Structural basis of drug binding to L Ca2+ channels

What can naturally occurring mutations tell us about Ca(v)1.x channel function?

Voltage-gated Ca^2 + channels allow for Ca^2 +-dependent intracellular signaling by directly mediating Ca^2 + ion influx, by physical coupling to intracellular Ca^2 + release channels or functional coupling to other ion channels such as Ca^2 + activated potassium channels. L-type Ca^2 + channels that comprise the family of Ca_v1 channels are expressed in many electrically excitable tissues and are characterized by their unique sensitivity to dihydropyridines. In this issue, we summarize genetic defects in L-type Ca^2 + channels and analyze their role in human diseases (Ca^2 + channelopathies); e.g. mutations in Ca_v1.2 α1 cause Timothy and Brugada syndrome, mutations in Ca_v1.3 α1 are linked to sinoatrial node dysfunction and deafness while mutations in Ca_v1.4 α1 are associated with X-linked retinal disorders such as an incomplete form of congenital stationary night blindness. Herein, we also put the mutations underlying the channel's dysfunction into the structural context of the pore-forming α1 subunit. This analysis highlights the importance of combining functional data with structural analysis to gain a deeper understanding for the disease pathophysiology as well as for physiological channel function. This article is part of a Special Issue entitled: Calcium channels.

Repertoire of high voltage-activated Ca2+ channels in the lateral superior olive: functional analysis in wild-type, Cav1.3−/−, and Cav1.2DHP−/− mice

Voltage-gated Ca2+ (Cav)1.3 α-subunits of high voltage-activated Ca2+ channels (HVACCs) are essential for Ca2+ influx and transmitter release in cochlear inner hair cells and therefore for signal transmission into the central auditory pathway. Their absence leads to deafness and to striking structural changes in the auditory brain stem, particularly in the lateral superior olive (LSO). Here, we analyzed the contribution of various types of HVACCs to the total Ca2+ current ( ICa) in developing mouse LSO neurons to address several questions: do LSO neurons express functional Cav1.3 channels? What other types of HVACCs are expressed? Are there developmental changes? Do LSO neurons of Cav1.3−/− mice show any compensatory responses, namely, upregulation of other HVACCs? Our electrophysiological and pharmacological results showed the presence of functional Cav1.3 and Cav1.2 channels at both postnatal days 4 and 12. Aside from these L-type channels, LSO neurons also expressed functional P/Q-type, N-type, and, most likely, R-type channels. The relative contribution of the four different subtypes to ICa appeared to be 45%, 29%, 22%, and 4% at postnatal day 12, respectively. The physiological results were flanked and extended by quantitative RT-PCR data. Altogether, LSO neurons displayed a broad repertoire of HVACC subtypes. Genetic ablation of Cav1.3 resulted in functional reorganization of some other HVACCs but did not restore normal ICa properties. Together, our results suggest that several types of HVACCs are of functional relevance for the developing LSO. Whether on-site loss of Cav1.3, i.e., in LSO neurons, contributes to the recently described malformation of the LSO needs to be determined by using tissue-specific Cav1.3−/− animals.

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

Studies on the genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into the molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. In this review we focus on the role of mutated CaV2.1 (i.e., P/Q-type) voltage-activated Ca2+ channels, and on the ultimate consequences that mutations causing familial hemiplegic migraine type-1 (FHM1) have in neurotransmitter release. Transgenic mice harboring the human pathogenic FHM1 mutation R192Q or S218L (KI) have been used as models to study neurotransmission at several central and peripheral synapses. FHM1 KI mice are a powerful tool to explore presynaptic regulation associated with expression of CaV2.1 channels. Mutated CaV2.1 channels activate at more hyperpolarizing potentials and lead to a gain-of-function in synaptic transmission. This gain-of-function might underlie alterations in the excitatory/ inhibitory balance of synaptic transmission, favoring a persistent state of hyperexcitability in cortical neurons that would increase the susceptibility for cortical spreading depression (CSD), a mechanism believed to initiate the attacks of migraine with aura.

Calcium channels of schistosomes: unresolved questions and unexpected answers

Parasitic flatworms of the genus Schistosoma are the causative agents of schistosomiasis, a highly prevalent, neglected tropical disease that causes significant morbidity in hundreds of millions of people worldwide. The current treatment of choice against schistosomiasis is praziquantel (PZQ), which is known to affect Ca(2+) homeostasis in schistosomes, but which has an undefined molecular target and mode of action. PZQ is the only available antischistosomal drug in most parts of the world, making reports of PZQ resistance particularly troubling. Voltage-gated Ca(2+) (Ca(v)) channels have been proposed as possible targets for PZQ, and, given their central role in the neuromuscular system, may also serve as targets for new anthelmintic therapeutics. Indeed, ion channels constitute the majority of targets for current anthelmintics. Ca(v) channel subunits from schistosomes and other platyhelminths have several unique properties that make them attractive as potential drug targets, and that could also provide insights into structure-function relationships in, and evolution of, Ca(v) channels.

Distinct properties of amlodipine and nicardipine block of the voltage-dependent Ca2+ channels Cav1.2 and Cav2.1 and the mutant channels Cav1.2/dihydropyridine insensitive and Cav2.1/dihydropyridine sensitive

The binding site within the L-type Ca(2+) channel Ca(v)1.2 for neutral dihydropyridines is well characterized. However, the contributions of the alkylamino side chains of charged dihydropyridines such as amlodipine and nicardipine to channel block are not clear. We tested the hypothesis that the distinct locations of the charged side chains on amlodipine and nicardipine would confer distinct properties of channel block by these two drugs. Using whole-cell voltage clamp, we investigated block of wild type Ca(v) 2.1, wild type Ca(v)1.2, and Ca(v)1.2/Dihydropyridine insensitive, a mutant channel insensitive to neutral DHPs, by amlodipine and nicardipine. The potency of nicardipine and amlodipine for block of closed (stimulation frequency of 0.05 Hz) Ca(v)1.2 channels was not different (IC(50) values of 60 nM and 57 nM, respectively), but only nicardipine block was enhanced by increasing the stimulation frequency to 1 Hz. The frequency-dependent block of Ca(v)1.2 by nicardipine is the result of a strong interaction of nicardipine with the inactivated state of Ca(v)1.2. However, nicardipine block of Ca(v)1.2/Dihydropyridine insensitive was much more potent than block by amlodipine (IC(50) values of 2.0 μM and 26 μM, respectively). A mutant Ca(v)2.1 channel containing the neutral DHP binding site (Ca(v)2.1/Dihydropyridine sensitive) was more potently blocked by amlodipine (IC(50)=41 nM) and nicardipine (IC(50)=175 nM) than the parent Ca(v)2.1 channel. These data suggest that the alkylamino group of nicardipine and amlodipine project into distinct regions of Ca(v)1.2 such that the side chain of nicardipine, but not amlodipine, contributes to the potency of closed-channel block, and confers frequency-dependent block.

