Mechanism of calcium channel blockade by verapamil, D600, diltiazem and nitrendipine in single dialysed heart cells

Bioelectric patterning during oogenesis: stage-specific distribution of membrane potentials, intracellular pH and ion-transport mechanisms in Drosophila ovarian follicles

Background

Bioelectric phenomena have been found to exert influence on various developmental and regenerative processes. Little is known about their possible functions and the cellular mechanisms by which they might act during Drosophila oogenesis. In developing follicles, characteristic extracellular current patterns and membrane-potential changes in oocyte and nurse cells have been observed that partly depend on the exchange of protons, potassium ions and sodium ions. These bioelectric properties have been supposed to be related to various processes during oogenesis, e. g. pH-regulation, osmoregulation, cell communication, cell migration, cell proliferation, cell death, vitellogenesis and follicle growth. Analysing in detail the spatial distribution and activity of the relevant ion-transport mechanisms is expected to elucidate the roles that bioelectric phenomena play during oogenesis.

Results

To obtain an overview of bioelectric patterning along the longitudinal and transversal axes of the developing follicle, the spatial distributions of membrane potentials (V_mem), intracellular pH (pH_i) and various membrane-channel proteins were studied systematically using fluorescent indicators, fluorescent inhibitors and antisera. During mid-vitellogenic stages 9 to 10B, characteristic, stage-specific V_mem-patterns in the follicle-cell epithelium as well as anteroposterior pH_i-gradients in follicle cells and nurse cells were observed. Corresponding distribution patterns of proton pumps (V-ATPases), voltage-dependent L-type Ca²⁺-channels, amiloride-sensitive Na⁺-channels and Na⁺,H⁺-exchangers (NHE) and gap-junction proteins (innexin 3) were detected. In particular, six morphologically distinguishable follicle-cell types are characterized on the bioelectric level by differences concerning V_mem and pH_i as well as specific compositions of ion channels and carriers. Striking similarities between V_mem-patterns and activity patterns of voltage-dependent Ca²⁺-channels were found, suggesting a mechanism for transducing bioelectric signals into cellular responses. Moreover, gradients of electrical potential and pH were observed within single cells.

Conclusions

Our data suggest that spatial patterning of V_mem, pH_i and specific membrane-channel proteins results in bioelectric signals that are supposed to play important roles during oogenesis, e. g. by influencing spatial coordinates, regulating migration processes or modifying the cytoskeletal organization. Characteristic stage-specific changes of bioelectric activity in specialized cell types are correlated with various developmental processes.

Trial watch: Cardiac glycosides and cancer therapy

Cardiac glycosides (CGs) are natural compounds sharing the ability to operate as potent inhibitors of the plasma membrane Na⁺/K⁺-ATPase, hence promoting—via an indirect mechanism—the intracellular accumulation of Ca²⁺ ions. In cardiomyocytes, increased intracellular Ca²⁺ concentrations exert prominent positive inotropic effects, that is, they increase myocardial contractility. Owing to this feature, two CGs, namely digoxin and digitoxin, have extensively been used in the past for the treatment of several cardiac conditions, including distinct types of arrhythmia as well as contractility disorders. Nowadays, digoxin is approved by the FDA and indicated for the treatment of congestive heart failure, atrial fibrillation and atrial flutter with rapid ventricular response, whereas the use of digitoxin has been discontinued in several Western countries. Recently, CGs have been suggested to exert potent antineoplastic effects, notably as they appear to increase the immunogenicity of dying cancer cells. In this Trial Watch, we summarize the mechanisms that underpin the unsuspected anticancer potential of CGs and discuss the progress of clinical studies that have evaluated/are evaluating the safety and efficacy of CGs for oncological indications.

Structural basis of ion permeation gating in Slo2.1 K+ channels

The activation gate of ion channels controls the transmembrane flux of permeant ions. In voltage-gated K⁺ channels, the aperture formed by the S6 bundle crossing can widen to open or narrow to close the ion permeation pathway, whereas the selectivity filter gates ion flux in cyclic-nucleotide gated (CNG) and Slo1 channels. Here we explore the structural basis of the activation gate for Slo2.1, a weakly voltage-dependent K⁺ channel that is activated by intracellular Na⁺ and Cl⁻. Slo2.1 channels were heterologously expressed in Xenopus laevis oocytes and activated by elevated [NaCl]_i or extracellular application of niflumic acid. In contrast to other voltage-gated channels, Slo2.1 was blocked by verapamil in an activation-independent manner, implying that the S6 bundle crossing does not gate the access of verapamil to its central cavity binding site. The structural basis of Slo2.1 activation was probed by Ala scanning mutagenesis of the S6 segment and by mutation of selected residues in the pore helix and S5 segment. Mutation to Ala of three S6 residues caused reduced trafficking of channels to the cell surface and partial (K256A, I263A, Q273A) or complete loss (E275A) of channel function. P271A Slo2.1 channels trafficked normally, but were nonfunctional. Further mutagenesis and intragenic rescue by second site mutations suggest that Pro271 and Glu275 maintain the inner pore in an open configuration by preventing formation of a tight S6 bundle crossing. Mutation of several residues in S6 and S5 predicted by homology modeling to contact residues in the pore helix induced a gain of channel function. Substitution of the pore helix residue Phe240 with polar residues induced constitutive channel activation. Together these findings suggest that (1) the selectivity filter and not the bundle crossing gates ion permeation and (2) dynamic coupling between the pore helix and the S5 and S6 segments mediates Slo2.1 channel activation.

Multilayer spheroids to quantify drug uptake and diffusion in 3D

There is a need for new quantitative in vitro models of drug uptake and diffusion to help assess drug toxicity/efficacy as well as new more predictive models for drug discovery. We report a three-dimensional (3D) multilayer spheroid model and a new algorithm to quantitatively study uptake and inward diffusion of fluorescent calcein via gap junction intercellular communication (GJIC). When incubated with calcein-AM, a substrate of the efflux transporter P-glycoprotein (Pgp), spheroids from a variety of cell types accumulated calcein over time. Accumulation decreased in spheroids overexpressing Pgp (HEK-MDR) and was increased in the presence of Pgp inhibitors (verapamil, loperamide, cyclosporin A). Inward diffusion of calcein was negligible in spheroids that lacked GJIC (OVCAR-3, SK-OV-3) and was reduced in the presence of an inhibitor of GJIC (carbenoxolone). In addition to inhibiting Pgp, verapamil and loperamide, but not cyclosporin A, inhibited inward diffusion of calcein, suggesting that they also inhibit GJIC. The dose response curves of verapamil’s inhibition of Pgp and GJIC were similar (IC₅₀: 8 μM). The method is amenable to many different cell types and may serve as a quantitative 3D model that more accurately replicates in vivo barriers to drug uptake and diffusion.

