Receptor pharmacology of calcium entry blocking agents

Calcium-channel-blockers exhibit divergent regulation of cancer extravasation through the mechanical properties of cancer cells and underlying vascular endothelial cells

Cardiovascular and cancer illnesses often co-exist, share pathological pathways, and complicate therapy. In the context of the potential oncological role of cardiovascular-antihypertensive drugs (AHD), here we examine the role of calcium-channel blocking drugs on mechanics of extravasating cancer cells, choosing two clinically-approved calcium-channel blockers (CCB): Verapamil-hydrochloride and Nifedipine, as model AHD to simultaneously target cancer cells (MCF7 and or MDA231) and an underlying monolayer of endothelial cells (HUVEC). First, live-cell microscopy shows that exposure to Nifedipine increases the spreading-area, migration-distance, and frequency of transmigration of MCF-7 cells through the HUVEC monolayer, whereas Verapamil has the opposite effect. Next, impedance-spectroscopy shows that for monolayers of either endothelial or cancer cells, Nifedipine-treatment alone decreases the impedance of both cases, suggesting compromised cell-cell integrity. Furthermore, upon co-culturing MCF-7 on the HUVEC monolayers, Nifedipine-treated MCF-7 cells exhibit weaker impedance than Verapamil-treated MCF-7 cells. Following, fluorescent staining of CCB-treated cytoskeleton, focal adhesions, and cell-cell junction also indicated that Nifedipine treatment diminished the cell-cell integrity, whereas verapamil treatment preserved the integrity. Since CCBs regulate intracellular Ca²⁺, we next investigated if cancer cell's exposure to CCBs regulates calcium-dependent processes critical to extravasation, specifically traction and mechanics of plasma membrane. Towards this end, first, we quantified the 2D-cellular traction of cells in response to CCBs. Results show that exposure to F-actin depolymerizing drug decreases traction stress significantly only for Nifedipine-treated cells, suggesting an actin-independent mechanism of Verapamil activity. Next, using an optical tweezer to quantify the mechanics of plasma membrane (PM), we observe that under constant, externally-applied tensile strain, PM of Nifedipine-treated cells exhibits smaller relaxation-time than Verapamil and untreated cells. Finally, actin depolymerization significantly decreases MSD only for Verapamil treated cancer-cells and endothelial cells and not for Nifedipine-treated cells. Together, our results show that CCBs can have varied, mechanics-regulating effects on cancer-cell transmigration across endothelial monolayers. A judicious choice of CCBs is critical to minimizing the pro-metastatic effects of antihypertension therapy.

Ethyl 2,7,7-trimethyl-4-(1-methyl-1H-indol-3-yl)-5-oxo-1,4,5,6,7,8-hexa-hydro-quinoline-3-carboxyl-ate

In the title mol­ecule, C₂₄H₂₈N₂O₃, the cyclo­hexene ring is in a sofa conformation and the 1,4-dihydro­pyridine ring is in a slight boat conformation. In the indole ring system, the pyrrole and benzene rings form a dihedral angle of 2.63 (7)°. In the crystal, N—H⋯O hydrogen bonds connect the mol­ecules into C(6) chains parallel to the b axis and pairs of weak C—H⋯O hydrogen bonds link inversion-related chains into a ladder motif through R₂²(18) rings. A weak intra­molecular C—H⋯O hydrogen bond is also observed.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

Dihydropyridines: evaluation of their current and future pharmacological applications

The 1,4-dihydropyridines (DHPs), a class of drugs, possess a wide variety of biological and pharmacological actions, have represented one of the most important groups of calcium-channel-modulating agents and have experienced widespread use in the treatment of cardiovascular disease. Moreover, it has been demonstrated that DHPs could prove to be highly important as multidrug-resistance-reversing agents in cancer chemotherapy. Recent reports suggest that this class also has other notable activities, particularly as antimycobacterial and anticonvulsant agents. Finally, it might be possible for the DHP motif to serve as a scaffold for other pharmacological applications.

Antireproductive effect of calcium channel blockers on male rats

Introduction

Drugs have been shown to adversely affect male fertility and recently anti-hypertensive drugs were added to the list. The anti-fertility effects of nifedipine and similar calcium channel blockers are well-illustrated in in vitro experiments but not in vivo.

Purpose

The present study was designed to experimentally elucidate the sub-chronic effect of nifedipine, verapamil and diltiazem on sperm functions and reproductive hormone levels in vivo.

Methods

Male rats (150-200 g) were divided into four groups of ten rats each. Group 1 (control) received distilled water; Group 2 received nifedipine 0.57 mg/kg BW; Group 3 were given verapamil 3.40 mg/kg BW and Group 4 were given diltiazem 2.57 mg/kg BW. Each drug-treated group had its own recovery group from which treatment was discontinued for 30 days before the animals were sacrificed. Blood samples were collected for hormonal assay of FSH, LH and testosterone. Semen evaluation was done and the testes, seminal vesicle, epididymis and prostate were removed, and weighed immediately.

Results

Nifedipine, verapamil and diltiazem significantly decreased (P < 0.05) sperm count and motility in drug treated groups. The weight of the epididymis was significantly reduced (P < 0.05) in the drug treated rats. Semen parameters and other associated changes were restored after 30 days of drug withdrawal.