Interaction of diltiazem with an intracellularly accessible binding site on Ca(V)1.2

BACKGROUND AND PURPOSE

Diltiazem inhibits Ca_V1.2 channels and is widely used in clinical practice to treat cardiovascular diseases. Binding determinants for diltiazem are located on segments IIIS6, IVS6 and the selectivity filter of the pore forming α₁ subunit of Ca_V1.2. The aim of the present study was to clarify the location of the diltiazem binding site making use of its membrane-impermeable quaternary derivative d-cis-diltiazem (qDil) and mutant α₁ subunits.

EXPERIMENTAL APPROACH

Ca_V1.2 composed of α1, α2-δ and β2a subunits were expressed in tsA-201 cells and barium currents through Ca_V1.2 channels were recorded using the patch clamp method in the whole cell configuration. qDil was synthesized and applied to the intracellular side (via the patch pipette) or to the extracellular side of the membrane (by bath perfusion).

KEY RESULTS

Quaternary derivative d-cis-diltiazem inhibited Ca_V1.2 when applied to the intracellular side of the membrane in a use-dependent manner (59 ± 4% at 300 µM) and induced only a low level of tonic (non-use-dependent) block (16 ± 2% at 300 µM) when applied to the extracellular side of the membrane. Mutations in IIIS6 and IVS6 that have previously been shown to reduce the sensitivity of Ca_V1.2 to tertiary diltiazem also had reduced sensitivity to intracellularly applied qDil.

CONCLUSION AND IMPLICATIONS

The data show that use-dependent block of in Ca_V1.2 by diltiazem occurs by interaction with a binding site accessible via a hydrophilic route from the intracellular side of the membrane.

Alternative splicing modulates diltiazem sensitivity of cardiac and vascular smooth muscle Ca(v)1.2 calcium channels

BACKGROUND AND PURPOSE

As a calcium channel blocker, diltiazem acts mainly on the voltage-gated calcium channels, Ca(v)1.2, for its beneficial effects in cardiovascular diseases such as hypertension, angina and/or supraventricular arrhythmias. However, the effects of diltiazem on different isoforms of Ca(v)1.2 channels expressed in heart and vascular smooth muscles remain to be investigated. Here, we characterized the effects of diltiazem on the splice variants of Ca(v)1.2 channels, predominant in cardiac and vascular smooth muscles.

EXPERIMENTAL APPROACH

Cardiac and smooth muscle isoforms of Ca(v)1.2 channels were expressed in human embryonic kidney cells and their electrophysiological properties were characterized using whole-cell patch-clamp techniques.

KEY RESULTS

Under closed-channel and use-dependent block (0.03 Hz), cardiac splice variant Ca(v)1.2CM was less sensitive to diltiazem than two major smooth muscle splice variants, Ca(v)1.2SM and Ca(v)1.2b. Ca(v)1.2CM has a more positive half-inactivation potential than the smooth muscle channels, and diltiazem shifted it less to negative potential. Additionally, the current decay was slower in Ca(v)1.2CM channels. When we modified alternatively spliced exons of cardiac Ca(v)1.2CM channels into smooth muscle exons, we found that all three loci contribute to the different diltiazem sensitivity between cardiac and smooth muscle splice isoforms.

CONCLUSIONS AND IMPLICATIONS

Alternative splicing of Ca(v)1.2 channels modifies diltiazem sensitivity in the heart and blood vessels. Gating properties altered by diltiazem are different in the three channels.

Inclusion complexes of amlodipine besylate and cyclodextrins

In this paper the procedure for the preparation of inclusion complexes of amlodipine besylate with β-cyclodextrin (β-CD) and 2-hydrohypropyl-β-cyclodextrin (HPβ-CD) and their structural characterization was described. Molecular inclusion complexes of amlodipine besylate are prepared by the coprecipitation method and characterised by the application of spectroscopic methods FTIR, 1H-NMR and XRD. The photosensitivity of amlodipine besylate in the inclusion complexes was also determined with respect to uncomplexed agent. DSC curves indicate the loss of the clear peak due to melting of amlodipine besylate at about 200°C, while on XR diffractograms certain reflections are lost belonging to amlodipine besylate in complexes. This indicates its inclusion in the vacancies of the host. The inclusion of amlodipine besylate with cyclodextrins increases the stability, i.e. decreases the photosensitivity of amlodipine besylate.

Effect of otilonium bromide on contractile patterns in the human sigmoid colon

BACKGROUND

The mechanism of action of the spasmolytic compound otilonium bromide (OB) on human colonic motility is not understood. The aim of our study was to characterize the pharmacological effects of OB on contractile patterns in the human sigmoid colon.

METHODS

Circular sigmoid strips were studied in organ baths. Isolated smooth muscle cells from human sigmoid colon were examined using the calcium imaging technique.

KEY RESULTS

Otilonium bromide inhibited by 85% spontaneous non-neural rhythmic phasic contractions (RPCs), (IC(50) = 49.9 nmol L(-1)) and stretch-induced tone (IC(50) = 10.7 nmol L(-1)) with maximum effects at micromolar range. OB also inhibited by 50% both on- (IC(50) = 38.0 nmol L(-1)) and off-contractions induced by electrical stimulation of excitatory motor neurons. In contrast, the inhibitory latency period prior to off-contractions was unaffected by OB. OB inhibited acetylcholine-, substance P-, and neurokinin A-induced contractions. The L-type Ca(2+) channel agonist BayK8644 reversed the effects of OB on RPCs, on- and off-contractions. Hexamethonium, atropine, the NK(2) antagonist, or depletion of intracellular Ca(2+) stores by thapsigargin did not prevent the inhibitory effect of OB on RPCs and electrical contractions. KCl-induced calcium transients in isolated smooth muscle cells were also inhibited by OB (IC(50) = 0.2 micromol L(-1)).

CONCLUSIONS & INFERENCES

Otilonium bromide strongly inhibited the main patterns of human sigmoid motility in vitro by blocking calcium influx through L-type calcium channels on smooth muscle cells. This pharmacological profile may mediate the clinically observed effects of the drug in patients with irritable bowel syndrome.

