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Cardiomyocytes are contractile cells that regulate heart contraction. Ca

The application of mechanical stimuli to cells often induce increases in intracellular calcium, affecting the regulation of a variety of cell functions. Although the mechanism of mechanotransduction-induced calcium increases has not been fully resolved, the involvement of mechanosensitive ion channels in the plasma membrane and the endoplasmic reticulum has been reported. Here, we demonstrate that voltage-gated L-type calcium channels play a critical role in the mechanosensitive calcium response in H9c2 rat cardiomyocytes. The intracellular calcium level in H9c2 cells increased in a reproducible dose-dependent manner in response to uniaxial stretching. The stretch-activated calcium response (SICR) completely disappeared in calcium-free medium, whereas thapsigargin and cyclopiazonic acid, inhibitors of sarcoendoplasmic reticulum calcium ATPase, partially reduced the SICR. These findings suggest that both calcium influx across the cell membrane and calcium release from the sarcoendoplasmic reticulum are involved in the SICR. Nifedipine, diltiazem, and verapamil, inhibitors of L-type calcium channels, reduced the SICR in a dose-dependent manner. Furthermore, small interfering RNA against the L-type calcium channel α1c subunit diminished the SICR dramatically. Nifedipine also diminished the mechanosensitivity of Langendorff-perfused rat heart. These results suggest that the SICR in H9c2 cardiomyocytes involves the activation of L-type calcium channels and subsequent calcium release from the sarcoendoplasmic reticulum.

Mitochondria are an important source of reactive oxygen species (ROS). Although it has been reported that myocardial stretch increases cellular ROS production by activating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2), referred to as X-ROS signalling, the involvement of mitochondria in X-ROS is not clear. Mitochondria are organelles that generate adenosine triphosphate (ATP) for cellular energy needs, which are mechanical-load-dependent. Therefore, it would not be surprising if these organelles had mechano-sensitive functions associated with stretch-induced ROS production. In the present study, we investigated the relation between X-ROS and mitochondrial stretch-sensitive responses in isolated mouse cardiac myocytes. The cells were subjected to 10% axial stretch using computer-controlled, piezo-manipulated carbon fibres attached to both cell ends. Cellular ROS production and mitochondrial membrane potential (Δψm) were assessed optically by confocal microscopy. The axial stretch increased ROS production and hyperpolarised Δψm. Treatment with a mitochondrial metabolic uncoupler, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), at 0.5 μM did not suppress stretch-induced ROS production, whereas treatment with a respiratory Complex III inhibitor, antimycin A (5 μM), blunted the response. Although NOX inhibition by apocynin abrogated the stretch-induced ROS production, it did not suppress stretch-induced hyperpolarisation of Δψm. These results suggest that stretch causes activation of the respiratory chain to hyperpolarise Δψm, followed by NOX activation, which increases ROS production.

Azelnidipine and amlodipine are dihydropyridine-type Ca(2+) channel blockers for the treatment of hypertension. Although these drugs have high vasoselectivity and small negative inotropic effects in vivo, little is known regarding their direct effects on cellular contractility without humoral regulation or the additive effects of these drugs with other antihypertensive drugs on myocardial contractility. To investigate the effects of Ca(2+) channel blockers on single cell mechanics, mouse cardiomyocytes were enzymatically isolated, and a pair of carbon fibers was attached to opposite cell-ends to stretch the cells. Cells were paced at 4 Hz superfused in normal Tyrode solution at 37°C. Cell length and active/passive force calculated from carbon fiber bending were recorded in 6 different preload conditions. Slopes of end-systolic force-length relation curves (maximum elastance) were measured as an index of contractility before and after drugs were administered. Azelnidipine at 10nM and 100 nM did not change maximum elastance, while amlodipine at 100 nM did decrease maximum elastance. The combination of RNH-6270 (active form of angiotensin II receptor blocker, olmesartan, 10nM) and either amlodipine (10nM) or azelnidipine (10nM) did not affect maximum elastance. Although both amlodipine and azelnidipine can be used safely at therapeutically relevant concentrations even in combination with olmesartan, the present results suggest that azelnidipine has a less negative inotropic action compared to amlodipine.