Conclusion

Calcium channel blockers appear to have a reversible anti-fertility effect on male rats which does not occur through inhibition of the pituitary-gonadal axis.

Pediatric Toxicology

Pediatric patients present unique concerns in the field of medical toxicology. First, there are medicines that are potentially dangerous to small children, even when they are exposed to very small amounts. Clinicians should be wary of these drugs even when young patients present with accidental ingestions of apparently insignificant amounts. Next, over-the-counter laxatives and syrup of ipecac, although not commonly considered abused substances, may be misused in both the setting of Munchausen's syndrome by proxy and in adolescents who have eating disorders. Their use should be considered in any gastrointestinal illness of uncertain origin. Finally, as the use of syrup of ipecac at home now has been discouraged by many, some have explored using activated charcoal at home as a new method of prehospital gastrointestinal decontamination. The literature examining activated charcoal and its use in this capacity is discussed.

Syntheses and Biological Activities of Chalcone and 1,5-Benzothiazepine Derivatives: Promising New Free-Radical Scavengers, and Esterase, Urease, and?-Glucosidase Inhibitors

A series of 2,4-diaryl-2,3,4,5-tetrahydro- (36-40) and 2,4-diaryl-2,3-dihydro-1,5-benzothiazepines (25-35) have been synthesized from the corresponding chalcones 1-24. Both the benzothiazepines and chalcones were evaluated as DPPH free-radical scavengers and as inhibitors of cholinesterases, urease, and alpha-glucosidase. Compounds 2, 5, 6, 7, 10, 13, 18, 21, 36a, 37a, 37b, and 39a showed significant cholinesterase inhibiting activities. Among the 15 dihydro-1,5-benzothiazepines, 26, 32, and 35 exhibited significant radical-scavenging activities; and six tetrahydro-1,5-benzothiazepines (35, 36a, 36b, 37a, 37b, and 39a) were found to be inhibitors of AChE and BChE. Compounds 22, 25, 26, 33, 35, 36a, 37b, and 39a inhibited urease, and 25 and 27-31 were found to be potent inhibitors of alpha-glucosidase.

Nantenine blocks muscle contraction and Ca2+ transient induced by noradrenaline and K+ in rat vas deferens

The effect of nantenine, an aporphine alkaloid isolated from Ocotea macrophylla H.B.K., was studied on contractions and Ca(2+) translocation induced by noradrenaline, Ca(2+), or K(+) in the isolated rat vas deferens from reserpinized animals. Concentration-response curves of calcium chloride (CaCl(2)) were performed in the vas deferens, in a Ca(2+)-free nutrient solution, using potassium chloride (KCl, 80 mM) as a depolarizing agent. In these conditions, nantenine (2.35 x 10(-4) and 4.7 x 10(-4) M) significantly reduced the maximum contractions (E(max)) of Ca(2+) (IC(50)=2.6 x 10(-4) M) and noradrenaline (IC(50)=2.9 x 10(-4) M). The contractile responses were totally recovered after the withdrawal of nantenine. In addition, experiments performed to measure simultaneously the contraction and the increase of intracellular Ca(2+) induced by noradrenaline (10(-5) M) or KCl (80 mM) showed that nantenine (2.35 x 10(-4) and 4.7 x 10(-4) M) significantly decreased both effects. The results suggest that a reversible block of Ca(2+) entry could be involved on the non-competitive-like antagonism of nantenine in rat vas deferens.

Activation of L-type calcium channel by tolazoline derivatives: role of isothiocyanate moiety

Studies have investigated the pharmacologic mechanism of 2-(4'-isothiocyanatobenzyl) imidazoline (IBI) and analogs for interaction with imidazoline receptors (IRs), alpha-adrenergic receptors (alpha-ARs), and calcium channels in cardiovascular muscle systems. IBI differs from tolazoline by substitution of an electrophilic isothiocyanato (NCS) group. Unlike tolazoline, which is a partial alpha-AR agonist, IBI produced an irreversible, slow-onset, and sustained contraction of rat aorta with an median effective concentration (EC50) value of 5 microM, and a maximal contraction (116%) greater than that of phenylephrine (100%) and tolazoline (59%). The IBI-induced contractions were dependent on calcium channels and independent of alpha-ARs or IRs. Similarly, structure-activity relation studies in rat aortic smooth muscles on a series of synthesized IBI analogs indicated that NCS analogs, but not those without the NCS group, exhibited effects by a non-alpha-AR, non-IR, but a calcium channel-dependent mechanism. Thus the presence of an intact IBI ring in these analogs is not a requirement for these activities. Further, IBI inhibited dihydropyridine (DHP, [3H]PN 200-110 and [3H]Bay K 8644) binding to L-type calcium channels of T-tubule membranes in rabbit skeletal muscle. In contrast to nifedipine, IBI and NCS derivatives (nifedipine-NCS, naphazoline-NCS) only partially (50-88%) displaced specific binding of these radioligands. A single site of noncooperative interaction was observed for nifedipine (nH = 0.97), whereas tolazoline-NCS (IBI, nH = 1.46) and nifedipine-NCS (nH = 1.37) exhibited a positive cooperativity in binding to DHP sites. These receptor-binding data indicate that NCS derivatives bind to L-type calcium channels and interact allosterically with DHP-binding sites. Direct binding of the NCS group to specific nucleophilic protein sites of the calcium channel may be responsible for its activation and the subsequent contractile effects of IBI.