Uncoupling of Cav1.2 from Ca(2+)-induced Ca(2+) release and SK channel regulation in pancreatic β-cells

We investigated the role of Cav1.2 in pancreatic β-cell function by expressing a Cav1.2 II-III loop/green fluorescent protein fusion in INS-1 cells (Cav1.2/II-III cells) to disrupt channel-protein interactions. Neither block of KATP channels nor stimulation of membrane depolarization by tolbutamide was different in INS-1 cells compared with Cav1.2/II-III cells, but whole-cell Cav current density was significantly increased in Cav1.2/II-III cells. Tolbutamide (200 μM) stimulated insulin secretion and Ca(2+) transients in INS-1 cells, and Cav1.2/II-III cells were completely blocked by nicardipine (2 μM), but thapsigargin (1 μM) blocked tolbutamide-stimulated secretion and Ca(2+) transients only in INS-1 cells. Tolbutamide-stimulated endoplasmic reticulum [Ca(2+)] decrease was reduced in Cav1.2/II-III cells compared with INS-1 cells. However, Ca(2+) transients in both INS-1 cells and Cav1.2/II-III cells were significantly potentiated by 8-pCPT-2'-O-Me-cAMP (5 μM), FPL-64176 (0.5 μM), or replacement of extracellular Ca(2+) with Sr(2+). Glucose (10 mM) + glucagon-like peptide-1 (10 nM) stimulated discrete spikes in [Ca(2+)]i in the presence of verapamil at a higher frequency in INS-1 cells than in Cav1.2/II-II cells. Glucose (18 mM) stimulated more frequent action potentials in Cav1.2/II-III cells and primary rat β-cells expressing the Cav1.2/II-II loop than in control cells. Further, apamin (1 μM) increased glucose-stimulated action potential frequency in INS-1 cells, but not Cav1.2/II-III cells, suggesting that SK channels were not activated under these conditions in Cav1.2/II-III loop-expressing cells. We propose the II-III loop of Cav1.2 as a key molecular determinant that couples the channel to Ca(2+)-induced Ca(2+) release and activation of SK channels in pancreatic β-cells.

Disruption of calcium homeostasis in cardiomyocytes underlies cardiac structural and functional changes in severe sepsis

Sepsis, a major cause of morbidity/mortality in intensive care units worldwide, is commonly associated with cardiac dysfunction, which worsens the prognosis dramatically for patients. Although in recent years the concept of septic cardiomyopathy has evolved, the importance of myocardial structural alterations in sepsis has not been fully explored. This study offers novel and mechanistic data to clarify subcellular events that occur in the pathogenesis of septic cardiomyopathy and myocardial dysfunction in severe sepsis. Cultured neonatal mice cardiomyocytes subjected to serum obtained from mice with severe sepsis presented striking increment of [Ca²⁺]_i and calpain-1 levels associated with decreased expression of dystrophin and disruption and derangement of F-actin filaments and cytoplasmic bleb formation. Severe sepsis induced in mice led to an increased expression of calpain-1 in cardiomyocytes. Moreover, decreased myocardial amounts of dystrophin, sarcomeric actin, and myosin heavy chain were observed in septic hearts associated with depressed cardiac contractile dysfunction and a very low survival rate. Actin and myosin from the sarcomere are first disassembled by calpain and then ubiquitinated and degraded by proteasome or sequestered inside specialized vacuoles called autophagosomes, delivered to the lysosome for degradation forming autophagolysosomes. Verapamil and dantrolene prevented the increase of calpain-1 levels and preserved dystrophin, actin, and myosin loss/reduction as well cardiac contractile dysfunction associated with strikingly improved survival rate. These abnormal parameters emerge as therapeutic targets, which modulation may provide beneficial effects on future vascular outcomes and mortality in sepsis. Further studies are needed to shed light on this mechanism, mainly regarding specific calpain inhibitors.

Capacity and plasticity of potassium channels and high-affinity transporters in roots of barley and Arabidopsis

The role of potassium (K(+)) transporters in high- and low-affinity K(+) uptake was examined in roots of intact barley (Hordeum vulgare) and Arabidopsis (Arabidopsis thaliana) plants by use of (42)K radiotracing, electrophysiology, pharmacology, and mutant analysis. Comparisons were made between results from barley and five genotypes of Arabidopsis, including single and double knockout mutants for the high-affinity transporter, AtHAK5, and the Shaker-type channel, AtAKT1. In Arabidopsis, steady-state K(+) influx at low external K(+) concentration ([K(+)]ext = 22.5 µm) was predominantly mediated by AtAKT1 when high-affinity transport was inhibited by ammonium, whereas in barley, by contrast, K(+) channels could not operate below 100 µm. Withdrawal of ammonium resulted in an immediate and dramatic stimulation of K(+) influx in barley, indicating a shift from active to passive K(+) uptake at low [K(+)]ext and yielding fluxes as high as 36 µmol g (root fresh weight)(-1) h(-1) at 5 mm [K(+)]ext, among the highest transporter-mediated K(+) fluxes hitherto reported. This ammonium-withdrawal effect was also established in all Arabidopsis lines (the wild types, atakt1, athak5, and athak5 atakt1) at low [K(+)]ext, revealing the concerted involvement of several transport systems. The ammonium-withdrawal effect coincided with a suppression of K(+) efflux and a significant hyperpolarization of the plasma membrane in all genotypes except athak5 atakt1, could be sustained over 24 h, and resulted in increased tissue K(+) accumulation. We discuss key differences and similarities in K(+) acquisition between two important model systems and reveal novel aspects of K(+) transport in planta.