Conformational changes induced in voltage-gated calcium channel Cav1.2 by BayK 8644 or FPL64176 modify the kinetics of secretion independently of Ca2+ influx

The role of the L-type calcium channel (Cav1.2) as a molecular switch that triggers secretion prior to Ca(2+) transport has previously been demonstrated in bovine chromaffin cells and rat pancreatic beta cells. Here, we examined the effect of specific Cav1.2 allosteric modulators, BayK 8644 (BayK) and FPL64176 (FPL), on the kinetics of catecholamine release, as monitored by amperometry in single bovine chromaffin cells. We show that 2 microm BayK or 0.5 microm FPL accelerates the rate of catecholamine secretion to a similar extent in the presence either of the permeable Ca(2+) and Ba(2+) or the impermeable charge carrier La(3+). These results suggest that structural rearrangements generated through the binding of BayK or FPL, by altering the channel activity, could affect depolarization-evoked secretion prior to cation transport. FPL also accelerated the rate of secretion mediated by a Ca(2+)-impermeable channel made by replacing the wild type alpha(1)1.2 subunit was replaced with the mutant alpha(1)1.2/L775P. Furthermore, BayK and FPL modified the kinetic parameters of the fusion pore formation, which represent the initial contact between the vesicle lumen and the extracellular medium. A direct link between the channel activity and evoked secretion lends additional support to the view that the voltage-gated Ca(2+) channels act as a signaling molecular switch, triggering secretion upstream to ion transport into the cell.

Three-dimensional structure of the alpha1-beta complex in the skeletal muscle dihydropyridine receptor by single-particle electron microscopy

The dihydropyridine receptor (DHPR) is a protein complex that consists of five distinct subunits of alpha(1), alpha(2), beta, gamma and delta and functions as a voltage-dependent L-type Ca(2+) channel. Here we purified the alpha(1)-beta complex (approximately 250 kDa) from the rabbit skeletal muscle DHPR and reconstructed its three-dimensional (3D) structure to 38 A resolution by single particle analysis of negative staining electron microscopy. The alpha(1)-beta structure exhibited two unique regions: a pseudo-4-fold petaloid region and an elongated region. X-ray crystallographic models of a homologous voltage-dependent K(+) channel and the beta subunit fit well into the individual regions of the alpha(1)-beta structure, revealing that the two regions correspond to the transmembrane alpha(1) and the cytoplasmic beta subunits, respectively. In addition, 3D reconstruction and immuno-electron microscopic analysis performed on the independently purified DHPR demonstrated that the alpha(1)-beta complex was located in the large globular portion of the DHPR, and the N-terminal region of the beta subunit was extended to the leg-shaped protrusion of the DHPR, which includes the alpha(2)delta subunits. Our results propose a model in which the beta subunit may regulate ion channel function by acting as a hinge between alpha(1) and alpha(2)delta subunits of the DHPR.

Roscovitine binds to novel L-channel (CaV1.2) sites that separately affect activation and inactivation

L-type (Ca(V)1.2) calcium channel antagonists play an important role in the treatment of cardiovascular disease. (R)-Roscovitine, a trisubstituted purine, has been shown to inhibit L-currents by slowing activation and enhancing inactivation. This study utilized molecular and pharmacological approaches to determine whether these effects result from (R)-roscovitine binding to a single site. Using the S enantiomer, we find that (S)-roscovitine enhances inactivation without affecting activation, which suggests multiple sites. This was further supported in studies using chimeric channels comprised of N- and L-channel domains. Those chimeras containing L-channel domains I and IV showed (R)-roscovitine-induced slowed activation like that of wild type L-channels, whereas chimeric channels containing L-channel domain I responded to (R)-roscovitine with enhanced inactivation. We conclude that (R)-roscovitine binds to distinct sites on L-type channels to slow activation and enhance inactivation. These sites appear to be unique from other calcium channel antagonist sites that reside within domains III and IV and are thus novel sites that could be exploited for future drug development. Trisubstituted purines could become a new class of drugs for the treatment of diseases related to hyperfunction of L-type channels, such as Torsades de Pointes.

Engineering proteins for custom inhibition of Ca(V) channels

The influx of Ca(2+) ions through voltage-dependent calcium (Ca(V)) channels links electrical signals to physiological responses in all excitable cells. Not surprisingly, blocking Ca(V) channel activity is a powerful method to regulate the function of excitable cells, and this is exploited for both physiological and therapeutic benefit. Nevertheless, the full potential for Ca(V) channel inhibition is not being realized by currently available small-molecule blockers or second-messenger modulators due to limitations in targeting them either to defined groups of cells in an organism or to distinct subcellular regions within a single cell. Here, we review early efforts to engineer protein molecule blockers of Ca(V) channels to fill this crucial niche. This technology would greatly expand the toolbox available to physiologists studying the biology of excitable cells at the cellular and systems level.

Nitric oxide suppresses cerebral vasomotion by sGC-independent effects on ryanodine receptors and voltage-gated calcium channels

BACKGROUND/AIMS

In cerebral arteries, nitric oxide (NO) release plays a key role in suppressing vasomotion. Our aim was to establish the pathways affected by NO in rat middle cerebral arteries.

METHODS

In isolated segments of artery, isometric tension and simultaneous measurements of either smooth muscle membrane potential or intracellular [Ca(2+)] ([Ca(2+)](SMC)) changes were recorded.

RESULTS

In the absence of L-NAME, asynchronous propagating Ca(2+) waves were recorded that were sensitive to block with ryanodine, but not nifedipine. L-NAME stimulated pronounced vasomotion and synchronous Ca(2+) oscillations with close temporal coupling between membrane potential, tone and [Ca(2+)](SMC). If nifedipine was applied together with L-NAME, [Ca(2+)](SMC) decreased and synchronous Ca(2+) oscillations were lost, but asynchronous propagating Ca(2+) waves persisted. Vasomotion was similarly evoked by either iberiotoxin, or by ryanodine, and to a lesser extent by ODQ. Exogenous application of NONOate stimulated endothelium-independent hyperpolarization and relaxation of either L-NAME-induced or spontaneous arterial tone. NO-evoked hyperpolarization involved activation of BK(Ca) channels via ryanodine receptors (RYRs), with little involvement of sGC. Further, in whole cell mode, NO inhibited current through L-type voltage-gated Ca(2+) channels (VGCC), which was independent of both voltage and sGC.

CONCLUSION

NO exerts sGC-independent actions at RYRs and at VGCC, both of which normally suppress cerebral artery myogenic tone.

Clinical benefits versus shortcomings of diltiazem once-daily in the chronotherapy of cardiovascular diseases

BACKGROUND

The introduction of chronotherapy (that is improving a drugs therapeutic efficacy by paralleling the drugs plasma levels to circadian rhythms) has recently become a focus of interest.

OBJECTIVE

This article addresses the efficacy and potential shortcomings of chronotherapy, and focuses on one specific type of chronotherapy: a novel long-acting diltiazem formulation, DTZ-LA.

METHODS

We reviewed the literature to assess the clinical benefits and shortcomings associated with DTZ-LA in the management of hypertension and angina.