The aim of this present study was to provide a scaffold as a tool for the investigation of the effect of mechanical stimulation on three-dimensionally cultured cells. For this purpose, we developed an artificial self-assembling peptide (SPG-178) hydrogel scaffold. The structural properties of the SPG-178 peptide were confirmed by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and transmission electron microscopy (TEM). The mechanical properties of the SPG-178 hydrogel were studied using rheology measurements. The SPG-178 peptide was able to form a stable, transparent hydrogel in a neutral pH environment. In the SPG-178 hydrogel, mouse skeletal muscle cells proliferated successfully (increased by 12.4 ± 1.5 times during 8 days of incubation; mean ± SEM). When the scaffold was statically stretched, a rapid phosphorylation of ERK was observed (increased by 2.8 ± 0.2 times; mean ± SEM). These results demonstrated that the developed self-assembling peptide gel is non-cytotoxic and is a suitable tool for the investigation of the effect of mechanical stimulation on three-dimensional cell culture.

We have previously reported the electrophysiological properties of sarcolemmal stretch-activated BKCa (SAKCA) channels cloned from cultured chick embryonic ventricular myocytes. However, the role of BKCa channels in the electrophysiology of the more mature heart is not clear. We have investigated the effects on the BKCa current of axial stretch in post-hatch ventricular myocytes. Whole-cell currents of ventricular myocytes isolated from 2-week-old chicks were recorded using the patch-clamp technique, while the cells were either held at resting length or stretched to cause a 10% increase in sarcomere length using a pair of carbon fibres attached to opposite ends of the cell. Stretch did not affect whole-cell currents immediately after the stretch was applied. However, sustained stretch for 3 min significantly increased outward currents. This stretch-induced change was reversed by applying 10 nm iberiotoxin, a specific BKCa channel blocker, or a Na+–Ca2+-free environment. These results were reproduced in a computer simulation study. The present study is the first report about the sarcolemmal BKCa current from post-hatch ventricular myocytes. The present results suggest that axial stretch activates BKCa channels via a stretch-induced increase in the cytosolic Na+ concentration followed by an increased Ca2+ influx.

Beta2-glycoprotein I (beta2-GPI) is a major antigen for anti-cardiolipin antibodies and their epitopes are cryptic. Conformation of each domain of beta2-GPI was optimized from its crystal structure by energy minimization and by molecular dynamics simulation. Three electrostatic interactions, i.e. D193-K246, D222-K317 and E228-K308, were observed between domains IV and V in the optimized structure that was constructed based on the consensus sequences obtained by the phage-displayed random peptide library. Antigenic structures determined by the epitope mapping mainly consisted of hydrophobic amino acids located on two discontinuous sequences in domain IV. These amino acid clusters, as an epitope, were covered by domain V and were of a hidden nature. A similar but incomplete counterpart to the epitopic clusters was found in domain I but was not in domains II or III. Binding of anti-beta2-GPI auto-antibodies to solid-phase beta2-GPI was significantly reduced either by L replacement for W235, a common amino acid component for the epitopes, or by V replacement for all of D193, D222 and E228. Structural analysis indicated a hypothesis that these electrostatic interactions between domains IV and V retained exposure to W235 and that epitope spreading occurred in the region surrounding W235. Thus, epitopic structures recognized by anti-beta2-GPI auto-antibodies are cryptic and inter-domain electrostatic interactions are involved in their in exposure.

beta(2)-Glycoprotein I (beta(2)-GPI) is a major antigen for antiphospholipid antibodies (Abs) present in patients with the antiphospholipid syndrome (APS). We previously reported that beta(2)-GPI specifically binds to oxidized low density lipoprotein (oxLDL), but not to native low density lipoprotein (LDL). In the present study, a ligand specific for beta(2)-GPI, oxLig-1, was purified from the extracted lipids of oxLDL. The structure of oxLig-1 was shown to be identical to that of synthesized 7-ketocholesteryl-9-carboxynonanoate by mass spectroscopy and nuclear magnetic resonance analyses. Both purified and synthesized oxLig-1 were recognized by beta(2)-GPI and subsequently by anti-beta(2)-GPI auto-Abs, either in enzyme-linked immunosorbent assay (ELISA) or in ligand blot analysis. Binding of liposomes containing oxLig-1 (oxLig-1-liposomes) to mouse macrophages, J774A.1 cells, was relatively low, as compared with that of phosphatidylserine (PS)-liposomes. In contrast, binding of oxLig-1-liposomes was enhanced more than 10-fold in the presence of both beta(2)-GPI and an anti-beta(2)-GPI auto-Ab (WB-CAL-1), derived from (NZW x BXSB) F1 mouse, an animal APS model. Anti-beta(2)-GPI auto-Abs derived from APS patients with episodes of arterial thrombosis were detected in ELISA, using a solid phase oxLig-1 complexed with beta(2)-GPI. We suggest that autoimmune atherogenesis linked to beta(2)-GPI interaction with oxLDL and Abs may be present in APS.

