Antagonist binding sites of voltage-dependent calcium channels

T- and L-Type Calcium Channels Maintain Calcium Oscillations in the Murine Zona Glomerulosa

BACKGROUND

The zona glomerulosa of the adrenal gland is responsible for the synthesis and release of the mineralocorticoid aldosterone. This steroid hormone regulates salt reabsorption in the kidney and blood pressure. The most important stimuli of aldosterone synthesis are the serum concentrations of angiotensin II and potassium. In response to these stimuli, voltage and intracellular calcium levels in the zona glomerulosa oscillate, providing the signal for aldosterone synthesis. It was proposed that the voltage-gated T-type calcium channel CaV3.2 is necessary for the generation of these oscillations. However, Cacna1h knock-out mice have normal plasma aldosterone levels, suggesting additional calcium entry pathways.

METHODS

We used a combination of calcium imaging, patch clamp, and RNA sequencing to investigate calcium influx pathways in the murine zona glomerulosa.

RESULTS

Cacna1h-/- glomerulosa cells still showed calcium oscillations with similar concentrations as wild-type mice. No calcium channels or transporters were upregulated to compensate for the loss of CaV3.2. The calcium oscillations observed were instead dependent on L-type voltage-gated calcium channels. Furthermore, we found that L-type channels can also partially compensate for an acute inhibition of CaV3.2 in wild-type mice. Only inhibition of both T- and L-type calcium channels abolished the increase of intracellular calcium caused by angiotensin II in wild-type.

CONCLUSIONS

Our study demonstrates that T-type calcium channels are not strictly required to maintain glomerulosa calcium oscillations and aldosterone production. Pharmacological inhibition of T-type channels alone will likely not significantly impact aldosterone production in the long term.

5-Oxo-hexahydroquinoline: an attractive scaffold with diverse biological activities

5-Oxo-hexahydroquinoline (5-oxo-HHQ) represents a biologically attractive fused heterocyclic core. Various synthetic analogs of 5-oxo-HHQ have been synthesized and assessed for different biological activities. Some derivatives have exhibited myorelaxant, analgesic, anticancer, antibacterial, antifungal, antitubercular, antimalarial, antioxidant, anti-inflammatory, multidrug resistance reversal, anti-Alzheimer, neuroprotective, antidiabetic, antidyslipidemic and antiosteoporotic activities. This review provides a comprehensive report regarding the preparation and pharmacological characterization of 5-oxo-HHQ derivatives that have been reported so far. This information will be beneficial for medicinal chemists in the field of drug discovery to design and develop new and potent therapeutical agents bearing the 5-oxo-HHQ nucleus.

Synthesis of fused 1,4-dihydropyridines as potential calcium channel blockers

Objective

The aim of this study was to synthesize ten 1,4-dihydropyridine (DHP) derivatives in which substituted cyclohexane rings were fused to the DHP ring and to determine how different ester groups and the benzoyl substituent introduced in 4-phenyl ring affected their calcium channel blocking activity.

Methods

A microwave-assisted one-pot method was applied for the synthesis of compound 1–5 according to a modified Hantzsch reaction. The benzoyl moiety was introduced in the 4-phenyl ring of these dihydropyridines by refluxing with benzoyl chloride in acetone in the presence of anhydrous potassium carbonate. Synthesized products were characterized by elemental analysis, IR, 1H-NMR and 13C-NMR spectroscopy. The inhibitory actions of compounds 1–10 on calcium channel blocking activity were tested on isolated rat aorta preparations.

Results

The obtained pharmacological results showed that although all compounds are potent relaxing agents on isolated rat aorta smooth muscle, introduction of a benzoyloxy substitiuent on the phenyl ring (compound 6–10) decreased the relaxant effect of these compunds.

Conclusion

The reported 1,4-DHP derivatives have calcium channel blocking activity on rat aorta smooth muscle.

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.

Depolarization-dependent contraction increase after birth and preservation following long-term hypoxia in sheep pulmonary arteries