Synthesis and calcium channel modulating effects of isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(thienyl)-5-pyridinecarboxylates

A group of racemic isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(thienyl)-5-pyridinecarboxylates++ + 7a-f were prepared using a modified Hantzsch reaction that involved the condensation of a thienylcarboxaldehyde 4a-f with isopropyl 3-aminocrotonate 5 and nitroacetone 6. In vitro calcium channel antagonist activities were determined using a guinea pig ileum longitudinal smooth muscle (GPILSM) assay. Compounds 7a-f exhibited weaker calcium channel antagonist activity (IC50 = 10(-5) to 10(-7) M range) than the reference drug nifedipine (IC50 = 1.43 x 10(-8) M). The point of attachment of the C-4 thienyl ring system was a determinant of antagonist activity [3-thienyl (7b) > 2-thienyl (7a)]. A 5-substituent in the 2-thienyl moiety influenced antagonist activity where the potency order was 5-bromo-2-thienyl 7f > or = 5-methyl-2-thienyl 7c > 2-thienyl 7a. Although the 5-methyl-2-thienyl 7c and 3-methyl-2-thienyl 7d isomers are equipotent antagonists, the 5-bromo-2-thienyl compound 7f appears to be marginally more active than the 4-bromo-2-thienyl isomer 7e. The 2-thienyl compound 7a, unlike the 3-thienyl isomer 7b, exhibited an agonist effect on GPILSM in the absence of the muscarinic agonist carbachol. Effects of the 2-thienyl 7a and 3-thienyl 7b isomers on the magnitude of calcium current were determined in guinea pig ventricular myocytes with voltage clamp techniques. Results showed that 2-thienyl 7a inhibited calcium current (antagonist) when voltage steps were made from a potential of -40 mV. However, when voltage steps were made from -60 mV, 7a enhanced calcium current (agonist). The 3-thienyl isomer 7b had little, if any, effect on calcium current.

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.

Synthesis and smooth muscle calcium channel effects of dialkyl 1,4-dihydro-2,6-dimethyl-4-aryl-3,5-pyridinedicarboxylates containing a nitrone moiety in the 4-aryl substituent

A group of dialkyl 1,4-dihydro-2,6-dimethyl-4-¿3-(or 4-)[[(Z)-N- oxo-N-[4-substituted-phenylmethylene (or vinylmethylene)]-lambda 5- azanyl]phenyl¿-3,5-pyridinedicarboxylates 7a-n were synthesized. Reaction of the C-4 nitrophenyl compounds 6a-d with an aryl Grignard reagent afforded the corresponding nitrone derivatives 7a-e. Alternatively, reaction of the aryl hydroxylamine compounds 8a-b prepared by reduction of the nitrophenyl compounds 6c-d with Zn/NH4Cl, or the aryl hydroxylamine compounds 8c-d prepared by reduction of the nitrophenyl compounds 6e-f with 5% rhodium-on-charcoal and 65% hydrazine hydrate, with a 4-substituted-benzaldehyde, benzaldehyde or acrolein afforded the respective nitrone compounds 7f-n. In vitro calcium channel (CC) antagonist activities were determined using the guinea pig ileum longitudinal smooth muscle assay. This class of compounds containing a nitrone moiety on the 1,4-dihydropyridine C-4 phenyl ring exhibited CC antagonist activities (10(-5) to 10(-9) M range) relative to the reference drug nifedipine (IC50 = 1.43 x 10(-8) M). Structure-activity relationships showed that the position of the nitrone moiety on the C-4 phenyl ring was a determinant of CC antagonist activity where the potency order was always meta-nitrone > para-nitrone. The effect of the ester alkyl substituent was variable depending upon whether the nitrone substituent was at the meta or para-position (meta-nitrone, Et > i-Pr approximately Me; para-nitrone, i-Pr > Me approximately Et). In the diethyl ester series of compounds having a meta-nitrone moiety, the difference in potency for the various R2-nitrone substituents varied by a factor of 15-fold (IC50 = 1.51 x 10(-7) to 9.84 x 10(-9) M range) (4-Cl-C6H4- > or = 4-Me-C6H4- approximately C6H5- > or = 4-O2N-C6H4- 4-F3C-C6H4- > > CH2 = CH-). Whole-cell voltage-clamp studies using isolated guinea pig ventricular myocytes indicated that the 4-¿3-[(Z)-N-oxo-N-(phenylmethylene)-lambda 5-azanyl]-phenyl¿ compound 7c (10 microM) is a calcium channel antagonist which decreased the calcium current (ICa).

Alleviation of the peripheral hemodynamic effects of dexmedetomidine by the calcium channel blocker isradipine

BACKGROUND

Alpha 2-adrenergic agonists have peripheral vasoconstrictive effects and central sympatholytic and sedative effects. Whereas the latter are the basis of their use in anesthesia, the former could limit their clinical application.