Understanding alternative splicing of Cav1.2 calcium channels for a new approach towards individualized medicine

Calcium channel blockers (CCBs) are widely used to treat cardiovascular diseases such as hypertension, angina pectoris, hypertrophic cardiomyopathy, and supraventricular tachycardia. CCBs selectively inhibit the inward flow of calcium ions through voltage-gated calcium channels, particularly Ca_v1.2, that are expressed in the cardiovascular system. Changes to the molecular structure of Ca_v1.2 channels could affect sensitivity of the channels to blockade by CCBs. Recently, extensive alternative splicing was found in Ca_v1.2 channels that generated wide phenotypic variations. Cardiac and smooth muscles express slightly different, but functionally important Ca_v1.2 splice variants. Alternative splicing could also modulate the gating properties of the channels and giving rise to different responses to inhibition by CCBs. Importantly, alternative splicing of Ca_v1.2 channels may play an important role to influence the outcome of many cardiovascular disorders. Therefore, the understanding of how alternative splicing impacts Ca_v1.2 channels pharmacology in various diseases and different organs may provide the possibility for individualized therapy with minimal side effects.

Crystal structure of a Ba(2+)-bound gating ring reveals elementary steps in RCK domain activation

RCK domains control activity of a variety of K(+) channels and transporters through binding of cytoplasmic ligands. To gain insight toward mechanisms of RCK domain activation, we solved the structure of the RCK domain from the Ca(2+)-gated K(+) channel, MthK, bound with Ba(2+), at 3.1 Å resolution. The Ba(2+)-bound RCK domain was assembled as an octameric gating ring, as observed in structures of the full-length MthK channel, and shows Ba(2+) bound at several positions. One of the Ba(2+) sites, termed C1, overlaps with a known Ca(2+)-activation site, determined by residues D184 and E210. Functionally, Ba(2+) can activate reconstituted MthK channels as observed in electrophysiological recordings, whereas Mg(2+) (up to 100 mM) was ineffective. Ba(2+) activation was abolished by the mutation D184N, suggesting that Ba(2+) activates primarily through the C1 site. Our results suggest a working hypothesis for a sequence of ligand-dependent conformational changes that may underlie RCK domain activation and channel gating.

Glucose-6-phosphate dehydrogenase and NADPH redox regulates cardiac myocyte L-type calcium channel activity and myocardial contractile function

We recently demonstrated that a 17-ketosteroid, epiandrosterone, attenuates L-type Ca²⁺ currents (I_Ca-L) in cardiac myocytes and inhibits myocardial contractility. Because 17-ketosteroids are known to inhibit glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, and to reduce intracellular NADPH levels, we hypothesized that inhibition of G6PD could be a novel signaling mechanism which inhibit I_Ca-L and, therefore, cardiac contractile function. We tested this idea by examining myocardial function in isolated hearts and Ca²⁺ channel activity in isolated cardiac myocytes. Myocardial function was tested in Langendorff perfused hearts and I_Ca-L were recorded in the whole-cell patch configuration by applying double pulses from a holding potential of −80 mV and then normalized to the peak amplitudes of control currents. 6-Aminonicotinamide, a competitive inhibitor of G6PD, increased pCO₂ and decreased pH. Additionally, 6-aminonicotinamide inhibited G6PD activity, reduced NADPH levels, attenuated peak I_Ca-L amplitudes, and decreased left ventricular developed pressure and ±dp/dt. Finally, dialyzing NADPH into cells from the patch pipette solution attenuated the suppression of I_Ca-L by 6-aminonicotinamide. Likewise, in G6PD-deficient mice, G6PD insufficiency in the heart decreased GSH-to-GSSG ratio, superoxide, cholesterol and acetyl CoA. In these mice, M-mode echocardiographic findings showed increased diastolic volume and end-diastolic diameter without changes in the fraction shortening. Taken together, these findings suggest that inhibiting G6PD activity and reducing NADPH levels alters metabolism and leads to inhibition of L-type Ca²⁺ channel activity. Notably, this pathway may be involved in modulating myocardial contractility under physiological and pathophysiological conditions during which the pentose phosphate pathway-derived NADPH redox is modulated (e.g., ischemia-reperfusion and heart failure).

Rapid membrane responses to dihydrotestosterone are sex dependent in growth plate chondrocytes

Sex steroids are important regulators for longitudinal growth, bone mass accrual, and sexual dimorphism of the skeleton. 17β-Estradiol regulates proliferation and differentiation of female chondrocytes via a membrane-associated signaling pathway in addition to its estrogen receptor (ER) mediated effects. In contrast, testosterone does not elicit a similar membrane response, either in male or female cells. Whereas female rat growth plate chondrocytes convert testosterone to 17β-estradiol, male chondrocytes produce 5α-dihydrotestosterone (DHT). Previously DHT was found to mediate sex-specific effects of testosterone in male cells, but it is not known if a membrane-signaling pathway is involved. In this study, we hypothesized that DHT can induce sex-specific rapid membrane effects similar to other steroid hormones. Confluent cultures of chondrocytes isolated from resting zones of growth plates of both male and female rats were treated with 10(-10)-10(-7)M testosterone or DHT for 3, 9, 90 and 270min and protein kinase C (PKC) and phospholipase A2 (PLA2) activities were measured. To examine potential signaling pathways involved in PKC activation, male chondrocytes were treated with 10(-7)M DHT for 9min in the presence or absence of the phospholipase C (PLC) inhibitor U73122, the secretory PLA2 inhibitor quinacrine or the cytosolic PLA2 inhibitor AACOCF3; the Gαi inhibitor pertussis toxin (PTX) or Gαs activator cholera toxin (CTX), and the general G-protein inhibitor GDPβS; thapsigargin, an inhibitor of a Ca-ATPase pump in the endoplasmic reticulum; verapamil and nifedipine, inhibitors of specific L type Ca2+ channels on the cell membrane; and cyproterone acetate (CPA), which is an inhibitor of the classical androgen receptor (AR); as well as the transcription inhibitor actinomycin D, or the translation inhibitor cycloheximide. DHT induced a dose-dependent increase in PKC and PLA2 activity in male cells with the highest increase at 10(-7)M DHT (p<0.05), whereas testosterone had no effect. PKC activity was augmented at 9 and 90 min, and then decreased to baseline at 270min. Neither testosterone nor DHT affected PKC in female cells. U73122, quinacrine, and AACOCF3 inhibited DHT-induced activation of PKC. DHT treatment for 9 min had no effect in [(3)H]-thymidine incorporation in quiescent confluent cultures but caused a dose dependent increase in alkaline phosphatase specific activity. Inhibition of PLC reduced the response of to DHT in a dose dependent manner, indicating that PLC is involved. In conclusion, our study indicates that DHT, but not testosterone, has sex-specific rapid membrane effects in male growth plate chondrocytes involving PLC and PLA2-mediated PKC signaling pathways. Together with previous observations showing that male cells convert testosterone to DHT, these results suggest that DHT might act in the membrane through an autocrine/paracrine mechanism.