Membrane depolarization is critical to pulmonary arterial (PA) contraction. Both L-type Ca²⁺ channels (Ca_L) and Rho-kinase are important signaling components of this process and mitochondrial and non-mitochondrial generated superoxides can be part of the signaling process. Maturation and long-term hypoxia (LTH) each can modify depolarization-dependent contraction and the role of superoxides. By the use of wire myography, we tested the hypothesis that maturation and LTH increase pulmonary arterial reactivity to high-K⁺-induced membrane depolarization through enhancements in the importance of Ca_Land Rho-kinase-dependent pathways. The data show that maturation, but not LTH, increases contraction to 125 mM KCl (high-K⁺) without altering the EC₅₀. High-K⁺-dependent contraction was inhibited to a similar extent in fetal and adult PA by multiple Ca_L blockers, including 10 μM diltiazem, 10 μM verapamil, and 10 μM nifedipine. Postnatal maturation increased the role for 10 μM nifedipine-sensitive Ca_L, and decreased that for 10 μM Y-27632-sensitive Rho-kinase. In all groups, the combination of nifedipine and Y-27632 effectively inhibited high-K⁺ contraction. Tempol (3 mM) but not 100 μM apocynin slightly reduced contraction in arteries from fetal hypoxic and adult normoxic and hypoxic sheep, indicating a limited role for non-mitochondrial derived superoxide to high-K⁺-induced contraction. Western immunoblot for alpha smooth muscle actin indicated small increases in relative abundance in the adult. The data suggest that while Ca_L therapies more effectively vasodilate PA in adults and rho-kinase therapies are more effective in newborns, combination therapies would provide greater efficacy in both young and mature patients regardless of normoxic or hypoxic conditions.

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.

Exploration of the Ca2+ Interaction Modes of the Nifedipine Calcium Channel Antagonist

A comprehensive study is carried out using quantum chemical computation and molecular dynamics (MD) simulations to gain insight into the interaction between Ca(2+) ions and the most important class of calcium channel antagonists--nifedipine. First, the chelating structures and energetic characters of nifedipine-Ca(2+) in the gas phase are explored, and 25 isomers are found. The most favorable chelating mode is a tridentate one, that is, Ca(2+) binds to two carbonyl O atoms and one nitryl O atom, where Ca(2+) is above the plane of the three O atoms to form a pyramidal structure. Accurate geometric structures, relative stabilities, vertical and adiabatic binding energies, and charge distributions are discussed. The differences in the geometries and energies among these isomers are analyzed from the contributions of chelating sites, electrostatics and polarizations, steric repulsions, and charge distributions. The interconversions among isomers with similar geometries and energies are also investigated because of the importance of the geometric transformation in the biological system. Furthermore, certain numbers of water molecules are added to the nifedipine-Ca(2+) system to probe the effect of water. A detailed study is performed on the hydrated geometries on the basis of the most stable isomer 1. Stepwise hydration can weaken the nifedipine-Ca(2+) interaction, and the chelating sites of nifedipine are gradually replaced by the added water molecules. Hexacoordination is found to be the most favorable geometry no matter how many water molecules were added, which can be verified by the MD simulations. The transfer of water molecules from the inner shell to the outer shell is also supported by MD simulations of the hexahydrated complexes.

Acetylcholine and calcium signalling regulates muscle fibre formation in the zebrafish embryo

Nerve activity is known to be an important regulator of muscle phenotype in the adult, but its contribution to muscle development during embryogenesis remains unresolved. We used the zebrafish embryo and in vivo imaging approaches to address the role of activity-generated signals, acetylcholine and intracellular calcium, in vertebrate slow muscle development. We show that acetylcholine drives initial muscle contraction and embryonic movement via release of intracellular calcium from ryanodine receptors. Inhibition of this activity-dependent pathway at the level of the acetylcholine receptor or ryanodine receptor did not disrupt slow fibre number, elongation or migration but affected myofibril organisation. In mutants lacking functional acetylcholine receptors myofibre length increased and sarcomere length decreased significantly. We propose that calcium is acting via the cytoskeleton to regulate myofibril organisation. Within a myofibre, sarcomere length and number are the key parameters regulating force generation; hence our findings imply a critical role for nerve-mediated calcium signals in the formation of physiologically functional muscle units during development.

Unique structure-activity relationship for 4-isoxazolyl-1,4-dihydropyridines

A series of 4-isoxazolyl-1,4-dihydropyridines (IDs) were prepared and characterized, and their interaction with the calcium channel was studied by patch clamp analysis. The structure-activity relationship (SAR) that emerges is distinct from the 4-aryldihydropyridines (DHPs), and affinity increases dramatically at higher holding potentials. Thus, among the 3'-arylisoxazolyl analogues p-Br > p-Cl >> p-F, and p-Cl > m-Cl > o-Cl >> o-MeO. Four of the analogues were examined by single-crystal X-ray diffractometry, and all were found to adopt an O-exo conformation in the solid state. The calculated barrier to rotation, however, suggests that rotation about the juncture between the heterocyclic rings is plausible under physiological conditions. A variable-temperature NMR study confirmed the computation. With Striessnig's computational sequence homologation procedure, a working hypothesis was derived from the data that explains the unique SAR for IDs.