RESULTS/CONCLUSIONS

The clinical benefits of DTZ-LA outweigh its disadvantages when surrogate outcomes are evaluated, but it still remains to be determined whether chronotherapy benefits hard clinical outcomes. Nonetheless, chronotherapy has the potential to address the cardiovascular triggers that peak in the early morning hours when the preponderance of cardiovascular events occur, as well as providing better target organ protection compared with non-chronotherapeutic therapy.

Differential sensitivities of CaV1.2 IIS5-S6 mutants to 1,4-dihydropyridine analogs

1,4-Dihydropyridines (DHPs), L-type calcium channel (Ca(V)1) blockers, are known to interact with Ca(V)1.2 subunits through their binding site located at IIIS5-S6 and IVS6 regions. We recently identified two domain II residues (S666 and A752) critical for nifedipine blockade (Kwok et al., 2008). In this study, we examined the blockade effects of two DHP analogues, nemadipine and nicardipine, on wildtype, M1161A (in IIIS6), S666V (in IIS5) and A752T (in IIS6) mutants of the rat alpha(1C) subunit transiently expressed with beta(2a) and alpha(2)delta in cultured tsA201 cells. We found that the IC(50) ratio of the mutants to the wildtype channel was similar in S666V and M1161A mutants for both drugs, but in A752T it was lower for nemadipine than nicardipine (P<0.05). At saturating drug concentrations, not all the current was completely blocked in the mutants. The residual current recorded in 100 microM nemadipine was approximately 10% of the total current for the A752T channel, which was significantly higher than that in 100 microM nicardipine (approximately 2%). In wildtype, S666V and M1161A, there was no significant difference in residual current between nemadipine and nicardipine, although it was greater in S666V (approximately 15%) and M1161A approximately 30%) as compared to the wildtype channel (<5%). Taken together, our findings suggest that the domain II residues alter the DHP effect in a structure-specific manner and may be involved in a pathway downstream of DHP binding.

Orientation of the calcium channel beta relative to the alpha(1)2.2 subunit is critical for its regulation of channel activity

BACKGROUND

The Ca(v)beta subunits of high voltage-activated Ca(2+) channels control the trafficking and biophysical properties of the alpha(1) subunit. The Ca(v)beta-alpha(1) interaction site has been mapped by crystallographic studies. Nevertheless, how this interaction leads to channel regulation has not been determined. One hypothesis is that betas regulate channel gating by modulating movements of IS6. A key requirement for this direct-coupling model is that the linker connecting IS6 to the alpha-interaction domain (AID) be a rigid structure.

METHODOLOGY/PRINCIPAL FINDINGS

The present study tests this hypothesis by altering the flexibility and orientation of this region in alpha(1)2.2, then testing for Ca(v)beta regulation using whole cell patch clamp electrophysiology. Flexibility was induced by replacement of the middle six amino acids of the IS6-AID linker with glycine (PG6). This mutation abolished beta2a and beta3 subunits ability to shift the voltage dependence of activation and inactivation, and the ability of beta2a to produce non-inactivating currents. Orientation of Ca(v)beta with respect to alpha(1)2.2 was altered by deletion of 1, 2, or 3 amino acids from the IS6-AID linker (Bdel1, Bdel2, Bdel3, respectively). Again, the ability of Ca(v)beta subunits to regulate these biophysical properties were totally abolished in the Bdel1 and Bdel3 mutants. Functional regulation by Ca(v)beta subunits was rescued in the Bdel2 mutant, indicating that this part of the linker forms beta-sheet. The orientation of beta with respect to alpha was confirmed by the bimolecular fluorescence complementation assay.

CONCLUSIONS/SIGNIFICANCE

These results show that the orientation of the Ca(v)beta subunit relative to the alpha(1)2.2 subunit is critical, and suggests additional points of contact between these subunits are required for Ca(v)beta to regulate channel activity.

Oxyhemoglobin-induced expression of R-type Ca2+ channels in cerebral arteries

BACKGROUND AND PURPOSE

Cerebral vasospasm after subarachnoid hemorrhage (SAH) is a major contributor to mortality and morbidity after aneurysm rupture. Recently, R-type voltage-dependent Ca(2+) channel (VDCC) expression has been associated with increased cerebral artery constriction in a rabbit model of SAH. The goal of the present study was to examine whether the blood component oxyhemoglobin (oxyHb) can mimic the ability of SAH to cause R-type VDCC expression in the cerebral vasculature.

METHODS

Rabbit cerebral arteries were organ cultured in serum-free media for up to 5 days in the presence or absence of purified oxyHb (10 micromol/L). Diameter changes in response to diltiazem, (L-type VDCC antagonist) and SNX-482 (R-type VDCC antagonist) were recorded at day 1, 3, or 5 in arteries constricted by elevated extracellular potassium. RT-PCR was performed on RNA extracted from arteries cultured for 5 days (+/-oxyHb) to assess VDCC expression.

RESULTS

After 5 days, oxyHb-treated arteries were less sensitive and partially resistant to diltiazem compared to similar arteries organ cultured in the absence of oxyHb. Further, SNX-482 dilated arteries organ cultured for 5 days in the presence, but not in the absence, of oxyHb. RT-PCR revealed that oxyHb treated arteries expressed R-type VDCCs (Ca(V) 2.3) in addition to L-type VDCCs (Ca(V) 1.2), whereas untreated arteries expressed only Ca(V) 1.2.

CONCLUSIONS

These results demonstrate that oxyhemoglobin exposure for 5 days induces the expression of Ca(V) 2.3 in cerebral arteries. We propose that oxyhemoglobin contributes to enhanced cerebral artery constriction after SAH via the emergence of R-type VDCCs.

The auxiliary subunit gamma 1 of the skeletal muscle L-type Ca2+ channel is an endogenous Ca2+ antagonist

Ca2+ channels play crucial roles in cellular signal transduction and are important targets of pharmacological agents. They are also associated with auxiliary subunits exhibiting functions that are still incompletely resolved. Skeletal muscle L-type Ca2+ channels (dihydropyridine receptors, DHPRs) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored Ca2+. The skeletal muscle-specific gamma subunit of the DHPR (gamma 1) down-modulates availability by altering its steady state voltage dependence. The effect resembles the action of certain Ca2+ antagonistic drugs that are thought to stabilize inactivated states of the DHPR. In the present study we investigated the cross influence of gamma 1 and Ca2+ antagonists by using wild-type (gamma+/+) and gamma 1 knockout (gamma-/-) mice. We studied voltage-dependent gating of both L-type Ca2+ current and Ca2+ release and the allosteric modulation of drug binding. We found that 10 microM diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by gamma 1 elimination; 5 muM devapamil [(-)D888], a phenylalkylamine Ca2+ antagonist, approximately reversed the right-shifted voltage dependence of availability and the accelerated recovery kinetics of Ca2+ current and Ca2+ release. Moreover, the presence of gamma 1 altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that gamma 1 and the drug do not act independently of each other. We propose that the gamma 1 subunit of the DHPR functions as an endogenous Ca2+ antagonist whose task may be to minimize Ca2+ entry and Ca2+ release under stress-induced conditions favoring plasmalemma depolarization.