Benidipine persistently inhibits delayed rectifier K(+)-channel currents in murine thymocytes

Lymphocytes predominantly express delayed rectifier K(+)-channels (Kv1.3) in their plasma membranes, and the channels play crucial roles in the lymphocyte activation and proliferation. Since 1,4-dihydropyridine (DHP) Ca(2+) channel blockers (CCBs), which are highly lipophilic, exert relatively stronger immunomodulatory effects than the other types of CCBs, they would affect the Kv1.3-channel currents in lymphocytes. In the present study, employing the standard patch-clamp whole-cell recording technique in murine thymocytes, we examined the effects of benidipine, one of the most lipophilic DHPs, on the channel currents and the membrane capacitance and compared them with those of nifedipine. Both drugs significantly suppressed the peak and the pulse-end currents of the channels with significant decreases in the membrane capacitance. However, the effects of benidipine were more marked than those of nifedipine and were irreversible after the drug withdrawal. This study demonstrated for the first time that DHP CCBs, such as nifedipine and benidipine, exert inhibitory effects on thymocyte Kv1.3-channel currents. The persistent effect of benidipine was thought to be associated with its sustained accumulation in the plasma membranes as detected by the long-lasting decrease in the membrane capacitance.

Two mechanistically distinct effects of dihydropyridine nifedipine on CaV1.2 L-type Ca²⁺ channels revealed by Timothy syndrome mutation

Dihydropyridine Ca(2+) channel antagonists (DHPs) block Ca(V)1.2 L-type Ca(2+) channels (LTCCs) by stabilizing their voltage-dependent inactivation (VDI); however, it is still not clear how DHPs allosterically interact with the kinetically distinct (fast and slow) VDI. Thus, we analyzed the effect of a prototypical DHP, nifedipine on LTCCs with or without the Timothy syndrome mutation that resides in the I-II linker (L(I)-(II)) of Ca(V)1.2 subunits and impairs VDI. Whole-cell Ba(2+) currents mediated by rabbit Ca(V)1.2 with or without the Timothy mutation (G436R) (analogous to the human G406R mutation) were analyzed in the presence and absence of nifedipine. In the absence of nifedipine, the mutation significantly impaired fast closed- and open-state VDI (CSI and OSI) at -40 and 0 mV, respectively, but did not affect channels' kinetics at -100 mV. Nifedipine equipotently blocked these channels at -80 mV. In wild-type LTCCs, nifedipine promoted fast CSI and OSI at -40 and 0 mV and promoted or stabilized slow CSI at -40 and -100 mV, respectively. In LTCCs with the mutation, nifedipine resumed the impaired fast CSI and OSI at -40 and 0 mV, respectively, and had the same effect on slow CSI as in wild-type LTCCs. Therefore, nifedipine has two mechanistically distinct effects on LTCCs: the promotion of fast CSI/OSI caused by L(I-II) at potentials positive to the sub-threshold potential and the promotion or stabilization of slow CSI caused by different mechanisms at potentials negative to the sub-threshold potential.

A novel frequency analysis method for assessing K(ir)2.1 and Na (v)1.5 currents

Voltage clamping is an important tool for measuring individual currents from an electrically active cell. However, it is difficult to isolate individual currents without pharmacological or voltage inhibition. Herein, we present a technique that involves inserting a noise function into a standard voltage step protocol, which allows one to characterize the unique frequency response of an ion channel at different step potentials. Specifically, we compute the fast Fourier transform for a family of current traces at different step potentials for the inward rectifying potassium channel, K_ir2.1, and the channel encoding the cardiac fast sodium current, Na_v1.5. Each individual frequency magnitude, as a function of voltage step, is correlated to the peak current produced by each channel. The correlation coefficient vs. frequency relationship reveals that these two channels are associated with some unique frequencies with high absolute correlation. The individual IV relationship can then be recreated using only the unique frequencies with magnitudes of high absolute correlation. Thus, this study demonstrates that ion channels may exhibit unique frequency responses.

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.

A method to measure myocardial calcium handling in adult Drosophila

RATIONALE

Normal cardiac physiology requires highly regulated cytosolic Ca(2+) concentrations and abnormalities in Ca(2+) handling are associated with heart failure. The majority of approaches to identifying the components that regulate intracellular Ca(2+) dynamics rely on cells in culture, mouse models, and human samples. However, a genetically robust system for unbiased screens of mutations that affect Ca(2+) handling remains a challenge.

OBJECTIVE

We sought to develop a new method to measure myocardial Ca(2+) cycling in adult Drosophila and determine whether cardiomyopathic fly hearts recapitulate aspects of diseased mammalian myocardium.

METHODS AND RESULTS

Using engineered transgenic Drosophila that have cardiac-specific expression of Ca(2+)-sensing fluorescent protein, GCaMP2, we developed methods to measure parameters associated with myocardial Ca(2+) handling. The following key observations were identified: (1) Control w(1118) Drosophila hearts have readily measureable Ca(2+)-dependent fluorescent signals that are dependent on L-type Ca(2+) channels and SR Ca(2+) stores and originate from rostral and caudal pacemakers. (2) A fly mutant, held-up(2) (hdp(2)), that has a point mutation in troponin I and has a dilated cardiomyopathic phenotype demonstrates abnormalities in myocardial Ca(2+) handling that include increases in the duration of the 50% rise in intensity to peak intensity, the half-time of fluorescence decline from peak, the full duration at half-maximal intensity, and decreases in the linear slope of decay from 80% to 20% intensity decay. (3) Hearts from hdp(2) mutants had reductions in caffeine-induced Ca(2+) increases and reductions in ryanodine receptor (RyR) without changes in L-type Ca(2+) channel transcripts in comparison with w(1118).

