA while non-specific dye accumulation has been ruled out by using Bodipy-FL as control. Co-localization of fluorescence from DM-Bodipy-PAA binding sites (ex: 504 nm; em: 511 nm) and nile red fluorescing oil bodies (ex: 552 nm; em: 636 nm) has been undertaken by epifluorescence and confocal laser scanning microscopy (CLSM). It revealed the affinity of PAA-sensitive ion channels for the oil body surface. Findings from the current investigations highlight the significance of calcium and calcium channel proteins during oil body mobilization in sunflower.

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.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Studies on ion channel antagonist-binding sites in sunflower protoplasts

The cytological location of ion channel antagonist-binding sites was studied in sunflower protoplasts using the fluorescent probes DM-Bodipy-PAA and DM-Bodipy-DHP. The binding specificity of the probes was established by competition experiments with Bepridil, phenylalkylamine (Verapamil) and dihydropyridine (Nifedipine) which are known as calcium and potassium channel antagonists. Quantitative image analysis of the fluorescence emitted by the protoplasts showed the existence of interactions between PAA- and DHP-binding sites. Moreover, studies on the cytolocalization of the PAA receptors by confocal imaging showed that in freshly isolated protoplasts, DM-Bodipy-PAA binds exclusively at sites located in the cortical region of the cell.

Docking of verapamil in a synthetic Ca2+ channel: formation of a ternary complex involving Ca2+ ions

The mechanism by which diverse drugs modulate voltage-dependent Ca2+ channels is ill-understood. We have approached this problem by examining the interaction of verapamil with a 97-residue synthetic channel peptide (SCP) that exhibits functional similarities to authentic L-type Ca2+ channels in terms of cation selectivity and permeation as well as interaction with channel-activating and blocking drugs (Grove et al. (1991) Proc. Natl. Acad. Sci. USA 88, 6418). Different possibilities of binding of verapamil inside the Ca(2+)-bound SCP were simulated using the Monte Carlo-with-energy-minimization method. In the optimal mode of the binding, verapamil adopted a folded conformation and fit snugly in the pore. The dimethoxyphenyl groups of the drug interacted with two Ca2+ ions coordinated to the acidic residues of SCP, thus forming a ternary complex of the drug, Ca2+, and channel. The isopropyl group of verapamil abetted a ring of four Ile residues constituting the putative SCP gate. The occlusion of this gate by verapamil in this manner was strikingly similar to that accomplished by the methyl group of dihydropyridine drugs. In conjunction with an earlier study on SCP bound to dihydropyridine drugs (Zhorov and Ananthanarayanan (1996) Biophys. J. 70, 22), our data suggest that, in general, drug modulation of SCP would involve the interaction of the ligands with the pore-bound Ca2+ and with the hydrophobic gate. In light of the functional similarity between SCP and L-type Ca2+ channel, it is likely that the latter would also interact with drugs in a similar fashion.

L-type calcium channels: binding domains for dihydropyridines and benzothiazepines are located in close proximity to each other

We investigated the binding of a fluorescent diltiazem analogue (3R,4S)-cis-1-[2-[[3-[[3-[4,4-difluoro-3a,4-dihydro-5,7-dimethyl-4-bo ra-3a,4a-diaza-s-indacen-3-yl]propionyl]amino]propyl]amin o]ethy]-1,3,4,5-tetrahydro-3-hydroxy-4-(4-methoxyphenyl)-6-(triflu oromethyl)-2H-1-benzazepin-2-one (DMBODIPY-BAZ) to L-type Ca2+ channels in the presence of different 1,4-dihydropyridines (DHPs) by using fluorescence resonance energy transfer (FRET) [Brauns, T., Cai, Z.-W., Kimball, S. D., Kang, H.-C., Haugland, R. P., Berger, W., Berjukov, S., Hering, S., Glossmann, H., & Striessnig, J. (1995) Biochemistry 34, 3461]. When channels are occupied with DMBODIPY-BAZ, a rapid fluorescence change occurred upon addition of different DHPs. The direction of this intensity modulation was found to be only dependent on the chemical composition of the dihydropyridine employed. DHPs containing a nitro group decreased, whereas others (e.g., isradipine) enhanced the fluorescence signal. In addition, all DHPs markedly decreased the association rate constant for DMBODIPY-BAZ without affecting equilibrium binding. Both observations together are best explained by a steric model where the DHP binding site is located in close proximity to the accession pathway of DMBODIPY-BAZ.

