OBJECTIVECurrents contributing repolarization in rabbit ventricular myocyte are very complex since the I(To.s) covers almost the whole repolarization phase of the action potential. The other components of repolarizing currents, as I(Kr) and I(Ks) are small. The purpose of this study is to investigate whether or not there are other currents in rabbit ventricular repolarization.

METHODSIon currents of rabbit ventricular myocyte were recorded using the whole-cell patch-clamp technique.

RESULTSIn the present work, an nonselective cation current was identified by replacing the K(+) with Cs(+) in the bathing and pipette solutions. The outward current elicited by depolarizing potentials could be inhibited by Gd(3+), an effective inhibitor of nonselective cation currents. Depleting Ca(2+) and Mg(2+) in the bathing solution, the amplitudes of this outward current increased by 40%-116% at +60 mV, and adding 2 micromol/L insulin to the solution (with normal concentration of Ca(2+) and Mg(2+) in Tyrode's solution), the amplitude increased by 30%-60% at +60 mV.

CONCLUSIONIt is suggested that a nonselective cation current in rabbit ventricular myocytes may play an important role in the repolarization of the action potential in rabbit ventricle. Changes of nonselective cation current will lead to induce or inhibit arrhythmia.

Animals ; Membrane Potentials ; Myocytes, Cardiac ; physiology ; Patch-Clamp Techniques ; Rabbits

Molecular Probe Techniques ; Myocytes, Cardiac ; physiology ; Regeneration ; Stem Cells

Myocardial Infarction ; physiopathology ; Myocardium ; Myocytes, Cardiac ; physiology ; Regeneration ; Tissue Engineering

AIMTo study the effects of the 50 Hz-filter circuit in a microelectrode amplifier on cardiac action potential waveform and parameters.

METHODSCardiac action potential signals were fed into a microcomputer through a glass microelectrode, a microelectrode amplifier, a differentiator and A/D converter. The cardiac action potential signals were recorded and analyzed with 50 Hz-filter circuit and without it, and the frequency spectrum in the signals was analyzed with the fast Fourier transformation.

RESULTSWhen the 50 Hz-filter circuit was used, the phase 0 of the potential waveform was seriously distorted and prolonged. The maximal rate of depolarization at the phase 0 was cut down, while the other parameters were not effected.

CONCLUSIONThere has already been much 50 Hz element in the action potential waveform. During amplifying the cardiac action potential signal, the 50 Hz-filter circuit should not be turned on. Otherwise, the experiment results will be effected.

Action Potentials ; physiology ; Amplifiers, Electronic ; Animals ; Guinea Pigs ; Myocytes, Cardiac ; physiology ; Papillary Muscles ; cytology ; physiology

This study is aimed to investigate the effects of high intracellular Mg²⁺ on L-type calcium channel in guinea-pig ventricular myocytes. The cardiomyocytes were acutely isolated with enzyme digestion method. By adopting inside-out configuration of patch clamp technique, single channel currents of the L-type calcium channel were recorded under different intracellular Mg²⁺ concentrations ([Mg²⁺]i). In control group, which was treated with 0.9 mmol/L Mg²⁺, the relative activity of calcium channel was (176.5 ± 34.1)% (n = 7). When [Mg²⁺]i was increased from 0.9 to 8.1 mmol/L (high Mg²⁺ group), the relative activities of calcium channel decreased to (64.8 ± 18.1)% (n = 6, P < 0.05). Moreover, under 8.1 mmol/L Mg²⁺, the mean open time of calcium channel was shortened to about 25% of that under control condition (P < 0.05), but the mean close time of calcium channel was not altered. These results suggest that high intracellular Mg²⁺ may inhibit the activities of L-type calcium channel, which is mainly due to the shortening of the mean open time of single L-type calcium channel.
Animals ; Calcium Channels, L-Type ; physiology ; Guinea Pigs ; Magnesium ; physiology ; Myocytes, Cardiac ; physiology ; Patch-Clamp Techniques