CONCLUSIONS

Our results show that the cardiac-specific expression of GCaMP2 provides a means of characterizing propagating Ca(2+) transients in adult fly hearts. Moreover, the adult fruit fly heart recapitulates several aspects of Ca(2+) regulation observed in mammalian myocardium. A mutation in Drosophila that causes an enlarged cardiac chamber and impaired contractile function is associated with abnormalities in the cytosolic Ca(2+) transient as well as changes in transcript levels of proteins associated with Ca(2+) handling. This new methodology has the potential to permit an examination of evolutionarily conserved myocardial Ca(2+)-handing mechanisms by applying the vast resources available in the fly genomics community to conduct genetic screens to identify new genes involved in generated Ca(2+) transients and arrhythmias.

Neuromuscular synaptic patterning requires the function of skeletal muscle dihydropyridine receptors

Developing skeletal myofibers in vertebrates are intrinsically “pre-patterned” for motor nerve innervation. However, the intrinsic factors that regulate muscle pre-patterning remain unknown. Here we show that a functional skeletal muscle dihydropyridine receptor (DHPR, the L-type Ca²⁺ channel in muscle) is required for muscle pre-patterning during the development of the neuromuscular junction (NMJ). Targeted deletion of the β1 subunit of DHPR (Cacnb1) in mice leads to muscle pre-patterning defects, aberrant innervation and precocious maturation of the NMJ. Reintroducing the Cacnb1 gene into Cacnb1^−/− muscles reverses the pre-patterning defects and restores normal development of the NMJ. The mechanism by which DHPRs govern muscle pre-patterning is independent of their role in excitation-contraction coupling (E-C coupling), but requires Ca²⁺ influx through the L-type Ca²⁺ channel. Our findings demonstrate that the skeletal muscle DHPR retrogradely regulates the patterning and formation of the NMJ.

Recapitulation of an ion channel IV curve using frequency components

INTRODUCTION

Presently, there are no established methods to measure multiple ion channel types simultaneously and decompose the measured current into portions attributable to each channel type. This study demonstrates how impedance spectroscopy may be used to identify specific frequencies that highly correlate with the steady state current amplitude measured during voltage clamp experiments. The method involves inserting a noise function containing specific frequencies into the voltage step protocol. In the work presented, a model cell is used to demonstrate that no high correlations are introduced by the voltage clamp circuitry, and also that the noise function itself does not introduce any high correlations when no ion channels are present. This validation is necessary before the technique can be applied to preparations containing ion channels. The purpose of the protocol presented is to demonstrate how to characterize the frequency response of a single ion channel type to a noise function. Once specific frequencies have been identified in an individual channel type, they can be used to reproduce the steady state current voltage (IV) curve. Frequencies that highly correlate with one channel type and minimally correlate with other channel types may then be used to estimate the current contribution of multiple channel types measured simultaneously.

METHODS

Voltage clamp measurements were performed on a model cell using a standard voltage step protocol (-150 to +50 mV, 5mV steps). Noise functions containing equal magnitudes of 1-15 kHz frequencies (zero to peak amplitudes: 50 or 100mV) were inserted into each voltage step. The real component of the Fast Fourier transform (FFT) of the output signal was calculated with and without noise for each step potential. The magnitude of each frequency as a function of voltage step was correlated with the current amplitude at the corresponding voltages.

RESULTS AND CONCLUSIONS

In the absence of noise (control), magnitudes of all frequencies except the DC component correlated poorly (|R|<0.5) with the IV curve, whereas the DC component had a correlation coefficient greater than 0.999 in all measurements. The quality of correlation between individual frequencies and the IV curve did not change when a noise function was added to the voltage step protocol. Likewise, increasing the amplitude of the noise function also did not increase the correlation. Control measurements demonstrate that the voltage clamp circuitry by itself does not cause any frequencies above 0 Hz to highly correlate with the steady-state IV curve. Likewise, measurements in the presence of the noise function demonstrate that the noise function does not cause any frequencies above 0 Hz to correlate with the steady-state IV curve when no ion channels are present. Based on this verification, the method can now be applied to preparations containing a single ion channel type with the intent of identifying frequencies whose amplitudes correlate specifically with that channel type.

Inhibition of recombinant L-type voltage-gated calcium channels by positive allosteric modulators of GABAA receptors

Benzodiazepines (BDZs) depress neuronal excitability via positive allosteric modulation of inhibitory GABA(A) receptors (GABA(A)R). BDZs and other positive GABA(A)R modulators, including barbiturates, ethanol, and neurosteroids, can also inhibit L-type voltage-gated calcium channels (L-VGCCs), which could contribute to reduced neuronal excitability. Because neuronal L-VGCC function is up-regulated after long-term GABA(A)R modulator exposure, an interaction with L-VGCCs may also play a role in physical dependence. The current studies assessed the effects of BDZs (diazepam, flurazepam, and desalkylflurazepam), allopregnanolone, pentobarbital, and ethanol on whole-cell Ba(2+) currents through recombinant neuronal Ca(v)1.2 and Ca(v)1.3 L-VGCCs expressed with β(3) and α(2)δ-1 in HEK293T cells. Allopregnanolone was the most potent inhibitor (IC(50), ∼10 μM), followed by BDZs (IC(50), ∼50 μM), pentobarbital (IC(50), 0.3-1 mM), and ethanol (IC(50), ∼300 mM). Ca(v)1.3 channels were less sensitive to pentobarbital inhibition than Ca(v)1.2 channels, similar to dihydropyridine (DHP) L-VGCC antagonists. All GABA(A)R modulators induced a negative shift in the steady-state inactivation curve of Ca(v)1.3 channels, but only BDZs and pentobarbital induced a negative shift in Ca(v)1.2 channel inactivation. Mutation of the high-affinity DHP binding site (T1039Y and Q1043M) in Ca(v)1.2 channels reduced pentobarbital potency. Despite the structural similarity between benzothiazepines and BDZs, mutation of an amino acid important for diltiazem potency (I1150A) did not affect diazepam potency. Although L-VGCC inhibition by BDZs occurred at concentrations that are possibly too high to be clinically relevant and is not likely to play a role in the up-regulation of L-VGCCs during long-term treatment, pentobarbital and ethanol inhibited L-VGCCs at clinically relevant concentrations.

