Measuring single cardiac myocyte contractile force via moving a magnetic bead

One of the biggest problems of heart failure is the heart's inability to effectively pump blood to meet the body's demands, which may be caused by disease-induced alterations in contraction properties (such as contractile force and Young's modulus). Thus, it is very important to measure contractile properties at single cardiac myocyte level that can lay the foundation for quantitatively understanding the mechanism of heart failure and understanding molecular alterations in diseased heart cells. In this article, we report a novel single cardiac myocyte contractile force measurement technique based on moving a magnetic bead. The measuring system is mainly composed of 1), a high-power inverted microscope with video output and edge detection; and 2), a moving magnetic bead based magnetic force loading module. The main measurement procedures are as follows: 1), record maximal displacement of single cardiac myocyte during contraction; 2), attach a magnetic bead on one end of the myocyte that will move with myocyte during the contraction; 3), repeat step 1 and record contraction processes under different magnitudes of magnetic force loading by adjusting the magnetic field applied on the magnetic bead; and 4), derive the myocyte contractile force base on the maximal displacement of cell contraction and magnetic loading force. The major advantages of this unique approach are: 1), measuring the force without direct connections to the cell specimen (i.e., "remote sensing", a noninvasive/minimally invasive approach); 2), high sensitivity and large dynamic range (force measurement range: from pico Newton to micro Newton); 3), a convenient and cost-effective approach; and 4), more importantly, it can be used to study the contractile properties of heart cells under different levels of external loading forces by adjusting the magnitude of applied magnetic field, which is very important for studying disease induced alterations in contraction properties. Experimental results demonstrated the feasibility of proposed approach.

Turning on stem cell cardiogenesis with extremely low frequency magnetic fields

Modulation of stem cell differentiation is an important assignment for cellular engineering. Embryonic stem (ES) cells can differentiate into cardiomyocytes, but the efficiency is typically low. Here, we show that exposure of mouse ES cells to extremely low frequency magnetic fields triggered the expression of GATA-4 and Nkx-2.5, acting as cardiac lineage-promoting genes in different animal species, including humans. Magnetic fields also enhanced prodynorphin gene expression, and the synthesis and secretion of dynorphin B, an endorphin playing a major role in cardiogenesis. These effects occurred at the transcriptional level and ultimately ensued into a remarkable increase in the yield of ES-derived cardiomyocytes. These results demonstrate the potential use of magnetic fields for modifying the gene program of cardiac differentiation in ES cells without the aid of gene transfer technologies and may pave the way for novel approaches in tissue engineering and cell therapy.

Evaluation of Epicardial Microwave Ablation Lesions: Histology Versus Electrophysiology

BACKGROUND

Pulmonary vein isolation is a hallmark in current surgical ablation for atrial fibrillation. However, validation of isolation remains cumbersome. We evaluated electrophysiologic and not histologic means to test isolation.

METHODS

In 16 mongrel dogs, robot-assisted epicardial beating-heart microwave ablation (FLEX 10) was performed around the pulmonary veins. Electrophysiologic isolation was tested by pacing at 4 times threshold values inside and outside the pulmonary veins (exit and entrance block). The histology of lesions was studied for transmurality and continuity of the lesion lines. In 5 dogs, lesions were studied at various time intervals.

RESULTS

Histologic evaluation of the lesions showed incomplete (48% +/- 20%) circumferential myocardial damage in all dogs with acute lesions. Electrophysiologic evaluation showed completion of the box (entrance and exit block) in 8 dogs and in another 5 dogs after repeated ablation (p < 0.01 compared with histologic evaluation). Electrophysiologic evaluation of the dogs with chronic lesions showed completed lesions in 4 of 5 dogs directly after ablation. At follow-up (1 to 3 weeks), the isolations remained electrophysiologically complete. Histologic evaluation of the lesions 1 to 3 weeks after ablation showed complete (100%) circumferential lesions in all 4 dogs (p < 0.001 compared with the histology of dogs with acute lesions).

CONCLUSIONS

Directly after treatment, ablation lesions are best evaluated electrophysiologically, because complete (transmural and circumferential) lesions are not shown by histologic evaluation in the acute stage. After 1 to 3 weeks, the histology is in accordance with the electrophysiology. To obtain a complete isolation, online electrophysiologic evaluation during pulmonary vein microwave ablation is necessary to optimize the results.

Sarcoplasmic reticulum Ca2+ refilling controls recovery from Ca2+-induced Ca2+ release refractoriness in heart muscle

In cardiac muscle Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) is initiated by Ca2+ influx via L-type Ca2+ channels. At present, the mechanisms underlying termination of SR Ca2+ release, which are required to ensure stable excitation-contraction coupling cycles, are not precisely known. However, the same mechanism leading to refractoriness of SR Ca2+ release could also be responsible for the termination of CICR. To examine the refractoriness of SR Ca2+ release, we analyzed Na+-Ca2+ exchange currents reflecting cytosolic Ca2+ signals induced by UV-laser flash-photolysis of caged Ca2+. Pairs of UV flashes were applied at various intervals to examine the time course of recovery from CICR refractoriness. In cardiomyocytes isolated from guinea-pigs and mice, beta-adrenergic stimulation with isoproterenol-accelerated recovery from refractoriness by approximately 2-fold. Application of cyclopiazonic acid at moderate concentrations (<10 micromol/L) slowed down recovery from refractoriness in a dose-dependent manner. Compared with cells from wild-type littermates, those from phospholamban knockout (PLB-KO) mice exhibited almost 5-fold accelerated recovery from refractoriness. Our results suggest that SR Ca2+ refilling mediated by the SR Ca2+-pump corresponds to the rate-limiting step for recovery from CICR refractoriness. Thus, the Ca2+ sensitivity of CICR appears to be regulated by SR Ca2+ content, possibly resulting from a change in the steady-state Ca2+ sensitivity and in the gating kinetics of the SR Ca2+ release channels (ryanodine receptors). During Ca2+ release, the concomitant reduction in Ca2+ sensitivity of the ryanodine receptors might also underlie Ca2+ spark termination by deactivation.