METHODS

To study whether a vasodilator could alleviate the systemic and coronary vasoconstrictor effects of dexmedetomidine without influencing the central sympatholytic effects, the calcium channel blocker isradipine was infused after a high dose of dexmedetomidine in anesthetized dogs.

RESULTS

Dexmedetomidine 10 micrograms.kg-1 decreased plasma concentrations of norepinephrine and epinephrine by more than 90%, heart rate by 39%, cardiac output by 64%, dp/dtmax by 29% and increased mean arterial pressure by 55% and the left ventricular end-diastolic pressure (LVEDP) 4-fold as compared to baseline. In addition, coronary blood flow decreased by 52% and coronary venous oxygen saturation by 51%. Isradipine could completely antagonize all the coronary and systemic hemodynamic changes induced by dexmedetomidine, but only partially he increase in LVEDP. Isradipine caused no changes in plasma catecholamine levels.

CONCLUSION

Isradipine could alleviate the peripheral hemodynamic actions of dexmedetomidine while having no effect on its central sympatholytic properties.

Diltiazem derivatives modulate the dihydropyridine-binding to intact rat ventricular myocytes

To examine whether the modulation of the 1,4-dihydropyridine-binding by diltiazem derivatives, which has been shown in cardiac and skeletal muscle membranes, takes place in intact cardiac myocytes, effects of diltiazem on the specific binding of [3H](+)-PN200-110 to freshly isolated adult rat ventricular myocytes were investigated in normal Tyrode solution at 37 degrees C. Diltiazem consistently potentiated the [3H](+)-PN200-110-binding in a concentration-dependent manner, while DTZ323 (3-(acetyloxy)-5-[2-[[2- (3,4-dimethoxyphenyl)ethyl]-methylamino]ethyl]-2,3-dihydro-2(-4 methoxyphenyl)-1,5-benzothiazepin-4-(5H)-one), a potent diltiazem derivative, inhibited it in a non-competitive manner. In saturation studies, 100 microM decreased the Kd value of the 3[H](+)-PN200-110-binding (control, 0.102 +/- 0.008 vs. diltiazem, 0.074 +/- 0.004 (nM, n = 6), P < 0.05) without significant effect on Bmax (control, 65.7 +/- 6.4 vs. diltiazem, 76.7 +/- 4.4 (fmol/mg protein, n = 6). Moreover, membrane-impermeant quaternary diltiazem also potentiated the [3H](+)-PN200-110-binding in intact myocytes. These results suggest that diltiazem modulates the 1,4-dihydro-pyridine-binding even in intact cardiac myocytes, and the binding site of diltiazem is accessible from the extracellular side of the L-type Ca2+ channels.

Differential and interactive effects of calcium channel blockers and cholesterol content of the diet on jejunal uptake of lipids in rabbits

The present study was undertaken to determine the effects of two classes of calcium channel blockers (CCB), nisoldipine (N) and verapamil (V), on the jejunal uptake of lipids in rabbits. The uptake of cholesterol and long-chain fatty acids into rabbit jejunum was examined after 6 and 36 min of exposure to N or V in vitro ("acute" studies), and after 3-wk feeding of N or V ("chronic" studies). Animals were fed either a low (0.08%) cholesterol diet (LCD) or a high (2.8%) cholesterol diet (HCD), with or without N or V added. Acute in vitro exposure of the jejunum to N or V did not affect the uptake of cholesterol or palmitic acid in rabbits fed LCD or HCD. The effect of N or V feeding depended upon the cholesterol content of the diet; adding N or V to LCD increased cholesterol uptake while adding N or V to HCD enhanced or lowered cholesterol uptake, respectively. Both N and V increased the uptake of stearic acid in LCD. N in HCD had no effect on fatty acid uptake, whereas V lowered the uptake of stearic and linoleic acids and increased the uptake of oleic acid. These changes in lipid uptake were not due to variation in the animals' food intake, body weight gain, or intestinal mucosal surface area. The chronic administration of N or V results in an intestinal adaptative process that alters the jejunal uptake of lipids, the direction of which is influenced by the class of CCB, and by the cholesterol content of the diet. (ABSTRACT TRUNCATED AT 250 WORDS)

Unique pharmacologic properties of amlodipine

Amlodipine is an intrinsically long-acting, vasoselective calcium antagonist structurally related to nifedipine, but with unique binding and pharmacologic properties that distinguish it from other agents of this class. Pharmacokinetic studies in animal models demonstrate a more prolonged half-life, high volume of distribution, and gradual elimination of amlodipine compared with that of other calcium antagonists. The presence of a basic side chain at the 2-position of the dihydropyridine ring renders the molecule > 90% ionized at physiologic pH and is believed to be primarily responsible for its markedly different pharmacologic and pharmacokinetic properties. Amlodipine has slow receptor binding kinetics that result in a gradual onset of action and may allow for less dependence on instantaneous plasma levels. Although amlodipine appears to bind to additional calcium channel recognition sites blocked by diltiazem and verapamil, it does not significantly depress heart rate nor does it produce significant negative inotropic effects or electrophysiologic disturbances. Preclinical studies indicate that amlodipine is a potent antihypertensive agent with natriuretic and diuretic properties that may enhance its ability to reduce blood pressure without attendant fluid retention.

