Mechanical restitution (MR) represents the time recovery of the heart muscle's ability to contract. Despite intensive research, some aspects of MR remain unclear. To describe MR in guinea pig cardiac muscle, we modified our published mathematical model of guinea pig ventricular cardiomyocyte and supplemented it with a description of cellular contraction. To achieve the best agreement between the model simulations and available experimental data, some model parameters were optimised. The model enables the simulation of the experimentally observed fast onset of recovery of action potential duration, L-type Ca2+ current amplitude, and isometric force. The performed simulations and analyses of model data showed that the high time constant of voltage-dependent inactivation of L-type Ca2+ channels used in previously published models (~ 600 ms at resting voltage) is not consistent with the initial steep rise of the MR curve in guinea pig cardiomyocytes. It also suggests that the adaptation rate of ryanodine receptors, which was set differently in the previous models, is fast (~ 100 s- 1). Finally, analysis of the effect of a 50% reduction in membrane currents on MR revealed a marked dependence on stimulation frequency. At 1 Hz, only the reduction of INaCa and INaK significantly affected the MR course.

• Klíčová slova
• Calcium current, Guinea pig cardiomyocyte, Mathematical model, Mechanical restitution, Ryanodine receptor,
• MeSH
• akční potenciály fyziologie   MeSH
• kardiomyocyty * fyziologie   metabolismus   MeSH
• kontrakce myokardu * fyziologie   MeSH
• modely kardiovaskulární * MeSH
• morčata MeSH
• počítačová simulace MeSH
• ryanodinový receptor vápníkového kanálu metabolismus   MeSH
• vápníkové kanály - typ L metabolismus   MeSH
• zvířata MeSH
• Check Tag
• morčata MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• ryanodinový receptor vápníkového kanálu MeSH
• vápníkové kanály - typ L MeSH

The transverse-axial tubular system (tubular system) of cardiomyocytes plays a key role in excitation-contraction coupling. To determine the area of the tubular membrane in relation to the area of the surface membrane, indirect measurements through the determination of membrane capacitances are currently used in addition to microscopic methods. Unlike existing electrophysiological methods based on an irreversible procedure (osmotic shock), the proposed new approach uses a reversible short-term intermittent increase in the electrical resistance of the extracellular medium. The resulting increase in the lumen resistance of the tubular system makes it possible to determine separate capacitances of the tubular and surface membranes. Based on the analysis of the time course of the capacitive current, computational relations were derived to quantify the elements of the electrical equivalent circuit of the measured cardiomyocyte including both capacitances. The exposition to isotonic low-conductivity sucrose solution is reversible which is the main advantage of the proposed approach allowing repetitive measurements on the same cell under control and sucrose solutions. Experiments on rat ventricular cardiomyocytes (n = 20) resulted in the surface and tubular capacitance values implying the fraction of tubular capacitance/area of 0.327 ± 0.018. We conclude that the newly proposed method provides results comparable to the data obtained by the currently used detubulation method and, in addition, by being reversible, allows repeated evaluation of surface and tubular membrane parameters on the same cell.

• Klíčová slova
• Cardiomyocyte, Novel method, Sucrose, Tubular membrane capacitance, Tubular system,
• MeSH
• elektrická vodivost MeSH
• kardiomyocyty * MeSH
• krysa rodu Rattus MeSH
• osmotický tlak MeSH
• sacharosa MeSH
• spřažení excitace a kontrakce * MeSH
• zvířata MeSH
• Check Tag
• krysa rodu Rattus MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• sacharosa MeSH

The transverse-axial tubular system (t-tubules) plays an essential role in excitation-contraction coupling in cardiomyocytes. Its remodelling is associated with various cardiac diseases. Numerous attempts were made to analyse characteristics essential for proper understanding of the t-tubules and their impact on cardiac cell function in health and disease. The currently available methodical approaches related to the fraction of the t-tubular membrane area produce diverse data. The widely used detubulation techniques cause irreversible cell impairment, thus, distinct cell samples have to be used for estimation of t-tubular parameters in untreated and detubulated cells. Our proposed alternative method is reversible and allows repetitive estimation of the fraction of t-tubular membrane (f t) in cardiomyocytes using short-term perfusion of the measured cell with a low-conductive isotonic sucrose solution. It results in a substantial increase in the electrical resistance of t-tubular lumen, thus, electrically separating the surface and t-tubular membranes. Using the whole-cell patch-clamp measurement and the new approach in enzymatically isolated rat atrial and ventricular myocytes, a set of data was measured and evaluated. The analysis of the electrical equivalent circuit resulted in the establishment of criteria for excluding measurements in which perfusion with a low conductivity solution did not affect the entire cell surface. As expected, the final average f t in ventricular myocytes (0.337 ± 0.017) was significantly higher than that in atrial myocytes (0.144 ± 0.015). The parameter f t could be estimated repetitively in a particular cell (0.345 ± 0.021 and 0.347 ± 0.023 in ventricular myocytes during the first and second sucrose perfusion, respectively). The new method is fast, simple, and leaves the measured cell intact. It can be applied in the course of experiments for which it is useful to estimate both the surface and t-tubular capacitance/area in a particular cell.