Block of Ca(2+)-channels by alpha-endosulphine inhibits insulin release

1. alpha-Endosulphine, isolated as an endogenous equivalent for sulphonylureas, is a 121-amino acids protein of 19 kDa apparent molecular mass, member of a cyclic AMP-regulated phosphoprotein family. We have previously shown that alpha-endosulphine inhibits sulphonylurea binding and K(ATP) channel activity, thereby stimulating basal insulin secretion. 2. We now describe that in the perfused rat pancreas, no stimulation was detected and that alpha-endosulphine inhibited glucose stimulated insulin release. This inhibition was dose-dependent and affected both phases of insulin secretion. 3. This inhibitory effect of alpha-endosulphine also occurred on MIN6 beta-cells when insulin release was stimulated either by glucose, sulphonylureas or a high K(+) depolarization. Inhibition was concentration-dependent with a half-maximal inhibition at 0.5 microM and was mirrored by inhibition of calcium influx. 4. Electrophysiological experiments demonstrated, in comparison to the effects of the sulphonylurea tolbutamide, that these inhibitory effects were linked to a direct inhibition of L-type Ca(2+)-channels and were independent from a regulation of K(ATP) channels. 5. Although alpha-endosulphine is able to stimulate insulin release under specific conditions acting via modulation of K(ATP) channel activity, the present study suggests that, under physiological conditions, the peptide mainly acts to block voltage-gated Ca(2+)-channels. This block leads to the inhibition of calcium influx and triggers inhibition of insulin release. 6. We conclude that alpha-endosulphine is not exclusively an endogenous equivalent for sulphonylureas and not solely a K(ATP) channel regulator.

Characterization of voltage-gated calcium currents in freshly isolated smooth muscle cells from rat tail main artery

The aim of the present study was to characterize voltage-gated Ca2+ currents in smooth muscle cells freshly isolated from rat tail main artery in the presence of 5 mmol L(-1) external Ca2+. Calcium currents were identified on the basis of their voltage dependencies and sensitivity to nifedipine, Ni2+ and cinnarizine. In the majority of the cells studied, T- and L-type currents were observed, while the remaining cells showed predominantly L-type currents. In the latter group of cells, holding potential change from -50 to either -70 or -90 mV increased the corresponding inward current amplitude while its voltage activation threshold remained unchanged. The steady state inactivation of L-type Ca2+ channels showed half-maximal inactivation at -38 mV. A Ca2+-dependent inactivation was also evident. Nifedipine (3 micromol L(-1)) blocked L-type but not T-type Ca2+ currents. Ni2+ (50 micromol L(-1)) as well as cinnarizine (1 micromol L(-1)) suppressed the nifedipine-resistant, T-type component of the currents. At higher concentrations, both Ni2+ (0.3-1 mmol L(-1)) and cinnarizine (10 micromol L(-1)) blocked the net inward current. Replacement of Ca2+ with 10 mmol L(-)1 Ba2+ significantly increased the amplitude of L-type Ca2+ currents. These results demonstrate that smooth muscle cells freshly isolated from rat tail main artery may be divided into two populations, one expressing both L- and T-type and the other only L-type Ca2+ channels. Furthermore, this report shows that in arterial smooth muscle cells cinnarizine potently inhibited T-type currents at low concentrations (1 micromol L(-1)) but also blocked L-type Ca2+ currents at higher concentrations (10 micromol L(-1)).

Interactions between presynaptic Ca2+ channels, cytoplasmic messengers and proteins of the synaptic vesicle release complex

Influx of Ca(2+) through presynaptic voltage-gated Ca(2+) channels is a key step in rapid neurotransmitter release. The amount of Ca(2+) entering through these channels is modulated by a plethora of intracellular messenger molecules, including betagamma-subunits of G proteins, and protein kinases. In addition, Ca(2+) channels bind physically to proteins of the vesicle-release machinery in a Ca(2+)-dependent manner, which can, in turn, regulate the activity of Ca(2+) channels. Recent evidence suggests that second messengers and presynaptic vesicle-release proteins do not regulate Ca(2+) channel activity as independent entities, but that there is extensive crosstalk between these two mechanisms. The complex interactions between second messengers, vesicle-release proteins and voltage-gated Ca(2+) channels might provide multiple avenues for fine-tuning Ca(2+) entry into the presynaptic terminal and, consequently, neurotransmission.