Enantioselective syntheses and calcium channel modulating effects of (+)- and (-)-3-isopropyl 5-(4-methylphenethyll) 1,4-dihydro-2,6-dimethyl-4-(2-pyridyl)-3,5-pyridinedicarboxylates

The (+)- and (-)-enantiomers of 3-isopropyl 5-(4-methylphenethyl) 1,4-dihydro-2,6-dimethyl-4-(2-pyridyl)-3,5-pyridinedicarboxylate were synthesized using an efficient highly enantioselective (ee > or = to 96%) variant of the Hantzsch dihydropyridine synthesis. The key step in this procedure involved the asymmetric Michael addition of a metalated chiral aminocrotonate, derived from D-valine or L-valine, respectively, to the Knoevenagel acceptor (Z)-2-isopropoxycarbonyl-1-(2-pyridyl)-but-1-en-3-one. Both enantiomers exhibited a dual cardioselective partial calcium channel agonist (positive inotropic)/smooth muscle selective calcium channel antagonist effect. The relative in vitro smooth muscle calcium channel antagonist activities of the (-):(+) enantiomers was 26:1. In contrast, the (+)-enantiomer exhibited a greater in vitro positive inotropic effect on guinea pig left atrium where the contractile force was maximally increased by 14.8% at a concentration of 1.63 x 10(-8)M.

Azidobutyryl clentiazem, a new photoactivatable diltiazem analog, labels benzothiazepine binding sites in the alpha 1 subunit of the skeletal muscle calcium channel

[3H]Azidobutyryl clentiazem, a new photoactivable diltiazem derivative, has a higher binding affinity than azidobutyryl diltiazem. It can be covalently incorporated into the alpha 1 subunit of the skeletal muscle calcium channel by UV irradiation, which allows the benzothiazepine binding site to be determined. The photolabeled alpha 1 subunit and its proteolytic fragments were analyzed with a panel of sequence-directed antibodies. The results suggest that the linker region between segment S5 and S6 of domain IV is involved in benzothiazepine binding. This site is different from the dihydropyridine and verapamil binding sites.

Design of a functional calcium channel protein: inferences about an ion channel-forming motif derived from the primary structure of voltage-gated calcium channels

To identify sequence-specific motifs associated with the formation of an ionic pore, we systematically evaluated the channel-forming activity of synthetic peptides with sequence of predicted transmembrane segments of the voltage-gated calcium channel. The amino acid sequence of voltage-gated, dihydropyridine (DHP)-sensitive calcium channels suggests the presence in each of four homologous repeats (I-IV) of six segments (S1-S6) predicted to form membrane-spanning, alpha-helical structures. Only peptides representing amphipathic segments S2 or S3 form channels in lipid bilayers. To generate a functional calcium channel based on a four-helix bundle motif, four-helix bundle proteins representing IVS2 (T4CaIVS2) or IVS3 (T4CaIVS3) were synthesized. Both proteins form cation-selective channels, but with distinct characteristics: the single-channel conductance in 50 mM BaCl2 is 3 pS and 10 pS. For T4CaIVS3, the conductance saturates with increasing concentration of divalent cation. The dissociation constants for Ba2+, Ca2+, and Sr2+ are 13.6 mM, 17.7 mM, and 15.0 mM, respectively. The conductance of T4CaIVS2 does not saturate up to 150 mM salt. Whereas T4CaIVS3 is blocked by microM Ca2+ and Cd2+, T4CaIVS2 is not blocked by divalent cations. Only T4CaIVS3 is modulated by enantiomers of the DHP derivative BayK 8644, demonstrating sequence requirement for specific drug action. Thus, only T4CaIVS3 exhibits pore properties characteristic also of authentic calcium channels. The designed functional calcium channel may provide insights into fundamental mechanisms of ionic permeation and drug action, information that may in turn further our understanding of molecular determinants underlying authentic pore structures.

Identification of 1,4-dihydropyridine binding domains within the primary structure of the alpha 1 subunit of the skeletal muscle L-type calcium channel

Calcium channel blockers are drugs that bind to the alpha 1 subunit of L-type calcium channels and selectively inhibit ion movements through these channels. Determination of the mechanism of channel blockade requires localization of drug-binding sites within the primary structure of the receptor. In this study the 1,4-dihydropyridine-binding site of the membrane bound receptor has been identified. The covalently labeled receptor was purified and digested with trypsin. The labeled peptide fragments were immunoprecipitated with sequence-directed antibodies. The data indicate the existence of at least three distinct dihydropyridine-binding domains within the primary structure of the alpha 1 subunit.