Dihydropyridine receptors in transverse tubules from normal and dystrophic chicken skeletal muscle

Calcium overload is a fundamental pathogenic event associated with chronic muscle degeneration in muscular dystrophies. The possibility that L-type voltage-dependent calcium channels were involved in the etiology of chicken muscular dystrophy was investigated by studying the dihydropyridine receptors in transverse tubule membranes isolated from skeletal muscle of normal (line 412) and dystrophic (line 413) chickens. The yield of T-tubular protein from dystrophic muscle was considerably increased compared with that from normal muscle (2.51 +/- 0.18 vs 1.04 +/- 0.31 mg protein x 100 g muscle-1). The binding of the calcium channel antagonist (+) [3H]PN200-110 to the dihydropyridine receptor in transverse tubule preparations was relatively slow, markedly affected by temperature and required divalent cations. (+) [3H]PN200-110 equilibrium binding assays revealed a single class of high-affinity sites and showed that maximum binding capacity (Bmax) (3.17 +/- 0.47 for normal and 3.51 +/- 0.52 pmol x mg protein-1 for dystrophic transverse tubules) and dissociation constant (Kd) (0.32 +/- 0.07 and 0.26 +/- 0.09 nM, respectively) were not significantly different in normal and dystrophic membranes. Kinetic studies indicated that normal and dystrophic transverse tubules did not differ significantly in association (2.54 x 10(6) and 2.27 x 10(6) M(-1)s(-1), respectively) and dissociation (8.5 x 10(-4) and 9.3 x 10(-4)s(-1), respectively) rate constants. Since dissociation kinetics for both preparations were monoexponential under all the experimental conditions employed, no low-affinity binding sites for (+) [3H]PN200-110 could be detected in chicken transverse tubules membranes. However, immunoblot assay, using a monoclonal antibody, revealed that dystrophic transverse tubules as compared with normal membranes were enriched twofold with the alpha 1-subunit of the dihydropyridine receptor. Therefore, although dihydropyridine-binding sites were not altered in transverse tubule membranes from dystrophic chicken skeletal muscle, both the increased yield in T-tubule vesicles and the enhanced immunodetection of the alpha 1-subunit of the dihydropyridine receptor, suggest that total content in dihydropyridine receptor is higher in dystrophic than in normal muscle.

Coordination of Ca2+ by the pore region glutamates is essential for high-affinity dihydropyridine binding to the cardiac Ca2+ channel alpha 1 subunit

The molecular determinants for Ca2+ modulation of dihydropyridine (DHP) binding to cardiac Ca2+ channels were identified by mutational neutralization of the glutamate residues that comprise the Ca2+ channel selectivity filter. The binding activity of the DHP (+)-[3H]isradipine, monitored after expression of wild-type and mutant alpha 1 subunits in COS-7 cells, was markedly reduced in four single mutants and a double mutant. Evidence for decreased Ca2+ affinity was obtained for two single mutants in kinetic and equilibrium binding studies. Mutational destabilization of Ca2+ binding resulted in a concomitant decrease of (+)-[3H]isradipine binding affinity. Recovery of (+)-[3H]isradipine binding activity by the allosteric modulator (+)-tetrandrine in two single mutants was associated with a recovery of Ca2+ and DHP binding kinetics to wild-type values. Our findings demonstrate that high-affinity DHP binding is dependent on Ca2+ coordination by glutamate residues which form the selectivity filter of the channel pore.

Benzothiazepine binding domain of purified L-type calcium channels: direct labeling using a novel fluorescent diltiazem analogue

We have synthesized a series of N-propylamino-substituted benzazepinones (NPSBs) as specific probes for the benzothiazepinone (BTZ) binding domain of muscle L-type calcium channels (LTCCs). NPSBs were identified which possess high affinity for the channel after purification. We synthesized a fluorescent NPSB, DMBODIPY-BAZ, as the first benz(othi)azepinone derivative known to reversibly label partially purified LTCCs. DMBODIPY-BAZ binds to the partially purified channel with high affinity (Kd = 25 nM, Bmax = 580 pmol/mg of protein). Fluorescence resonance energy transfer (FRET) occurred between tryptophan residues of the channel protein and the DMBODIPY fluorophore upon specific drug binding. FRET was exploited to allow highly time-resolved detection of specific drug binding kinetics. We found that the dissociation half-life (t1/2) of DMBODIPY-BAZ decreased with the concentration of an unlabeled competitor, which indicates ligand-induced accelerated dissociation. In contrast, t1/2 was concentration-dependently increased by the dihydropyridine (DHP) (+)-isradipine. These kinetic properties of DMBODIPY-BAZ indicate that a high-affinity BTZ binding domain also exists on purified LTCCs. NPSBs represent novel tools to provide further insight into the molecular pharmacology of the BTZ binding domain on LTCCs.