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.

Calcium channel blockers differentially modulate cytokine production by peripheral blood mononuclear cells

BACKGROUND

Calcium channel blockers (CCB) are known to modulate immune reactions, so the present study was performed to examine the effects of various CCBs that have shown different effects on transcription factors and on the production of pro-inflammatory cytokines by human peripheral blood mononuclear cells (PBMC).

METHODS AND RESULTS

PBMC from healthy volunteers were isolated by Ficoll-paque density centrifugation. To study the effect of CCBs, the PBMC were stimulated with lipopolysaccharide or concanavalin A. After 24 h of incubation, the supernatants were harvested and the interleukin (IL)-1alpha, -1beta, and -6, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma levels were determined by specific enzyme-linked immunosorbent assay. The production of IL-1alpha and -1beta stimulated with lipopolysaccharide was significantly increased in the presence of amlodipine. In contrast, nifedipine and verapamil suppressed the production of IL-1beta, TNF-alpha, and IFN-gamma. Amlodipine and diltiazem significantly increased production of IL-1alpha stimulated with concanavalin A. Nifedipine inhibited production of IL-1alpha, IL-6, and IFN-gamma. Verapamil suppressed production of IFN-gamma.

CONCLUSIONS

Differential modulation of cytokine production was seen with various CCBs, and the suppressive effect of nifedipine was most prominent.

Structural model for phenylalkylamine binding to L-type calcium channels

Phenylalkylamines (PAAs), a major class of L-type calcium channel (LTCC) blockers, have two aromatic rings connected by a flexible chain with a nitrile substituent. Structural aspects of ligand-channel interactions remain unclear. We have built a KvAP-based model of LTCC and used Monte Carlo energy minimizations to dock devapamil, verapamil, gallopamil, and other PAAs. The PAA-LTCC models have the following common features: (i) the meta-methoxy group in ring A, which is proximal to the nitrile group, accepts an H-bond from a PAA-sensing Tyr_IIIS6; (ii) the meta-methoxy group in ring B accepts an H-bond from a PAA-sensing Tyr_IVS6; (iii) the ammonium group is stabilized at the focus of P-helices; and (iv) the nitrile group binds to a Ca(2+) ion coordinated by the selectivity filter glutamates in repeats III and IV. The latter feature can explain Ca(2+) potentiation of PAA action and the presence of an electronegative atom at a similar position of potent PAA analogs. Tyr substitution of a Thr in IIIS5 is known to enhance action of devapamil and verapamil. Our models predict that the para-methoxy group in ring A of devapamil and verapamil accepts an H-bond from this engineered Tyr. The model explains structure-activity relationships of PAAs, effects of LTCC mutations on PAA potency, data on PAA access to LTCC, and Ca(2+) potentiation of PAA action. Common and class-specific aspects of action of PAAs, dihydropyridines, and benzothiazepines are discussed in view of the repeat interface concept.

Optimization of Ca(v)1.2 screening with an automated planar patch clamp platform

INTRODUCTION

Ca(v)1.2 channels play an important role in shaping the cardiac action potential. Screening pharmaceutical compounds for Ca(v)1.2 block is very important in developing drugs without cardiac liability. Ca(v)1.2 screening has been traditionally done using fluorescence assays, but these assays have some limitations. Patch clamping is considered the gold standard for ion channel studies, but is very labor intensive. The purpose of this study was to develop a robust medium throughput Ca(v)1.2 screening assay in PatchXpress 7000A by optimizing cell isolation conditions, recording solutions and experimental parameters. Under the conditions established, structurally different standard Ca(v)1.2 antagonists and an agonist were tested.

METHODS

HEK-293 cells stably transfected with hCa(v)1.2 L-type Ca channel were used. For experiments, cells were isolated using 0.05% Trypsin. Currents were recorded in the presence of 30 mM extracellular Ba2+ and low magnesium intracellular recording solution to minimize rundown. Ca(v)1.2 currents were elicited from a holding potential of -60 mV at 0.05 Hz to increase pharmacological sensitivity and minimize rundown. Test compounds were applied at increasing concentrations for 5 min followed by a brief washout.

RESULTS

Averaged peak Ca(v)1.2 current amplitudes were increased from 10 pA/pF to 15 pA/pF by shortening cell incubation and trypsin exposure time from 2.5 min at 37 degrees C to 1 min at room temperature and adding 0.2 mM cAMP to the intracellular solution. Rundown was minimized from 2%/min to 0.5%/min by reducing the intracellular free Mg2+ from 2.7 mM to 0.2 mM and adding 100 nM Ca2+. Under the established conditions, we tested 8 structurally different antagonists and an agonist. The IC(50) values obtained ranked well against published values and results obtained using traditional clamp experiments performed in parallel using the expressed cell line and native myocytes.

DISCUSSION

This assay can be used as a reliable pharmacological screening tool for Ca(v)1.2 block to assess compounds for cardiac liability during lead optimization.

Studies of verapamil binding to human serum albumin by high-performance affinity chromatography

The binding of verapamil to the protein human serum albumin (HSA) was examined by using high-performance affinity chromatography. Many previous reports have investigated the binding of verapamil with HSA, but the exact strength and nature of this interaction (e.g. the number and location of binding sites) is still unclear. In this study, frontal analysis indicated that at least one major binding site was present for R- and S-verapamil on HSA, with estimated association equilibrium constants on the order of 10(4)M(-1) and a 1.4-fold difference in these values for the verapamil enantiomers at pH 7.4 and 37 degrees C. The presence of a second, weaker group of binding sites on HSA was also suggested by these results. Competitive binding studies using zonal elution were carried out between verapamil and various probe compounds that have known interactions with several major and minor sites on HSA. R/S-Verapamil was found to have direct competition with S-warfarin, indicating that verapamil was binding to Sudlow site I (i.e. the warfarin-azapropazone site of HSA). The average association equilibrium constant for R- and S-verapamil at this site was 1.4 (+/-0.1)x10(4)M(-1). Verapamil did not have any notable binding to Sudlow site II of HSA but did appear to have some weak allosteric interactions with l-tryptophan, a probe for this site. An allosteric interaction between verapamil and tamoxifen (a probe for the tamoxifen site) was also noted, which was consistent with the binding of verapamil at Sudlow site I. No interaction was seen between verapamil and digitoxin, a probe for the digitoxin site of HSA. These results gave good agreement with previous observations made in the literature and help provide a more detailed description of how verapamil is transported in blood and of how it may interact with other drugs in the body.