Dexmedetomidine-induced decrease in cerebral blood flow is attenuated by verapamil in rats: a laser Doppler study

This study was performed to examine the changes in local cortical blood flow (CoBF) after simultaneous administration of an alpha 2 adrenergic agonist (dexmedetomidine) and a calcium channel antagonist (verapamil) to urethane-anaesthetized rats. Dexmedetomidine (100 micrograms.kg-1) given intraperitoneally alone resulted in decreases in mean arterial blood pressure (MABP) (F[27,140] = 3.43; P < 0.01) and CoBF (F[27,140] = 4.22; P < 0.01), whereas the heart rate (HR) was increased (F[27,140] = 2.33; P < 0.01). Verapamil (2.5 mg.kg-1) given subcutaneously reduced the MABP (F[27,140] = 3.41; P < 0.01), but the HR and CoBF were not changed. Combined administration of the drugs decreased MAPB (F[27,140] = 5.37; P < 0.01), with no changes in CoBF and HR. The present data indicate that the calcium channel antagonist verapamil did not potentiate the haemodynamic effects of dexmedetomidine in rats, but rather attenuated the effect of dexmedetomidine on CoBF. This favourable interaction suggests a potential therapeutic role of these agents in maintaining cardiovascular stability during surgical interventions.

Effects of the Ca2+ chelators EGTA and EDTA on ethanol- or stress-induced gastric mucosal lesions and gastric secretion

Ca2+ modulates gastric function and dysfunction as well as the release of cysteine proteases and metalloproteinases which have been implicated in the pathogenesis of gastric mucosal lesions. We thus tested the hypothesis that pretreatment with the Ca2+ chelators, ethylene diamine tetraacetic acid (EDTA) and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) might reduce the experimental gastric mucosal damage induced by restraint cold stress or 1.0 ml of 75% ethanol. Other rats were prepared with chronic indwelling gastric cannulas and the effects of EDTA and EGTA on conscious basal gastric acid output were assessed. In addition, rats were pretreated with EGTA or EDTA prior to pylorus ligation and their effects on acid and pepsin output assessed. Both EDTA and EGTA reduced significantly the extent of ethanol-induced gastric mucosal damage as well as the degree of stress-induced gastric lesions. To further characterize the mechanism of Ca2+ chelator protection against ethanol-induced gastric lesions, some rats were vagotomized or adrenalectomized prior to treatment with EGTA or EDTA, followed by ethanol or stress. Both adrenalectomy and vagotomy abolished gastroprotection by EGTA and slightly reduced that induced by EDTA in both models of experimental gastric mucosal injury. Both EDTA and EGTA reduced significantly basal gastric acid output, an effect which persisted for at least 2 h following their administration. Both compounds also decreased significantly acid and pepsin output in pylorus-ligated rats. We conclude that Ca2+ chelators attenuate both acid-dependent and acid-independent gastric lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diltiazem and verapamil on (+)-PN 200-110 binding kinetics in dog cardiac membranes

The effects of d-cis-diltiazem (diltiazem) and verapamil on 1,4-dihydropyridine binding to dog cardiac membranes were studied in competition, saturation and kinetic binding experiments with [3H](+)-PN200-110. Diltiazem increased [3H](+)-PN200-110 binding with an observed maximal effect at 50 microM, while verapamil decreased [3H](+)-PN200-110 binding in a dose-dependent manner. Scatchard analysis of saturation binding data revealed that diltiazem (50 microM) increased the maximal binding site density and verapamil (100 microM) increased the dissociation constant (KD) of [3H](+)-PN200-110 binding. The kinetic experiments demonstrated that diltiazem significantly reduced both the association and the dissociation rate of [3H](+)-PN200-110 binding, resulting in no significant change in the apparent KD. In contrast, verapamil accelerated dissociation and slowed down association of [3H](+)-PN200-110 binding. Diltiazem appears to alter both the number of [3H](+)-PN200-110 binding sites and the characteristics of [3H](+)-PN200-110 binding.

A molecular blueprint for the pore-forming structure of voltage-gated calcium channels

A protein that imitates the sequence of a highly conserved segment predicted to line the pore of dihydropyridine-sensitive L-type calcium channels was designed and synthesized. Single-channel conductance properties were studied in planar lipid bilayers. The synthetic protein emulates the ionic conductance, ionic selectivity, and pharmacological properties of the authentic calcium channel, including the stereospecific action of agonist and antagonist enantiomers of the dihydropyridine BayK 8644. The identified sequence is identical in L-type calcium channels from skeletal muscle and isoforms of cardiac muscle, brain, and aorta. It is plausible that this structural motif represents the molecular blueprint for the pore-forming structure of voltage-gated calcium channels.

Calcium antagonism by KB-2796, a new diphenylpiperazine analogue, in dog vascular smooth muscle

The effects of KB-2796, a new diphenylpiperazine analogue, on [3H]nitrendipine ([3H]NTD) binding, KCl-induced contraction and 45Ca influx has been examined in dog vascular smooth muscle, and compared with those of other diphenylpiperazines. In the binding study, [3H]NTD was found to bind with a high affinity to a single class of sites on aortic membranes (Kd = 0.41 nM and Bmax = 31 fmol (mg protein)-1). KB-2796 inhibited specific [3H]NTD binding in a concentration-dependent manner, with a Ki value of 0.34 microM. The other diphenylpiperazine derivatives such as flunarizine and cinnarizine also inhibited binding in the same manner. Also, in the contraction study, all the diphenylpiperazines antagonized the 50 mM KCl-induced contraction of isolated mesenteric arteries concentration-dependently. The IC50 values of the compounds for KCl-induced contraction correlated strongly with the respective Ki values obtained in the [3H]NTD binding study. In the 45Ca influx study, KB-2796 also effectively inhibited KCl-induced 45Ca influx in mesenteric arteries, with an IC50 value of 0.14 microM. This was close to the IC50 value found in the KCl-induced contraction study. These findings suggest that calcium antagonism by KB-2796 is responsible for its vasorelaxing action in vascular smooth muscle.