Preconditioning protects by inhibiting the mitochondrial permeability transition

Mitochondrial permeability transition (mPT) is a crucial event in the progression to cell death in the setting of ischemia-reperfusion. We have used a model system in which mPT can be reliably and reproducibly induced to test the hypothesis that the profound protection associated with the phenomenon of myocardial preconditioning is mediated by suppression of the mPT. Adult rat myocytes were loaded with the fluorescent probe tetramethylrhodamine methyl ester, which generates oxidative stress on laser illumination, thus inducing the mPT (indicated by collapse of the mitochondrial membrane potential) and ATP depletion, seen as rigor contracture. The known inhibitors of the mPT, cyclosporin A (0.2 microM) and N-methyl-4-valine-cyclosporin A (0.4 microM), increased the time taken to induce the mPT by 1.8- and 2.9-fold, respectively, compared with control (P < 0.001) and rigor contracture by 1.5-fold compared with control (P < 0.001). Hypoxic preconditioning (HP) and pharmacological preconditioning, using diazoxide (30 microM) or nicorandil (100 microM), also increased the time taken to induce the mPT by 2.0-, 2.1-, and 1.5-fold, respectively (P < 0.001), and rigor contracture by 1.9-, 1.7-, and 1.5-fold, respectively, compared with control (P < 0.001). Effects of HP, diazoxide, and nicorandil were abolished in the presence of mitochondrial ATP-sensitive K(+) (K(ATP)) channel blockers glibenclamide (10 microM) and 5-hydroxydecanoate (100 microM) but were maintained in the presence of the sarcolemmal K(ATP) channel blocker HMR-1098 (10 microM). In conclusion, preconditioning protects the myocardium by reducing the probability of the mPT, which normally occurs during ischemia-reperfusion in response to oxidative stress.

Application of two-photon flash photolysis to reveal intercellular communication and intracellular Ca2+ movements

Two-photon excitation makes it possible to excite molecules in volumes of much less than 1 fl. In two-photon flash photolysis (TPFP) this property is used to release effector molecules from caged precursors with high three-dimensional resolution. We describe and examine the benefits of using TPFP in model solutions and in a number of cell systems to study their spatial and temporal properties. Using TPFP of caged fluorescein, we determined the free diffusion coefficient of fluorescein (D=4 x 0(-6) cm(2)/s at 20 degrees C, which is in close agreement with published values). TPFP of caged fluorescein in lens tissue in situ revealed spatial nonuniformities in intercellular fiber cell coupling by gap junctions. At the lens periphery, intercellular transport was predominantly directed along rows of cells, but was nearly isotropic further from the periphery. To test an algorithm aiming to reconstruct the Ca(2+) release flux underlying physiological Ca(2+) signals in heart muscle cells, TPFP of DM-Nitrophen was utilized to generate artificial microscopic Ca(2+) signals with known underlying Ca(2+) release flux. In an experiment with mouse oocytes, the recently developed Ca(2+) cage dimethoxynitrophenyl-ethyleneglycol-bis-(beta-aminoethylether)-N,N,N('),N(') tetraacetic acid-4 (DMNPE-4) was released in the oocyte cytosol and inside a nucleolus. Analysis of the resulting fluorescence changes suggested that the effective diffusion coefficient within the nucleolus was half of that in the cytosol. These experiments demonstrate the utility of TPFP as a novel tool for the optical study of biomedical systems.

Field stimulation of cardiac fibers with random spatial structure

Polarization of individual cells ("sawtooth") has been proposed as a mechanism for field stimulation and defibrillation. To date, the modeling work has concentrated on the myocardium with periodic spatial structure; this paper investigates potentials arising in cardiac fibers with random spatial structure. Ten different random fibers consisting of cells with varying length (l(c) = 100 +/- 50 microm), diameter (d(c) = 20 +/- 10 microm), thickness of extracellular space (t(e) = 1.18 +/- 0.59 microm), and junctional resistance (R(j) = 2 +/- 1 M(omega)) are studied. Simulations demonstrate that randomizing spatial structure introduces to the field-induced potential (V(m)) a randomly varying baseline, which arises due to polarization of groups of cells. This polarization appears primarily in response to randomizing t(e); R(j), l(c), and d(c) have less influence. The maximum V(m) increases from 3.5 mV in a periodic fiber to 20.5+/-4.7 mV in random fibers (1 V/cm field applied for 5 ms). Field stimulation threshold E(th) decreases from 6.9 to 1.59 +/- 0.43 V/cm, which is within the range of experimental measurements. Thresholds for normal and reversed field polarities are statistically equivalent: 1.59 +/- 0.43 versus 1.44 +/- 0.41 V/cm (p value = 0.453). Thus, V(m) arising due to random structure of the myocardium may play an important role in field stimulation and defibrillation.