In vitro characterization of micropatterned PLGA-PHBV8 blend films as temporary scaffolds for photoreceptor cells

In developed countries the aging population faces increasing risks of blinding retinal diseases, for which there are few effective treatments available. Photoreceptor transplantation represents one approach, but generally results have been disappointing. We hypothesize that micropatterned biodegradable poly(L-lactic acid-co-glycolic acid)/poly(hydroxybutyrate-co-hydroxyvaleric acid) (PLGA-PHBV8) blend films could deliver photoreceptor cells in a more organized manner than bolus injections. Blending of PLGA and PHBV8 was used to optimize the degradation rate of the temporary template. At the end of 8 weeks, for both thin and thick films of PLGA-PHBV8 a 50% decrease of their initial weight with increasing water uptake was observed. When photoreceptor cells were seeded onto micropatterned PLGA-PHBV8 films with parallel grooves (21- and 42-microm-wide grooves and 20 microm ridge width and depth), the cells preferred laminin-deposited grooves to ridges and expressed rod- and cone-specific markers such as rhodopsin and arrestin. A loss in photoreceptor viability of 50% was observed after 7 days in culture. The effects of either retinal pigment epithelium (RPE)-derived or Muller glial cell-derived conditioned media or bFGF on the survival of photoreceptor cells seeded on PLGA-PHBV8 films were investigated. Addition of either RPE- and Muller-conditioned media increased statistically (p < 0.01) the viability of photoreceptor cells after 7 days of incubation. Our results suggest that such biodegradable micropatterned PLGA-PHBV8 blend films have a potential to deliver photoreceptor cells to the subretinal space and ensure laminar organization and maintenance of differentiation, and that incorporation of intrinsic factors within the scaffold would enhance the survival rate of transplanted cells.

Hydrogen sulphide is an inhibitor of L-type calcium channels and mechanical contraction in rat cardiomyocytes

AIMS

Hydrogen sulphide (H(2)S) is an endogenously generated gaseous transmitter that has recently been suggested to regulate cardiovascular functions. To date, there is no direct evidence for a potential role of H(2)S in regulating calcium channels in the heart. The present study aims to examine the hypothesis that H(2)S is a novel inhibitor of the L-type calcium channel current (I(Ca,L)).

METHODS AND RESULTS

Electrophysiological measurements were performed in cardiomyocytes isolated from Wistar-Kyoto and spontaneously hypertensive rats. Bath application of 100 microM NaHS (a H(2)S donor) significantly reduced the time required for the repolarization of the action potential. Inhibition of the peak I(Ca,L) by NaHS was determined to be concentration-dependent (25, 50, 100, 200, and 400 microM). NaHS inhibited the recovery from depolarization-induced inactivation. Electric field-induced [Ca(2+)]i transients and contraction of single cardiomyocytes and isolated papillary muscles were reduced by NaHS treatment. In contrast, caffeine induced an increase in [Ca(2+)]i that was not altered by NaHS. NaHS had no effect on the K(ATP) current or on the levels of cAMP and cGMP in the current study.

CONCLUSION

H(2)S is a novel inhibitor of L-type calcium channels in cardiomyocytes. Moreover, H(2)S-induced inhibition of [Ca(2+)]i appears to be a secondary effect owing to its initial action towards I(Ca,L). The inhibitory effect of H(2)S on I(Ca,L) requires further investigation, particularly in the exploration of new pathways involved in cardiac calcium homeostasis and disease pathology.

Molecular modeling of benzothiazepine binding in the L-type calcium channel

Benz(othi)azepine (BTZ) derivatives constitute one of three major classes of L-type Ca(2+) channel ligands. Despite intensive experimental studies, no three-dimensional model of BTZ binding is available. Here we have built KvAP- and KcsA-based models of the Ca(v)1.2 pore domain in the open and closed states and used multiple Monte Carlo minimizations to dock representative ligands. In our open channel model, key functional groups of BTZs interact with BTZ-sensing residues, which were identified in previous mutational experiments. The bulky tricyclic moiety occupies interface between domains III and IV, while the ammonium group protrudes into the inner pore, where it is stabilized by nucleophilic C-ends of the pore helices. In the closed channel model, contacts with several ligand-sensing residues in the inner helices are lost, which weakens ligand-channel interactions. An important feature of the ligand-binding mode in both open and closed channels is an interaction between the BTZ carbonyl group and a Ca(2+) ion chelated by the selectivity filter glutamates in domains III and IV. In the absence of Ca(2+), the tricyclic BTZ moiety remains in the domain interface, while the ammonium group directly interacts with a glutamate residue in the selectivity filter. Our model suggests that the Ca(2+) potentiation involves a direct electrostatic interaction between aCa(2+) ion and the ligand rather than an allosteric mechanism. Energy profiles indicate that BTZs can reach the binding site from the domain interface, whereas access through the open activation gate is unlikely, because reorientation of the bulky molecule in the pore is hindered.

Compartmentalized regulations of ion channels in the heart

The rate and force of contraction of the heart are precisely controlled by compartmentalized regulation of cardiac ion channels which determine electrical activities. It is known that modulation of cardiac ion channels, which is caused by drug administration, sympathetic nervous system stimulation and gender difference, can increase risks of lethal arrhythmias in carriers of inherited disease mutations. These modulations are thought to also be involved in common cardiac arrhythmias. Because many signaling molecules are localized within single cells, an understanding of the molecular basis of compartmentalized regulation of cardiac channels is a key for understanding and treating the lethal arrhythmias. In this review, I will discuss molecular mechanisms of compartmentalized regulation of cardiac ion channels via drugs, cAMP and sex hormones.