Diltiazem and verapamil lower blood pressure in the unanaesthetized rat through CNS mechanisms involving endogenous opioids

1. To evaluate and compare the effects of the calcium channel blockers, diltiazem and verapamil, on CNS modulation of blood pressure, unanaesthetized and unrestrained rats with catheters previously inserted into the lateral cerebral ventricle and femoral artery received intracerebroventricular (i.c.v.) administration of diltiazem or verapamil, 10 or 50 micrograms/kg, or their diluent. 2. Diltiazem, at both 10 and 50 micrograms/kg i.c.v., produced significant (P less than 0.05) decreases in systolic and diastolic blood pressure and heart rate. Verapamil, at 50 micrograms/kg but not at 10 micrograms/kg i.c.v., produced a significant (P less than 0.05) decrease in blood pressure, while both doses significantly (P less than 0.05) decreased heart rate. 3. To examine the endogenous opioid systems as potential modulators of the effects of these calcium antagonists, the mu opioid antagonist naloxone, 20 micrograms/kg, was administered i.c.v. either before or after each calcium antagonist. Naloxone reversed and prevented the reduction in blood pressure produced by both agents. The decrease in heart rate produced by verapamil but not diltiazem was reversed by naloxone. 4. The results suggest that: (1) calcium channels in neuron membranes in the CNS play a role in blood pressure regulation; (2) at least part of the blood pressure reduction produced by calcium blockers may be effected in the CNS; and (3) central opioid mechanisms modulate part of the action of the calcium antagonists verapamil and diltiazem on blood pressure.

The effect of alteration of extracellular Na+ or Ca2+ and inhibition of Ca2+ entry, Na(+)-H+ exchange, and Na(+)-Ca2+ exchange by diltiazem, amiloride, and dichlorobenzamil on the response of cardiac cell aggregates to epidermal growth factor

The purpose of this study was to examine the effect of epidermal growth factor (EGF) on cardiac function and to explore ionic mechanisms as potential explanations for EGF-induced changes in cardiac contractile frequency. Cardiac cell aggregates were prepared from 7-day-old chick embryo hearts and were maintained in culture. EGF over a concentration range of 5 to 20 ng/ml produced a dose-dependent increase in cardiac contractile frequency. Inhibition of Na(+)-H+ exchange by amiloride antagonized the action of EGF. Inhibition of Na(+)-Ca2+ exchange by dichlorobenzamil prevented the effects of EGF. Inhibition of voltage-dependent calcium influx by diltiazem also antagonized the effect of EGF. The positive chronotropic action of EGF was significantly enhanced when the concentration of Na+ or Ca2+ was increased in the medium. These data indicate that EGF has a definite dose-dependent effect on the cardiac contractile frequency that is operative through ionic transport mechanisms that include increased calcium entry through voltage-dependent calcium channels and stimulation of Na(+)-H+ and Na(+)-Ca2+ exchange. The similarity in the effects of inhibition of these three ionic mechanisms suggests they are interrelated so that interference at any step in the process inhibits the action of EGF on cardiac myocytes.

Vascular effects of calcium antagonists: implications for hypertension and other risk factors for coronary heart disease

All calcium antagonists (CAs) so far developed are vasodilators, and this property is a most important component of their therapeutic potency in hypertension and angina pectoris. At a cellular level, the specific interaction of CAs with transmembranous calcium fluxes involves both potential and receptor-operated channels, respectively. Both alpha 2 and alpha 1 adrenoceptors when activated with an appropriate agonist can trigger the calcium influx through receptor-operated CA channels, alpha 2 adrenoceptors probably more readily so than alpha 1. More recently, angiotensin II receptors have also been demonstrated to be involved, although moderately, in the influx of calcium ions from the extracellular space. The hemodynamic profile of CAs is characterized by a particular specificity for the resistance vessels and for the coronary arterial system, as a useful basis for their therapeutic effect in hypertension and in angina pectoris. The weak natriuretic activity of CAs, probably the result of a tubular effect in the kidney, counteracts the fluid retention to be expected for vasodilator drugs. Interesting ancillary properties of CAs are their potentially favorable effects on the myocardial and vascular hypertrophy associated with long-standing hypertension, as well as their antiatherogenic activity that so far has only been demonstrated in animal models. Such additional properties are of potential benefit and deserve further research, since most large-scale hypertension trials have shown that vasodilatation and the reduction of elevated blood pressure as such, are probably not sufficient to adequately protect hypertensive patients against coronary events.