Complex molecular mechanism for dihydropyridine binding to L-type Ca(2+)-channels as revealed by fluorescence resonance energy transfer

We analyzed binding-induced changes in the fluorescence properties of the 1,4-dihydropyridine (DHP), DMBODIPY-DHP [(-)-1,4-dihydro-2,6-dimethyl-4-(2-trifluromethylphenyl)- 3,5-pyridinedicarboxylic acid 2-[4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-3- (s-indacene)propionylamino]ethylethyl ester)], to study the molecular mechanisms underlying the interaction of DHPs with the alpha 1-subunit of skeletal muscle L-type Ca2+ channels. The quantum yield of the fluorophore DMBODIPY was similar in solvents of different polarity. In contrast, the quantum yield of DMBODIPY-DHP was low in buffer but increased with solvent polarity and upon specific binding. This indicates the existence of binding-induced changes of intramolecular quenching of the fluorophore by the DHP moiety. Specific ligand binding also induced fluorescence resonance energy transfer (FRET) between one or more tryptophanes of the channel protein and the DMBODIPY-DHP fluorophore. The specific FRET signal was successfully used to directly measure DHP binding at high time resolution. It revealed complex association and dissociation kinetics of DMBODIPY-DHP although no site heterogeneity was detected in equilibrium experiments. We therefore fitted our data to a binding scheme considering one or more intermediate conformational states for the formation of the ligand-receptor complex. Such a step-wise binding mechanism explains previously observed differences in the binding site densities and the kinetic constants determined for different DHPs using conventional binding (for example filtration) assays.

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.

Calcium channels: the beta-subunit increases the affinity of dihydropyridine and Ca2+ binding sites of the alpha 1-subunit

A Ca2+ channel alpha 1-subunit derived from rabbit heart was transiently expressed in COS-7 cells. The dihydropyridine (+)-isradipine had low affinity (Ki = 34.3 nM) for the alpha 1-subunit in the absence of the beta-subunit due to rapid dissociation (k-1 = 0.11 min-1). Co-expression of the beta-subunit resulted in a > 35-fold increase in (+)-isradipine binding affinity (Ki = 0.9 nM) due to decreased dissociation (k-1 of 0.007 min-1). Higher DHP binding affinity was associated with an increase of the apparent affinity of Ca2+ ions for the channel. Our data suggest that the beta-subunit affects the coordination of Ca2+ ions with sites that are coupled to the dihydropyridine binding domain and by this mechanism increases the affinity for these ligands.

Synthesis and properties of fluorescent beta-adrenoceptor ligands

We describe the synthesis of bordifluoropyrromethene (BODIPY), fluorescein, and related fluorescent derivatives of the beta-adrenergic ligand CGP 12177. With these probes we screened insect (Sf9) cells stably transformed with the human beta 2-adrenoceptor gene and expressing (2-3.5) x 10(5) human beta 2-adrenoceptors per cell. Among these derivatives only BODIPY-CGP gave a receptor-specific signal sufficiently strong for measuring the on- and off-rate constants and the equilibrium dissociation constant of beta-adrenoceptor-specific binding by spectrofluorometry or photon counting. Similar KD values for BODIPY-CGP binding were obtained by kinetic measurements (approx. 250 pM) and under equilibrium conditions (400 +/- 180 pM), and these were in the same range as those obtained with [3H]CGP 12177 (200 +/- 32 pM). The cell-bound fluorescence could be quenched specifically with nonfluorescent CGP 12177 to near background levels. The disposition of the beta 2-adrenoceptors in BODIPY-CGP-stained Sf9 cells was mainly restricted to the cell surface at 4 and 30 degrees C. Hence, beta-adrenoceptor-expressing cells can be stained specifically with BODIPY-CGP, and beta-adrenoceptors on a single cell can be assessed by photon counting under the fluorescence microscope. Cells can also be scanned by fluorescence-activated flow cytometry.