Effects of phenylalkylamines and benzothiazepines on Cav1.3-mediated Ca2+ currents in neonatal mouse inner hair cells

Calcium currents (I(Ca)) in inner hair cells (IHCs) are carried by the Ca(v)1.3 subtype of L-type calcium channels. They play an important role in synaptic transmission of sound-evoked mechanical stimuli. L-type calcium channels are targets of the organic blocker classes dihydropyridines, phenylalkylamines and benzothiazepines. Previously a low sensitivity of the Ca(v)1.3 subtype towards dihydropyridines has been demonstrated. Therefore, this study evaluates the effect of two phenylalkylamines (verapamil and gallopamil) and the benzothiazepine diltiazem on I(Ca) through Ca(v)1.3 channels in mouse IHCs. Whole-cell I(Ca) was measured using the patch-clamp technique in mouse IHCs aged postnatal day 3-7 with 5 mM calcium as a charge carrier. The phenylalkylamines verapamil and gallopamil and the benzothiazepine diltiazem inhibited I(Ca) in IHCs in a concentration-dependent manner. This block was largely reversible. Dose-response curves revealed IC(50) values of 199+/-19 microM for verapamil, 466+/-151 microM for gallopamil and 326+/-67 microM for diltiazem. The inhibition of peak I(Ca) by phenylalkylamines and benzothiazepines was voltage-independent. Verapamil (300 microM) enhanced current inactivation from -20 to +20 mV while diltiazem (300 microM) did so only at very depolarised potentials (+20 mV). In conclusion, the concentrations of phenylalkylamines and benzothiazepine necessary to inhibit 50% of I(Ca) in IHCs were one order larger compared to concentrations which inhibited I(Ca) through Ca(v)1.2 channels in native cells or expression systems. However, inhibitory concentrations were in the same range as those required for block of I(Ca) in turtle hair cells.

Electromechanical characterization of cinnamophilin, a natural thromboxane A2 receptor antagonist with anti-arrhythmic activity, in guinea-pig heart

BACKGROUND AND PURPOSE

Cinnamophilin, a thromboxane A(2) receptor antagonist, has been identified as a prominent anti-arrhythmic agent in rat heart. This study aimed to determine its electromechanical and anti-arrhythmic effects in guinea-pig hearts.

EXPERIMENTAL APPROACH

Microelectrodes were used to study action potentials in ventricular papillary muscles. Fluo-3 fluorimetric ratio and whole-cell voltage-clamp techniques were used to record calcium transients and membrane currents in single ventricular myocytes, respectively. Intracardiac electrocardiograms were obtained and the anti-arrhythmic efficacy was determined from isolated perfused hearts.

KEY RESULTS

In papillary muscles, cinnamophilin decreased the maximal rate of upstroke (V(max)) and duration of action potential, and reduced the contractile force. In single ventricular myocytes, cinnamophilin reduced Ca(2+) transient amplitude. Cinnamophilin decreased the L-type Ca(2+) current (I(Ca,L))(IC(50)=7.5 microM) with use-dependency, induced a negative shift of the voltage-dependent inactivation and retarded recovery from inactivation. Cinnamophilin also decreased the Na(+) current (I(Na)) (IC(50)=2.7 microM) and to a lesser extent, the delayed outward (I(K)), inward rectifier (I(K1)), and ATP-sensitive (I(K,ATP)) K(+) currents. In isolated perfused hearts, cinnamophilin prolonged the AV nodal conduction interval and Wenckebach cycle length and the refractory periods of the AV node, His-Purkinje system and ventricle, while shortening the ventricular repolarization time. Additionally, cinnamophilin reduced the occurrence of reperfusion-induced ventricular fibrillation.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the promising anti-arrhythmic effect and the changes in the electromechanical function induced by cinnamophilin in guinea-pig heart can be chiefly accounted for by inhibition of I(Ca,L) and I(Na).

Molecular requirements for L-type Ca2+ channel blockade by testosterone

Despite being generally perceived as detrimental to the cardiovascular system, testosterone has marked beneficial vascular effects; most notably it acutely and directly causes vasodilatation. Indeed, men with hypotestosteronaemia can present with myocardial ischemia and angina which can be rapidly alleviated by infusion of testosterone. To date, however, in vitro studies have failed to provide a convincing mechanism to account for this clinically important effect. Here, using whole-cell patch-clamp recordings to measure current flow through recombinant human L-type Ca2+ channel alpha(1C) subunits (Ca(v)1.2), we demonstrate that testosterone inhibits such currents in a concentration-dependent manner. Importantly, this occurs over the physiological range of testosterone concentrations (IC50 34 nM), and is not mimicked by the metabolite 5alpha-androstan-17beta-ol-3-one (DHT), nor by progesterone or estradiol, even at high (10 microM) concentration. L-type Ca2+ channels in the vasculature are also important clinical targets for vasodilatory dihydropyridines. A single point mutation (T1007Y) almost completely abolishes nifedipine sensitivity in our recombinant expression system. Crucially, the same mutation renders the channels insensitive to testosterone. Our data strongly suggest, for the first time, the molecular requirements for testosterone binding to L-type Ca2+ channels, thereby supporting its beneficial role as an endogenous Ca2+ channel antagonist in the treatment of cardiovascular disease.

Effects of Thiopental on Ca2+ Currents and Intracellular Ca2+ Transient in Single Atrial Cells from Guinea Pig

The effects of thiopental on Ca2+ currents and intracellular Ca2+ transient in single atrial cells from guinea pigs were studied by means of a whole-cell voltage-clamp method and Ca2+-sensitive fluorescent dye. Thiopental inhibited L-type voltage-dependent Ca2+ currents in a concentration-dependent manner (IC50=2.8.10(-5) mol/l). Moreover, the mode of Ca2+ current inhibition by thiopental showed no use dependency. Electrical stimulation-induced intracellular Ca2+ transient was significantly suppressed by 10(-5) mol/l thiopental. However, the caffeine-sensitive Ca2+ releasing pathway from sarcoplasmic reticulum was not affected by thiopental. Our results indicate that thiopental inhibits L-type Ca2+ currents, but not release of Ca2+ from sarcoplasmic reticulum. These results suggest that the negative inotropic action of thiopental is mainly due to inhibition of L-type Ca2+ channels in guinea pig atrial myocytes.