Calcium channel modulators: effects on gastric function

Calcium is important in stimulus-secretion coupling in the gut. We therefore examined a dihydropyridine (nitrendipine), a phenylalkylamine (verapamil) and a benzothiazepine (diltiazem) calcium channel blocker as well as a calcium channel 'agonist', CGP 28392, in several models of gastric function. Nitrendipine significantly decreased stress-induced gastric lesions, but was far less efficacious against 100% ethanol-induced lesions. The anti-ulcer effects of nitrendipine were not reversible with indomethacin, sodium meclofenamate or N-ethylmaleimide. Nitrendipine also significantly reduced basal gastric acid output. Verapamil significantly reduced stress lesions in an indomethacin and meclofenamate-reversible manner, but worsened ethanol ulcers. Verapamil also decreased basal acid secretion. Diltiazem decreased stress lesions (indomethacin- and meclofenamate-reversible), worsened ethanol lesions and slightly reduced acid secretion. CGP 28392 exerted a modest stress gastroprotective effect, decreased ethanol lesions and reduced slightly basal acid secretion. A possible clinical role for gastric calcium channel blockade is raised by these results.

Diltiazem, verapamil, and nifedipine inhibit theophylline-enhanced diaphragmatic contractility

The ability of theophylline to limit diaphragmatic fatigue and to improve contractility appears to be dependent upon alterations in calcium metabolism. The mechanism responsible for these actions, however, remains unclear. We used perfused, contracting, intact rat diaphragm to measure the influence of therapeutic theophylline levels (10(-4) M) on tension development, in the presence or absence of the three most commonly used calcium channel blocking agents, or in the absence of calcium. Concentrations of diltiazem (10(-4) M), verapamil (10(-5) M), or nifedipine (10(-6) M) sufficient to completely block transmembrane calcium channels, were used. During the experiment, each diaphragm preparation was subjected to one of two fatiguing procedures: one to mimic that encountered during tachypnea and one to mimic that encountered during increased respiratory resistance. Our findings showed that therapeutic levels of theophylline were related to statistically significant (p less than 0.0005) increases in diaphragmatic contractility under control conditions and to statistically significant reductions in the sensitivity of the preparation to fatigue. All three calcium channel blockers negated the positive influence of theophylline. Zero calcium also prevented theophylline from enhancing the contractile properties of the diaphragm. It is our conclusion that theophylline enhances the contractile properties of the diaphragm by altering the transmembrane movement of calcium. Calcium antagonists, in turn, inhibit the beneficial effects of theophylline on diaphragm function.

Receptors for calcium antagonists

Calcium antagonists have been divided into 3 different subclasses represented by nifedipine, verapamil and diltiazem. These drugs have different pharmacologic effects and are not interchangeable. Previous studies suggested that all calcium antagonists bind to a 170 kd polypeptide (now called the alpha 2 subunit of the voltage-dependent calcium channel). The apparent molecular weight of this polypeptide characteristically decreased from 170 to 140 kd upon disulfide reduction as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Recent studies demonstrated that calcium antagonists bind to a previously unrecognized 165 kd polypeptide (alpha 1 subunit) that does not change its electrophoretic mobility on disulfide reduction. Because of their similar molecular weights, the 2 polypeptides may overlap each other on polyacrylamide gels. The primary structure of both polypeptides clearly shows, however, that they are different from each other and only the alpha 1 subunit has the features expected of an ion channel.

Effects of KB-2796, a new calcium antagonist, and other diphenylpiperazines on [3H]nitrendipine binding

The effect of KB-2796, a new diphenylpiperazine calcium antagonist, on [3H]nitrendipine ([3H]NTD) binding was investigated in synaptosomal membranes prepared from the guinea pig cerebral cortex. KB-2796 inhibited [3H]NTD binding in a dose-dependent manner with an IC50 value of 86 nM. In this respect, KB-2796 was the most potent among the diphenylpiperazine derivatives tested. Saturation binding data indicated that this inhibition resulted from a decrease in the binding affinity without changes in the maximal number of binding sites. KB-2796, however, significantly increased the dissociation rate constant of [3H]NTD from radiolabeled membranes. This finding suggests that KB-2796 inhibits [3H]NTD binding by a negative heterotropic allosteric mechanism. Other diphenylpiperazines tested also showed similar inhibitory properties. Diphenylpiperazines may act at a site, which is different from the 1,4-dihydropyridine binding site, on the voltage-dependent calcium channel.

The purified Ca2+ antagonist receptor from skeletal muscle: subunit structure, photoaffinity labeling and endogenous protein kinase activity

Tritiated analogues of the Ca2+ channel blockers such as [3H] PN200-110, [3H] verapamil and [3H] diltiazem have been used to identify and isolate Ca2+ antagonist receptors. The Ca2+ antagonist binding sites were solubilized from skeletal muscle transverse tubules with the detergent CHAPS and purified by wheat germ lectin column chromatography and sucrose density gradient centrifugation. The isolated proteins retained their ability to bind the various classes of Ca2+ channel blockers. Polypeptides of 170, 150, 108, 56, and 32 kDa were found to be present in the purified receptor fraction when analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis under non-reducing conditions. The apparent molecular weight of the 170 kDa polypeptide changed to 145 kDa in the presence of reducing agents, as where the apparent molecular weight of the 150, 108, 56 and 32 kDa peptides remained unchanged. An endogenous protein-kinase present in the original membranes, co-purified with the receptor and stimulated the phosphorylation of the 150 and 56 kDa polypeptides in the isolated fraction.