Roscovitine, a cyclin-dependent kinase inhibitor, affects several gating mechanisms to inhibit cardiac L-type (Ca(V)1.2) calcium channels

BACKGROUND AND PURPOSE

L-type calcium channels (Ca((V))1.2) play an important role in cardiac contraction. Roscovitine, a cyclin-dependent kinase inhibitor and promising anticancer drug, has been shown to affect Ca((V))1.2 by inhibiting current amplitude and slowing activation. This research investigates the mechanism by which roscovitine inhibits Ca((V))1.2 channels.

EXPERIMENTAL APPROACH

Ca((V))1.2 channels were transfected into HEK 293 cells, using the calcium phosphate precipitation method, and currents were measured using the whole-cell patch clamp technique.

KEY RESULTS

Roscovitine slows activation at all voltages, which precludes one previously proposed mechanism. In addition, roscovitine enhances voltage-dependent, but not calcium-dependent inactivation. This enhancement resulted from both an acceleration of inactivation and a slowing of the recovery from inactivation. Internally applied roscovitine failed to affect Ca((V))1.2 currents, which supports a kinase-independent mechanism and extracellular binding site. Unlike the dihydropyridines, closed state inactivation was not affected by roscovitine. Inactivation was enhanced in a dose-dependent manner with an IC(50)=29.5+/-12 microM, which is close to that for slow activation and inhibition.

CONCLUSIONS AND IMPLICATIONS

We conclude that roscovitine binds to an extracellular site on Ca((V))1.2 channels to inhibit current by both slowing activation and enhancing inactivation. Purine-based drugs could become a new option for treatment of diseases that benefit from L-channel inhibition such as cardiac arrhythmias and hypertension.

Simultaneous assessment of the effects of L-type current modulators on sensory and motor pathways of the mouse spinal cord in vitro

The effects of modulators of L-type currents in the processing of nociceptive stimuli across sensory and motor circuits were studied using an in vitro preparation of the young mouse spinal cord. Responses to repetitive C-fibre intensity stimuli delivered to a lumbar dorsal root were simultaneously recorded from motor axons in the corresponding ventral root and from putative sensory axons in the anterolateral pathway. L-current antagonists verapamil, diltiazem and nimodipine as well as the agonist Bay K8644 were superfused at a range of concentrations and their effects on responses to afferent stimulation were assessed. All antagonists produced a concentration-dependent depression of transmission across sensory and motor pathways by inhibiting sustained firing and wind-up. All antagonists showed concentration-dependent depression of evoked firing in anterolateral fibres with LogIC50 of -4.2 for verapamil, -4.1 for diltiazem and -4.9 for nimodipine. Applied at high concentrations (>or=100 microM) verapamil and diltiazem produced almost complete blockade of the ascending signals whereas nimodipine produced only partial depression. The effects of the antagonists on motor pathways were significantly greater and the LogIC50 decreased to -5 for verapamil, to -4.9 for diltiazem and to -5.3 for nimodipine. Bay K8644 applied at 2 microM produced only a slight potentiation of responses in anterolateral axons and a very large and long-lasting potentiation of responses from motor neurons. We conclude that mice motor pathways are more sensitive to L-type current modulators than the anterolateral pathway and that analgesic effects reported for some L-type antagonists may be due to a mixture of selective and non-selective effects of these agents on sensory neurones.

Competitive and Cooperative Effects of Bay K8644 on the L-Type Calcium Channel Current Inhibition by Calcium Channel Antagonists

Phenylalkylamines, benzothiazepines, and dihydropyridines bind noncompetitively to the L-type calcium channel. The molecular mechanisms of this interaction were investigated in enzymatically isolated rat ventricular myocytes using the whole-cell patch-clamp technique. When applied alone, felodipine, verapamil, and diltiazem inhibited the L-type calcium current with values of inhibitory constant (K(B)) of 11, 246, and 512 nM, respectively, whereas 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester (Bay K8644) activated I(Ca) with activation constant (K(A)) of 33 nM. Maximal activation of I(Ca) by 300 nM Bay K8644 strongly reduced the inhibitory potency of felodipine (apparent K(B) of 165 nM), significantly reduced the inhibitory potency of verapamil (apparent K(B) of 737 nM), but significantly increased the inhibitory potency of diltiazem (apparent K(B) of 310 nM). In terms of a new pseudoequilibrium two-drug binding model, the interaction between the dihydropyridine agonist Bay K8644 and the antagonist felodipine was found purely competitive. The interaction between Bay K8644 and verapamil or diltiazem was found noncompetitive, and it could be described only by inclusion of a negative interaction factor nu = -0.60 for verapamil and a positive interaction factor nu = +0.24 for diltiazem. These results suggest that at physiological membrane potentials, the L-type calcium channel cannot be simultaneously occupied by a dihydropyridine agonist and antagonist, whereas it can simultaneously bind a dihydropyridine agonist and a nondihydropyridine antagonist. Generally, the effects of the drugs on the L-type calcium channel support a concept of a channel domain responsible for binding of calcium channel antagonists and agonists changing dynamically with the membrane voltage and occupancy of individual binding sites.

Photophysics and Photochemical Studies of 1,4-Dihydropyridine Derivatives

The absorption and fluorescence properties of nifedipine (NPDHP), felodipine (CPDHP) and a series of structurally related 1,4-dihydropyridines were studied in aqueous solution and organic solvents of different properties. The absorption and fluorescence spectra were found to depend on the chemical nature of the substituents at the position 4 of the 1,4-dihydropyridine ring (DHP) and on solvent properties. In aqueous solution, the fluorescence spectra of 4-phenyl substituted compounds are blue-shifted with respect to the alkyl substituted compounds. The more fluorescent compound is CPDHP. Nifedipine is not fluorescent. All compounds, with the exception of CPDHP, present monoexponential fluorescence decay with very short lifetime (0.2-0.4 ns). CPDHP showed a biexponential emission decay with a long-lived component of 1.7 ns; this behavior is explained in terms of different conformers because of the hindered rotation of the phenyl group by the ortho-substitution. Analysis of the solvent effect on the maximum of the absorption spectrum by using the linear solvent-energy relation solvato-chromic equation indicates the redshifts are influenced by the polarizability, hydrogen bonding ability and the hydrogen bond acceptance of the solvent. Whereas, the fluorescence characteristics (spectra, quantum yields and lifetimes) are sensitive to the polarizabilty and hydrogen bond ability of the solvents. Photo-decomposition of nifedipine is dependent on the solvent properties. Faster decomposition rates were obtained in nonprotic solvents. The 4-carboxylic derivative goes to decarboxylation. Under similar conditions, the other DHP compounds did not show appreciable photodecomposition.

Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes

During the cardiac action potential, Ca(2+) entry through dyhidropyridine receptor L-type Ca(2+) channels (DHPRs) activates ryanodine receptors (RyRs) Ca(2+)-release channels, resulting in massive Ca(2+) mobilization from the sarcoplasmic reticulum (SR). This global Ca(2+) release arises from spatiotemporal summation of many localized elementary Ca(2+)-release events, Ca(2+) sparks. We tested whether DHPRs modulate Ca(2+)sparks in a Ca(2+) entry-independent manner. Negative modulation by DHPR of RyRs via physical interactions is accepted in resting skeletal muscle but remains controversial in the heart. Ca(2+) sparks were studied in cat cardiac myocytes permeabilized with saponin or internally perfused via a patch pipette. Bathing and pipette solutions contained low Ca(2+) (100 nM). Under these conditions, Ca(2+) sparks were detected with a stable frequency of 3-5 sparks.s(-1).100 microm(-1). The DHPR blockers nifedipine, nimodipine, FS-2, and calciseptine decreased spark frequency, whereas the DHPR agonists Bay-K8644 and FPL-64176 increased it. None of these agents altered the spatiotemporal characteristics of Ca(2+) sparks. The DHPR modulators were also without effect on SR Ca(2+) load (caffeine-induced Ca(2+) transients) or sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity (Ca(2+) loading rates of isolated SR microsomes) and did not change cardiac RyR channel gating (planar lipid bilayer experiments). In summary, DHPR modulators affected spark frequency in the absence of DHPR-mediated Ca(2+) entry. This action could not be attributed to a direct action of DHPR modulators on SERCA or RyRs. Our results suggest that the activity of RyR Ca(2+)-release units in ventricular myocytes is modulated by Ca(2+) entry-independent conformational changes in neighboring DHPRs.

Identification of small signalling molecules promoting cardiac-specific differentiation of mouse embryonic stem cells

Identification of signalling cascades involved in cardiomyogenesis is crucial for optimising the generation of cardiomyocytes from embryonic stem cells (ES cells) (in vitro). We used a transgenic ES cell lineage expressing enhanced green fluorescent protein (EGFP) under the control of the alpha-myosin heavy chain (alpha-MHC) promoter (palphaMHC-EGFP) to investigate the effects of 33 small molecules interfering with several signalling cascades on cardiomyogenesis. Interestingly, the L-Type Ca2+ channel blocker Verapamil as well as Cyclosporin, an inhibitor of the protein phosphatase 2B, exerted the most striking pro-cardiomyogenic effect. Forskolin (adenylate cyclase stimulator) exerted the most striking anti-cardiomyogenic effect. The cardiomyogenic effect of Cyclosporin and Verapamil correlated with an expression of early cardiac markers Nkx2.5 and GATA4. Compared to the effects on late developmental stage embryoid bodies (EBs) stimulation of early developmental stage EBs (1-day old) with Verapamil or Cyclosporin for 48 h resulted in a potent cardiomyogenic effect. Accordingly, enhanced expression of alpha-MHC mRNA and EGFP mRNA was observed after stimulation of the early developmental stage EBs for 48 h. No expression of alpha-smooth muscle actin or platelet endothelial cell adhesion molecule-1 (PECM-1) as well as of neuronal genes (Nestin, Neurofilament H) has been observed demonstrating a preferentially pro-cardiomyogenic effect by both molecules.

Ion channels and calcium signaling in cerebral arteries following subarachnoid hemorrhage

Entry of Ca(2+) through voltage-dependent calcium channels (VDCCs) is critical to the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in vascular smooth muscle and thus the control of cerebral artery diameter. Increased VDCC activity in cerebral artery myocytes may contribute to decreased cerebral blood flow and the accompanying neurological deficits associated with subarachnoid hemorrhage (SAH). This review will focus on the impact of SAH on VDCCs and K(+)-selective ion channels, two important classes of ion channels located in the plasma membrane of cerebral artery myocytes. SAH may act through a variety of direct and indirect mechanisms to increase the activity of VDCCs promoting cerebral artery constriction and reduced cerebral blood flow. Further, SAH may lead to suppression of K(+) channel activity to cause membrane potential depolarization to enhance VDCC activity. The ability of VDCC blockers or K(+) channel activators to alleviate SAH-induced vasospasm will also be examined.

Calcium channel antagonists: clinical uses--past, present and future

The calcium channel antagonists are a mature group of drugs directed at cardiovascular diseases including hypertension, angina, peripheral vascular disorders and some arrhythmic conditions. Their sites and mechanisms of actions have been well explored over the past two decades and their interactions at the alpha(1) subunit of L-type channels (Ca(V)1.1-1.4) have made them valuable molecular tools for channel classification and localization. With the realization that other members of the voltage-gated calcium channel family exist--Ca(V)2.1-2.3 and Ca(V)3.1-3.3--considerable effort has been directed to drug discovery at these channel types where therapeutic prospects exist for a variety of disorders including pain, epilepsy, affective disorders, neurodegenerative disorders, etc. In contrast to the situation with the L-type channel antagonists success in developing small molecule antagonists of therapeutic utility for these other channel types has thus far been lacking. The reasons for this are explored and potential new directions are indicated including male fertility, bone growth, immune disorders, cancer and schistosomiasis.

Spider venoms: a rich source of acylpolyamines and peptides as new leads for CNS drugs

Advances in NMR and mass spectrometry as well as in peptide biochemistry coupled to modern methods in electrophysiology have permitted the isolation and identification of numerous products from spider venoms, previously explored due to technical limitations. The chemical composition of spider venoms is diverse, ranging from low molecular weight organic compounds such as acylpolyamines to complex peptides. First, acylpolyamines (< 1000 Da) have an aromatic moiety linked to a hydrophilic lateral chain. They were characterized for the first time in spider venoms and are ligand-gated ion channel antagonists, which block mainly postsynaptic glutamate receptors in invertebrate and vertebrate nervous systems. Acylpolyamines represent the vast majority of organic components from the spider venom. Acylpolyamine analogues have proven to suppress hippocampal epileptic discharges. Moreover, acylpolyamines could suppress excitatory postsynaptic currents inducing Ca+ accumulation in neurons leading to protection against a brain ischemic insult. Second, short spider peptides (< 6000 Da) modulate ionic currents in Ca2+, Na+, or K+ voltage-gated ion channels. Such peptides may contain from three to four disulfide bridges. Some spider peptides act specifically to discriminate among Ca2+, Na+, or K+ ion channel subtypes. Their selective affinities for ion channel subfamilies are functional for mapping excitable cells. Furthermore, several of these peptides have proven to hyperpolarize peripheral neurons, which are associated with supplying sensation to the skin and skeletal muscles. Some spider N-type calcium ion channel blockers may be important for the treatment of chronic pain. A special group of spider peptides are the amphipathic and positively charged peptides. Their secondary structure is alpha-helical and they insert into the lipid cell membrane of eukaryotic or prokaryotic cells leading to the formation of pores and subsequently depolarizing the cell membrane. Acylpolyamines and peptides from spider venoms represent an interesting source of molecules for the design of novel pharmaceutical drugs.