Interaction of SNX482 with domains III and IV inhibits activation gating of alpha(1E) (Ca(V)2.3) calcium channels

We have investigated the action of SNX482, a toxin isolated from the venom of the tarantula Hysterocrates gigas, on voltage-dependent calcium channels expressed in tsa-201 cells. Upon application of 200 nM SNX482, R-type alpha(1E) calcium channels underwent rapid and complete inhibition, which was only poorly reversible upon washout. However, upon application of strong membrane depolarizations, rapid and complete recovery from inhibition was obtained. Tail current analysis revealed that SNX482 mediated an approximately 70 mV depolarizing shift in half-activation potential, suggesting that the toxin inhibits alpha(1E) calcium channels by preventing their activation. Experiments involving chimeric channels combining structural features of alpha(1E) and alpha(1C) subunits indicated that the presence of the domain III and IV of alpha(1E) is a prerequisite for a strong gating inhibition. In contrast, L-type alpha(1C) channels underwent incomplete inhibition at saturating concentrations of SNX482 that was paralleled by a small shift in half-activation potential and which could be rapidly reversed, suggesting a less pronounced effect of the toxin on L-type calcium channel gating. We conclude that SNX482 does not exhibit unequivocal specificity for R-type channels, but highly effectively antagonizes their activation.

Voltage-dependent calcium channels--beyond dihydropyridine antagonists

The blockade of L-type calcium channels by dihydropyridines, phenylalkylamines and benzothiazepines has been well described and forms the basis of a multibillion dollar market for the treatment of cardiovascular disease and migraine. More recently, neuron-specific calcium channels have become the subject of intense interest regarding their potential as therapeutic targets for the treatment of chronic and neuropathic pain. A number of recently described agents that selectively target neuronal calcium channels have been described and appear promising for a variety of pain conditions.

Block of voltage-dependent calcium channels by aliphatic monoamines

We have recently identified farnesol, an intermediate in the mevalonate pathway, as a potent endogenous modulator and blocker of N-type calcium channels (Roullet, J. B., R. L. Spaetgens, T. Burlingame, and G. W. Zamponi. 1999. J. Biol. Chem. 274:25439-25446). Here, we investigate the action of structurally related compounds on various types of voltage-dependent Ca(2+) channels transiently expressed in human embryonic kidney cells. 1-Dodecanol, despite sharing the 12-carbon backbone and headgroup of farnesol, exhibited a significantly lower blocking affinity for N-type Ca(2+) channels. Among several additional 12-carbon compounds tested, dodecylamine (DDA) mediated the highest affinity inhibition of N-type channels, indicating that the functional headgroup is a critical determinant of blocking affinity. This inhibition was concentration-dependent and relatively non-discriminatory among N-, L-, P/Q-, and R-Ca(2+) channel subtypes. However, whereas L-type channels exhibited predominantly resting channel block, the non-L-type isoforms showed substantial rapid open channel block manifested by a speeding of the apparent time course of current decay and block of the inactivated state. Consistent with these findings, we observed significant frequency-dependence of block and dependence on external Ba(2+) concentration for N-type, but not L-type, channels. We also systematically investigated the drug structural requirements for N-type channel inhibition. Blocking affinity varied with carbon chain length and showed a clear maximum at C12 and C13, with shorter and longer molecules producing progressively weaker peak current block. Overall, our data indicate that aliphatic monoamines may constitute a novel class of potent inhibitors of voltage-dependent Ca(2+) channels, with block being governed by rigid structural requirements and channel-specific state dependencies.

Modulation of neuronal voltage-gated calcium channels by farnesol

The modulation of presynaptic voltage-dependent calcium channels by classical second messenger molecules such as protein kinase C and G protein betagamma subunits is well established and considered a key factor for the regulation of neurotransmitter release. However, little is known of other endogenous mechanisms that control the activity of these channels. Here, we demonstrate a unique modulation of N-type calcium channels by farnesol, a dephosphorylated intermediate of the mammalian mevalonate pathway. At micromolar concentrations, farnesol acts as a relatively non-discriminatory rapid open channel blocker of all types of high voltage-activated calcium channels, with a mild specificity for L-type channels. However, at 250 nM, farnesol induces an N-type channel-specific hyperpolarizing shift in channel availability that results in approximately 50% inhibition at a typical neuronal resting potential. Additional experiments demonstrated the presence of farnesol in the brain (rodents and humans) at physiologically relevant concentrations (100-800 pmol/g (wet weight)). Altogether, our results indicate that farnesol is a selective, high affinity inhibitor of N-type Ca(2+) channels and raise the possibility that endogenous farnesol and the mevalonate pathway are implicated in neurotransmitter release through regulation of presynaptic voltage-gated Ca(2+) channels.