Mathematical simulations of the effects of altered AMP-kinase activity on I and the action potential in rat ventricle.

INTRODUCTION: Alterations in the activity of a so-called "metabolic switch" enzyme, adenosine monophosphate-activated protein kinase (AMP kinase), in mammalian heart contribute to the conduction abnormalities and rhythm disturbances in the settings of Wolff-Parkinson-White syndrome and ventricular pre-excitation. A recent study by Light et al. has shown that augmented AMP kinase activity can alter the biophysical properties of mammalian cardiac sodium currents. These experiments involved an electrophysiological analysis following heterologous expression of human Na(v)1.5 in tsA201 cells. Constitutive activation of AMP kinase followed by co-transfection caused: (i) a hyperpolarizing shift in the activation curve for I(Na), (ii) a small change in the voltage dependence of steady-state inactivation, and (iii) a significant slowing in the rate of inactivation of I(Na). METHODS AND RESULTS: We have attempted to simulate these results using our mathematical model of the membrane action potential of the adult rat ventricular myocyte. The changes in I(Na) produced by AMP kinase activation and/or overexpression can be reconstructed mathematically by altering two rate constants in a Markovian model that governs the I(Na) kinetics. Simulated macroscopic I(Na) records in which a fraction (10-100%) of the Na(+) channels had the appropriate rate constants for two state-dependent transitions increased by a factor of 100-fold exhibited: (i) slowed inactivation, (ii) a shift in steady-state activation to more hyperpolarized membrane potentials, and (iii) a very small change in the voltage dependence of steady-state inactivation. SUMMARY: Thus, straightforward modifications of a previously published kinetic scheme for the time and voltage dependence of mammalian heart I(Na), when incorporated into a mathematical model for the rat ventricular action potential can reproduce the main features of these AMP kinase-induced modifications in I(Na) in mammalian ventricle. Ongoing mathematical simulations are directed toward developing formulations that mimic the molecular mechanisms for the AMP kinase effects, e.g., changes in the kinetics of I(Na) resulting from selective phosphorylation/dephosphorylation of sites on the alpha or beta subunits which comprise human Na(v)1.5. Thereafter, incorporation of these changes into a mathematical model for the action potential of the human ventricular myocyte is planned.

Actions of emigrated neutrophils on Na(+) and K(+) currents in rat ventricular myocytes.

Interactions between neutrophils and the ventricular myocardium can contribute to tissue injury, contractile dysfunction and generation of arrhythmias in acute cardiac inflammation. Many of the molecular events responsible for neutrophil adhesion to ventricular myocytes are well defined; in contrast, the resulting electrophysiological effects and changes in excitation-contraction coupling have not been studied in detail. In the present experiments, rat ventricular myocytes were superfused with either circulating or emigrated neutrophils and whole-cell currents and action potential waveforms were recorded using the nystatin-perforated patch method. Almost immediately after adhering to ventricular myocytes, emigrated neutrophils caused a depolarization of the resting membrane potential and a marked prolongation of myocyte action potential. Voltage clamp experiments demonstrated that following neutrophil adhesion, there was (i) a slowing of the inactivation of a TTX-sensitive Na(+) current, and (ii) a decrease in an inwardly rectifying K(+) current. One cytotoxic effect of neutrophils appears to be initiated by enhanced Na(+) entry into the myocytes. Thus, manoeuvres that precluded activation of Na(+) channels, for example holding the membrane potential at -80 mV, significantly increased the time to cell death or prevented contracture entirely. A mathematical model for the action potential of rat ventricular myocytes has been modified and then utilized to integrate these findings. These simulations demonstrate the marked effects of (50-fold) slowing of the inactivation of 2-4% of the available Na(+) channels on action potential duration and the corresponding intracellular Ca(2+) transient. In ongoing studies using this combination of approaches, are providing significant new insights into some of the fundamental processes that modulate myocyte damage in acute inflammation.

Two-cell to N-cell heterogeneous, inhibitory networks: precise linking of multistable and coherent properties.

Inhibitory networks are now recognized as being the controllers of several brain rhythms. However, experimental work with inhibitory cells is technically difficult not only because of their smaller percentage of the neuronal population, but also because of their diverse properties. As such, inhibitory network models with tight links to the experimental data are needed to understand their contributions to population rhythms. However, mathematical analyses of network models with more than two cells is challenging when the cellular models involve biophysical details. We use bifurcation analyses and simulations to show that two-cell analyses can quantitatively predict N-cell (N = 20, 50, 100) network dynamics for heterogeneous, inhibitory networks. Interestingly, multistable states in the two-cell system are manifest as different and distinct coherent network patterns in the N-cell networks for the same parameter sets.

The kinetics and mechanisms of the reaction of Mesna with cisplatin, oxiplatin and carboplatin.

BACKGROUND: Mesna is a sulfohydrate administered as a supportive drug in conjunction with oxazaphosphorines to prevent bladder toxicity from metabolites. When oxazaphosphorines are given simultaneously with platinum drugs, Mesna binds with platinum drugs as well. Previously we showed in cell culture, that Mesna reduces the efficacy of some platinum drugs. Here we elucidate the chemical reaction mechanism. MATERIAL AND METHODS: Cisplatin, Carboplatin and a novel platinum agent Oxiplatin were incubated with Mesna and the rate of disappearance of Mesna was measured, using an oxidation/reduction reaction between MTT and Mesna. RESULTS: All three platinum agents reacted with Mesna, but the chemical details differed largely. The molar ratios were 3:1, 2:1, and 1:1 for the reactions of Mesna with Oxiplatin, Cisplatin, and Carboplatin, respectively. The speed of the reaction followed a similar pattern, being fastest for Oxiplatin and slowest for Carboplatin. CONCLUSION: When considering the pharmacokinetics of Mesna and these platinum compounds and their reactivity, it appears unlikely that the reaction of Mesna with Carboplatin will become clinically relevant, while Cisplatin, might react with Mesna in patient serum.