Therapeutic targets: progress of their exploration and investigation of their characteristics

Modern drug discovery is primarily based on the search and subsequent testing of drug candidates acting on a preselected therapeutic target. Progress in genomics, protein structure, proteomics, and disease mechanisms has led to a growing interest in and effort for finding new targets and more effective exploration of existing targets. The number of reported targets of marketed and investigational drugs has significantly increased in the past 8 years. There are 1535 targets collected in the therapeutic target database compared with approximately 500 targets reported in a 1996 review. Knowledge of these targets is helpful for molecular dissection of the mechanism of action of drugs and for predicting features that guide new drug design and the search for new targets. This article summarizes the progress of target exploration and investigates the characteristics of the currently explored targets to analyze their sequence, structure, family representation, pathway association, tissue distribution, and genome location features for finding clues useful for searching for new targets. Possible "rules" to guide the search for druggable proteins and the feasibility of using a statistical learning method for predicting druggable proteins directly from their sequences are discussed.

Defining the roles of Ca2+ — permeable channels in sperm

Ion channels exert a vital role in the dialogue between male and female gametes and thus in the generation of new individuals in many species. Intracellular Ca2+ is possibly the key messenger between gametes. Different Ca2+-permeable channels have been detected in the plasma membrane and in the organelle-like acrosome membrane of sperm, which play vital roles in determining sperm fertilizing ability. Recent reports from several laboratories have adequately documented that the Ca2+-permeable channels of a sperm control a variety of functions ranging from motility to the acrosome reaction. In this article, we have reviewed the data from our and other laboratories, and have documented the mechanisms of different Ca2+-permeable channels involved in the fertilization event.

Probing the architecture of an L-type calcium channel with a charged phenylalkylamine: evidence for a widely open pore and drug trapping

Voltage-gated calcium channels are in a closed conformation at rest and open temporarily when the membrane is depolarized. To gain insight into the molecular architecture of Ca(v)1.2, we probed the closed and open conformations with the charged phenylalkylamine (-)devapamil ((-)qD888). To elucidate the access pathway of (-)D888 to its binding pocket from the intracellular side, we used mutations replacing a highly conserved Ile-781 by threonine/proline in the pore-lining segment IIS6 of Ca(v)1.2 (1). The shifted channel gating of these mutants (by 30-40 mV in the hyperpolarizing direction) enabled us to evoke currents with identical kinetics at different potentials and thus investigate the effect of the membrane potentials on the drug access per se. We show here that under these conditions the development of channel block by (-)qD888 is not affected by the transmembrane voltage. Recovery from block at rest was, however, accelerated at more hyperpolarized voltages. These findings support the conclusion that Ca(v)1.2 must be opening widely to enable free access of the charged (-)D888 molecule to its binding site, whereas drug dissociation from the closed channel conformation is restricted by bulky channel gates. The functional data indicating a location of a trapped (-)D888 molecule close to the central pore region are supported by a homology model illustrating that the closed Ca(v)1.2 is able to accommodate a large cation such as (-)D888.

Maturation of rat cerebellar Purkinje cells reveals an atypical Ca2+ channel current that is inhibited by omega-agatoxin IVA and the dihydropyridine (-)-(S)-Bay K8644

To determine if the properties of Ca2+ channels in cerebellar Purkinje cells change during postnatal development, we recorded Ca2+ channel currents from Purkinje cells in cerebellar slices of mature (postnatal days (P) 40-50) and immature (P13-20) rats. We found that at P40-50, the somatic Ca2+ channel current was inhibited by omega-agatoxin IVA at concentrations selective for P-type Ca2+ channels (approximately 85%; IC50, <1 nM) and by the dihydropyridine (-)-(S)-Bay K8644 (approximately 70%; IC50, approximately 40 nM). (-)-(S)-Bay K8644 is known to activate L-type Ca2+ channels, but the decrease in current was not secondary to the activation of L-type channels because inhibition by (-)-(S)-Bay K8644 persisted in the presence of the L-type channel blocker (R,S)-nimodipine. By contrast, at P13-20, the current was inhibited by omega-agatoxin IVA (approximately 86%; IC50, approximately 1 nM) and a minor component was inhibited by (R,S)-nimodipine (approximately 8%). The dihydropyridine (-)-(S)-Bay K8644 had no clear effect when applied alone, but in the presence of (R,S)-nimodipine it reduced the current (approximately 40%), suggesting that activation of L-type channels by (-)-(S)-Bay K8644 masks its inhibition of non-L-type channels. Our findings indicate that Purkinje neurons express a previously unrecognized type of Ca2+ channel that is inhibited by omega-agatoxin IVA, like prototypical P-type channels, and by (-)-(S)-Bay K8644, unlike classical P-type or L-type channels. During maturation, there is a decrease in the size of the L-type current and an increase in the size of the atypical Ca2+ channel current. These changes may contribute to the maturation of the electrical properties of Purkinje cells.

Drugs, their targets and the nature and number of drug targets

What is a drug target? And how many such targets are there? Here, we consider the nature of drug targets, and by classifying known drug substances on the basis of the discussed principles we provide an estimation of the total number of current drug targets.

Effects of voltage-gated calcium channel subunit genes on calcium influx in cultured C. elegans mechanosensory neurons

Voltage-gated calcium channels (VGCCs) serve as a critical link between electrical signaling and diverse cellular processes in neurons. We have exploited recent advances in genetically encoded calcium sensors and in culture techniques to investigate how the VGCC alpha1 subunit EGL-19 and alpha2/delta subunit UNC-36 affect the functional properties of C. elegans mechanosensory neurons. Using the protein-based optical indicator cameleon, we recorded calcium transients from cultured mechanosensory neurons in response to transient depolarization. We observed that in these cultured cells, calcium transients induced by extracellular potassium were significantly reduced by a reduction-of-function mutation in egl-19 and significantly reduced by L-type calcium channel inhibitors; thus, a main source of touch neuron calcium transients appeared to be influx of extracellular calcium through L-type channels. Transients did not depend directly on intracellular calcium stores, although a store-independent 2-APB and gadolinium-sensitive calcium flux was detected. The transients were also significantly reduced by mutations in unc-36, which encodes the main neuronal alpha2/delta subunit in C. elegans. Interestingly, while egl-19 mutations resulted in similar reductions in calcium influx at all stimulus strengths, unc-36 mutations preferentially affected responses to smaller depolarizations. These experiments suggest a central role for EGL-19 and UNC-36 in excitability and functional activity of the mechanosensory neurons.