The proper intracellular Ca(2+) signaling is essential for normal cell functions and organ development, and the maintaining Ca(2+) homeostasis in cardiac myocytes is of functional importance for the intact heart. As the first functional organ in the vertebrate embryo, the heart is continuously remodeled and maintains its physiologic pumping function in response to increasing circulatory demands. The expressions of Ca(2+) handing proteins in the embryonic heart, however, are different from those in neonatal and adult hearts, which means that the regulation of Ca(2+) transients in embryonic cardiomyocytes is different from that in adult cardiac myocytes. Recent advances in molecular and cellular biology, as well as the application of embryonic stem cell differentiation system, have made progress in uncovering the regulation of Ca(2+) homeostasis during cardiomyogenesis. This paper briefly summarizes the Ca(2+) homeostasis during early development of cardiomyocytes and reviews current knowledge of the regulatory mechanisms controlling Ca(2+) homeostasis during cardiomyocyte development.
Calcium ; physiology ; Calcium Channels ; metabolism ; physiology ; Calcium Signaling ; Heart ; embryology ; Homeostasis ; physiology ; Humans ; Intracellular Fluid ; physiology ; Myocytes, Cardiac ; metabolism ; physiology

OBJECTIVETo observe the effects of repeated + Gz exposures on the apoptosis of myocyte in rats.

METHODSTwelve male Sprague-Dawley rats were randomly divided into three groups: Control group, + 6Gz group and + 10Gz group. The rats of + Gz groups were exposed to + 6Gz for 3 min, + 10Gz for 3 min respectively, 1 b/d, 1 week. Four control rats were kept at the Earth gravity (1G) in the room with the centrifuge. All animals were anaesthetized and anatomies 1 day after the last exposure. Ventricular myocardium was studied by electron microscopy and terminal deoxynucleotide transferase-mediated dUTP nick end labeling (TUNEL) method.

RESULTSThe apoptosis of myocyte in control group and + 6Gz group were scarcely observed by electron microscopy, while heterochromatin concentration and margination were observed in + 10Gz group. The apoptotic index of myocardium increased significantly in + 6Gz and + 10Gz group compared with that of the control group (P < 0.05) and showed the largest value in the + 10Gz group (P < 0.05).

CONCLUSIONRepeated + Gz exposures may induce apoptosis in myocyte, and the number of apoptosis in myocyte increases gradually with the increase of G value.

Acceleration ; Animals ; Apoptosis ; physiology ; Male ; Myocardium ; cytology ; Myocytes, Cardiac ; physiology ; Rats ; Rats, Sprague-Dawley

OBJECTIVETo explore the alterations of after depolarization and triggered activity in myocardiac muscle during progression of mouse aortic stenosis.

METHODSAfter the establishment of mouse aortic stenosis model, the heart was collected and intracellular potential was recorded using standard glass microelectrode. After the recording, the action potential (AP), early after depolarization (EAD), delayed after depolarization (DAD) in papillary muscles were analysed.

RESULTS(1) Compared with that of time-matched control mice, there were no changes of the action potoutial duration at 90% repolarization(APD90) in model mice at 2 and 5 weeks, whereas the APD90 was progressively prolonged at 9 and 13 weeks. (2) During the 30 minites of the experiment, spontaneous EADs and DADs were frequently present in medel mice at 9 and 13 weeks, were not in control and model mice at 2 and 5 weeks. (3) Compared with those of control group, EADs and DADs triggered by hypokalaemia or isoproterenol were increased significantly. The incidence rate of triggering EADs and DADs was progressively increased over time.

CONCLUSIONEAD, DAD and trigger activities increase gradually during the process of aortic stenosis, and it suggests that the myocardium electrical-instability is markedly increased in the insult hearts.