• Klíčová slova
• detubulation, membrane capacitance, new method, rat cardiomyocytes, sucrose, t-tubules,
• Publikační typ
• časopisecké články MeSH

Inward rectifier potassium currents (I Kir,x) belong to prominent ionic currents affecting both resting membrane voltage and action potential repolarization in cardiomyocytes. In existing integrative models of electrical activity of cardiac cells, they have been described as single current components. The proposed quantitative model complies with findings indicating that these channels are formed by various homomeric or heteromeric assemblies of channel subunits with specific functional properties. Each I Kir,x may be expressed as a total of independent currents via individual populations of identical channels, i.e., channels formed by the same combination of their subunits. Solution of the model equations simulated well recently observed unique manifestations of dual ethanol effect in rat ventricular and atrial cells. The model reflects reported occurrence of at least two binding sites for ethanol within I Kir,x channels related to slow allosteric conformation changes governing channel conductance and inducing current activation or inhibition. Our new model may considerably improve the existing models of cardiac cells by including the model equations proposed here in the particular case of the voltage-independent drug-channel interaction. Such improved integrative models may provide more precise and, thus, more physiologically relevant results.

• Klíčová slova
• Cardiomyocytes, Dual effect, Ethanol, I K1, Inward rectifier potassium currents, Quantitative model,
• MeSH
• akční potenciály * MeSH
• alosterická regulace MeSH
• draslíkové kanály dovnitř usměrňující chemie   metabolismus   MeSH
• ethanol farmakologie   MeSH
• kardiomyocyty účinky léků   metabolismus   fyziologie   MeSH
• krysa rodu Rattus MeSH
• modely kardiovaskulární MeSH
• multimerizace proteinu MeSH
• srdce - funkce komor MeSH
• srdeční komory cytologie   MeSH
• zvířata MeSH
• Check Tag
• krysa rodu Rattus MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• draslíkové kanály dovnitř usměrňující MeSH
• ethanol MeSH

We have developed a computer model of human cardiac ventricular myocyte (CVM), including t-tubular and cleft spaces with the aim of evaluating the impact of accumulation-depletion of ions in restricted extracellular spaces on transmembrane ion transport and ionic homeostasis in human CVM. The model was based on available data from human CVMs. Under steady state, the effect of ion concentration changes in extracellular spaces on [Ca2+]i-transient was explored as a function of critical fractions of ion transporters in t-tubular membrane (not documented for human CVM). Depletion of Ca2+ and accumulation of K+ occurring in extracellular spaces slightly affected the transmembrane Ca2+ flux, but not the action potential duration (APD90). The [Ca2+]i-transient was reduced (by 2%-9%), depending on the stimulation frequency, the rate of ion exchange between t-tubules and clefts and fractions of ion-transfer proteins in the t-tubular membrane. Under non-steady state, the responses of the model to changes of stimulation frequency were analyzed. A sudden increase of frequency (1-2.5 Hz) caused a temporal decrease of [Ca2+] in both extracellular spaces, a reduction of [Ca2+]i-transient (by 15%) and APD90 (by 13 ms). The results reveal different effects of activity-related ion concentration changes in human cardiac t-tubules (steady-state effects) and intercellular clefts (transient effects) in the modulation of membrane ion transport and Ca2+ turnover.

• MeSH
• akční potenciály MeSH
• biologické modely * MeSH
• extracelulární prostor metabolismus   MeSH
• iontový transport MeSH
• ionty chemie   metabolismus   MeSH
• kardiomyocyty cytologie   metabolismus   MeSH
• lidé MeSH
• sarkolema metabolismus   MeSH
• vápník metabolismus   MeSH
• Check Tag
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• ionty MeSH
• vápník MeSH