Role of voltage-gated L-type Ca2+ channel isoforms for brain function

Voltage-gated LTCCs (L-type Ca2+ channels) are established drug targets for the treatment of cardiovascular diseases. LTCCs are also expressed outside the cardiovascular system. In the brain, LTCCs control synaptic plasticity in neurons, and DHP (dihydropyridine) LTCC blockers such as nifedipine modulate brain function (such as fear memory extinction and depression-like behaviour). Voltage-sensitive Ca2+ channels Cav1.2 and Cav1.3 are the predominant brain LTCCs. As DHPs and other classes of organic LTCC blockers inhibit both isoforms, their pharmacological distinction is impossible and their individual contributions to defined brain functions remain largely unknown. Here, we summarize our recent experiments with two genetically modified mouse strains, which we generated to explore the individual biophysical features of Cav1.2 and Cav1.3 LTCCs and to determine their relative contributions to various physiological peripheral and neuronal functions. The results described here also allow predictions about the pharmacotherapeutic potential of isoform-selective LTCC modulators.

Calcium channel antagonists discovered by a multidisciplinary approach

A multidisciplinary project has led to the discovery of novel, structurally diverse, L-type calcium entry blockers (CEBs). The absolute configuration of a recently reported CEB has been determined by vibrational circular dichroism spectroscopy, to assign the stereospecificity of the ligand-channel interaction. Thereafter, a virtual screening procedure was performed with the aim of identifying novel chemotypes for CEBs, starting from a database of purchasable compounds; 340,000 molecules were screened in silico in order to prioritize structures of interest for bioscreening. As a result, 20 compounds were tested in vitro, and functional and binding assays revealed several hits with promising behavior as CEBs.

The actions of azelnidipine, a dihydropyridine-derivative Ca antagonist, on voltage-dependent Ba2+ currents in guinea-pig vascular smooth muscle

BACKGROUND AND PURPOSE

Although azelnidipine is used clinically to treat hypertension its effects on its target cells, Ca2+ channels, in smooth muscle have not been elucidated. Therefore, its effects on spontaneous contractions and voltage-dependent L-type Ca2+ channels were investigated in guinea-pig portal vein.

EXPERIMENTAL APPROACH

The inhibitory potency of azelnidipine on spontaneous contractions in guinea-pig portal vein was compared with those of other dihydropyridine (DHP)-derived Ca antagonists (amlodipine and nifedipine) by recording tension. Also its effects on voltage-dependent nifedipine-sensitive inward Ba2+ currents (IBa) in smooth muscle cells dispersed from guinea-pig portal vein were investigated by use of a conventional whole-cell patch-clamp technique.

KEY RESULTS

Spontaneous contractions in guinea-pig portal vein were reduced by all of the Ca antagonists (azelnidipine, Ki = 153 nM; amlodipine, Ki = 16 nM; nifedipine, Ki = 7 nM). In the whole-cell experiments, azelnidipine inhibited the peak amplitude of IBa in a concentration- and voltage-dependent manner (-60 mV, Ki = 282 nM; -90 mV, Ki = 2 microM) and shifted the steady-state inactivation curve of IBa to the left at -90 mV by 16 mV. The inhibitory effects of azelnidipine on IBa persisted after 7 min washout at -60 mV. In contrast, IBa gradually recovered after being inhibited by amlodipine, but did not return to control levels. Both azelnidipine and amlodipine caused a resting block of IBa at -90 mV. Only nifedipine appeared to interact competitively with S(-)-Bay K 8644.

CONCLUSIONS AND IMPLICATIONS

These results suggest that azelnidipine induces long-lasting vascular relaxation by inhibiting voltage-dependent L-type Ca2+ channels in vascular smooth muscle.

Nitric oxide synthase inhibition activates L- and T-type Ca2+channels in afferent and efferent arterioles

Previous studies have shown that L-type Ca2+channel (LCC) blockers primarily dilate resting and ANG II-constricted afferent arterioles (AA), but do not influence either resting or ANG II-constricted efferent arterioles (EA). In contrast, blockade of T-type Ca2+channels (TCC) dilate EA and prevent ANG II-mediated efferent constriction. The present study determined the role of LCC and TCC in mediating the AA and EA constriction following inhibition of nitric oxide synthase (NOS) and tested the hypothesis that inhibition of NOS increases the influence of LCC on EA. With the use of an isolated blood-perfused rat juxtamedullary nephron preparation, single AA or EA were visualized and superfused with a NOS inhibitor, N-nitro-l-arginine (l-NNA), with or without concomitant treatment with an LCC blocker, diltiazem, or a TCC blocker, pimozide. In response to l-NNA (1, 10, and 100 μmol/l), AA and EA diameters decreased significantly by 6.0 ± 0.3, 13.7 ± 1.7, and 19.9 ± 1.4%, and by 6.2 ± 0.5, 13.3 ± 1.1, and 19.0 ± 1.9%, respectively. During TCC blockade with pimozide (10 μmol/l), l-NNA did not significantly constrict afferent (0.9 ± 0.6, 1.5 ± 0.5, and 1.7 ± 0.5%) or efferent (0.4 ± 0.1, 2.1 ± 0.7, and 2.5 ± 1.0%) arterioles. In contrast to the responses with other vasoconstictors, the l-NNA-induced constriction of EA, as well as AA, was reversed by diltiazem (10 μmol/l). The effects were overlapping as pimozide superimposed on diltiazem did not elicit further dilation. When the effects of l-NNA were reversed by superfusion with an NO donor, SNAP (10 μmol/l), diltiazem did not cause significant efferent dilation. As a further test of LCC activity, 55 mmol/l KCl, which depolarizes and constricts AA, caused only a modest constriction in resting EA (8.7 ± 1.3%), but a stronger EA constriction during concurrent treatment with l-NNA (23.8 ± 4.8%). In contrast, norepinephrine caused similar constrictions in both l-NNA-treated and nontreated arterioles. These results provide evidence that NO inhibits LCC and TCC activity and that NOS inhibition-mediated arteriolar constriction involves activation of LCC and TCC in both AA and EA. The difference in responses to high KCl between resting and l-NNA-constricted EA and the ability of diltiazem to block EA constriction caused by l-NNA contrasts with the lack of efferent effects in resting and SNAP-treated l-NNA-preconstricted arterioles and during ANG II-mediated vasoconstriction, suggesting a recruitment of LCC in EA when NOS is inhibited. These data help explain how endothelial dysfunction associated with hypertension may lead to enhanced activity of LCC in postglomerular arterioles and increased postglomerular resistance.