Action Potentials ; physiology ; Animals ; Aortic Valve Stenosis ; physiopathology ; Disease Models, Animal ; Male ; Mice ; Myocytes, Cardiac ; physiology

In order to culture cardiomyocytes or to observe the contractile function of adult mouse cardiomyocytes, it is necessary to isolate high-yield and high-quality cardiomyocytes at first. The mouse was injected with heparin (5,000 IU/kg, i.p.) 20 min prior to the experimental protocol, then was sacrificed by cervical dislocation. The heart was excised and the aorta was cannulated rapidly. The cannulated heart was mounted on a Langendorff perfusion apparatus with constant flow and perfusion pressure was monitored. The initial perfusion pressure was maintained at 40 mmHg by regulating the flow rate. The heart was digested by 0.05 % crude collagenase I at 37 degrees C and the enzymatic digestion was terminated immediately when the perfusion pressure was lowered to 28 mmHg. The heart was then cut off the cannula and the atria and aorta dissected away. The ventricular tissue was chopped and the single myocyte was dispersed gently by a wide tipped pipette. The viability of freshly isolated cardiomyocytes was more than 70 %. The cardiomyocytes were kept in Joklik's minimum essential medium containing 1 % BSA and 10 mmol/L BDM, then extracellular calcium was restored step-wise to a final concentration of 1.25 mmol/L. The viability of cardiomyocytes reduced to (40-50) % after 4 h standing. More than 90 % of rod-shaped cardiomyocytes were quiescent and had visible cross striations and sharp edges. The amplitude of unloaded shortening in cardiomyocytes was (9.72+/-0.43) % during 1.0 Hz stimulation, (11.28+/-0.43) % at 2.0 Hz and (11.40+/-0.45) % at 5.0 Hz. These results indicate that high yield and high quality cardiomyocytes can be obtained. In addition, the standards of identifying cardiomyocyte quality are concise and are suitable to culture the cardiomyocytes or to study the physiological function of cardiomyocytes.
Animals ; Cell Separation ; methods ; Cells, Cultured ; Female ; Male ; Mice ; Myocardial Contraction ; physiology ; Myocytes, Cardiac ; cytology ; physiology

In the heart, gap junctions mediate electrical and chemical coupling between adjacent cardiomyocytes, forming the cell-to-cell pathways for orderly spread of the wave of electrical excitation responsible for a functional syncytium. Three principal connexins are expressed in cardiomyocytes, connexin 43 (CX43), CX40, and CX45. CX43 predominates in ventricular muscle cells. Most of the gap junctions, assembled from CX43, are located at the intercalated discs, often with larger junctional plaques at the disc periphery. The gap junctions are rarely distributed to the sides of the cardiomyocyte. The ischemia-reperfusion, cardiac hypertrophy, heart failure, hypercholesterolemia, and diabetes mellitus induce gap junction remodeling. The gap junction remodeling induced by above-mentioned diseases shows similar characteristics, including down-regulation of CX43, reduction in gap junction plaque size, increased heterogeneity and lateralization of gap junction distribution, and dephosphorylation of CX43. The elevated angiotensin II concentration in local myocardium may play an important role in the gap junction remodeling. The down-regulation of CX43 and lateralization of gap junction distribution alter anisotropic spread of the impulse of ventricular myocardium. The dephosphorylation of CX43 not only reduces electrical conductance, but also decreases permeability of chemicals between cardiomyocytes. The lateralization of gap junctions may increase the number of hemichannels formed by CX43. The opening of hemichannels induces ATP efflux and Na(+) influx, which forms a delayed after-depolarization. The gap junction remodeling in pathological condition produces arrhythmia substrate in the ventricles. In this review, the current knowledge on the relationship between the remodeling of cardiac gap junctions and arrhythmias were summarized.
Animals ; Arrhythmias, Cardiac ; physiopathology ; Cell Communication ; Connexin 43 ; metabolism ; physiology ; Connexins ; metabolism ; physiology ; Gap Junctions ; physiology ; Humans ; Myocytes, Cardiac ; metabolism ; physiology