Problem The present study assesses the clinical significance of anti-laminin-1 auto-antibodies (auto-Abs) in recurrent miscarriages. Method of study A total of 207 recurrent aborters with a history of two or more consecutive first-trimester miscarriages were tested for the presence of anti-laminin-1 Abs, beta2-glycoprotein I-dependent anticardiolipin Abs, lupus anticoagulants, anti-DNA Abs, and anti-nuclear Abs, before they had conceived again. Recurrent aborters then were followed up during subsequent pregnancies and their outcomes were evaluated relative to their blood test results prior to pregnancy. Results Fifty-five (31.1%) women out of 177 recurrent aborters were positive for IgG anti-laminin-1 auto-Abs. The levels of IgG anti-laminin-1 auto-Abs in recurrent aborters were significantly higher than those in healthy pregnant women and in healthy non-pregnant women (P = 0.0043 and 0.0073, respectively). The live birth rate of subsequent pregnancies in IgG anti-laminin-1 auto-Abs-positive recurrent aborters was significantly lower than the IgG anti-laminin-1 auto-Abs-negative recurrent aborters (P = 0.0320). There were no specifically significant relationships observed between IgG anti-laminin-1 auto-Abs and other tested auto-Abs. Conclusion IgG anti-laminin-1 auto-Abs are associated with recurrent miscarriages and the subsequent pregnancy outcome of recurrent aborters.

We have previously reported the electrophysiological properties of sarcolemmal stretch-activated BK(Ca) (SAKCA) channels cloned from cultured chick embryonic ventricular myocytes. However, the role of BK(Ca) channels in the electrophysiology of the more mature heart is not clear. We have investigated the effects on the BK(Ca) current of axial stretch in post-hatch ventricular myocytes. Whole-cell currents of ventricular myocytes isolated from 2-week-old chicks were recorded using the patch-clamp technique, while the cells were either held at resting length or stretched to cause a 10% increase in sarcomere length using a pair of carbon fibres attached to opposite ends of the cell. Stretch did not affect whole-cell currents immediately after the stretch was applied. However, sustained stretch for 3 min significantly increased outward currents. This stretch-induced change was reversed by applying 10 nm iberiotoxin, a specific BK(Ca) channel blocker, or a Na(+)-Ca(2+)-free environment. These results were reproduced in a computer simulation study. The present study is the first report about the sarcolemmal BK(Ca) current from post-hatch ventricular myocytes. The present results suggest that axial stretch activates BK(Ca) channels via a stretch-induced increase in the cytosolic Na(+) concentration followed by an increased Ca(2+) influx.

To clarify the consequences of the valine/leucine polymorphism at position 247 of the beta(2)-glycoprotein I (beta(2)GPI) gene in patients with antiphospholipid syndrome (APS), by investigating the correlation between genotypes and the presence of anti-beta(2)GPI antibody. The reactivity of anti-beta(2)GPI antibodies was characterized using recombinant Val(247) and Leu(247) beta(2)GPI.Sixty-five Japanese patients with APS and/or systemic lupus erythematosus who were positive for antiphospholipid antibodies and 61 controls were analyzed for the presence of the Val/Leu(247) polymorphism of beta(2)GPI. Polymorphism assignment was determined by polymerase chain reaction followed by restriction enzyme digestion. Recombinant Val(247) and Leu(247) beta(2)GPI were established to compare the reactivity of anti-beta(2)GPI antibodies to beta(2)GPI between these variants. The variants were prepared on polyoxygenated plates or cardiolipin-coated plates, and the reactivity of a series of anti-beta(2)GPI antibodies (immunized anti-human beta(2)GPI monoclonal antibodies [Cof-19-21] and autoimmune anti-beta(2)GPI monoclonal antibodies [EY1C8, EY2C9, and TM1G2]) and IgGs purified from patient sera was investigated.A positive correlation between the Val(247) allele and the presence of anti-beta(2)GPI antibodies was observed in the patient group. Human monoclonal/polyclonal anti-beta(2)GPI autoantibodies showed higher binding to recombinant Val(247) beta(2)GPI than to Leu(247) beta(2)GPI, although no difference in the reactivity of the immunized anti-beta(2)GPI between these variants was observed. Conformational optimization showed that the replacement of Leu(247) by Val(247) led to a significant alteration in the tertiary structure of domain V and/or the domain IV-V interaction.The Val(247) beta(2)GPI allele was associated with both a high frequency of anti-beta(2)GPI antibodies and stronger reactivity with anti-beta(2)GPI antibodies compared with the Leu(247) beta(2)GPI allele, suggesting that the Val(247) beta(2)GPI allele may be one of the genetic risk factors for development of APS.