Determination of intracellular potassium ion concentration in isolated rat ventricular myocytes

A Parameter Representing Missing Charge Should Be Considered when Calibrating Action Potential Models

Computational models of the electrical potential across a cell membrane are longstanding and vital tools in electrophysiology research and applications. These models describe how ionic currents, internal fluxes, and buffering interact to determine membrane voltage and form action potentials (APs). Although this relationship is usually expressed as a differential equation, previous studies have shown it can be rewritten in an algebraic form, allowing direct calculation of membrane voltage. Rewriting in this form requires the introduction of a new parameter, called Γ₀ in this manuscript, which represents the net concentration of all charges that influence membrane voltage but are not considered in the model. Although several studies have examined the impact of Γ₀ on long-term stability and drift in model predictions, there has been little examination of its effects on model predictions, particularly when a model is refit to new data. In this study, we illustrate how Γ₀ affects important physiological properties such as action potential duration restitution, and examine the effects of (in)correctly specifying Γ₀ during model calibration. We show that, although physiologically plausible, the range of concentrations used in popular models leads to orders of magnitude differences in Γ₀, which can lead to very different model predictions. In model calibration, we find that using an incorrect value of Γ₀ can lead to biased estimates of the inferred parameters, but that the predictive power of these models can be restored by fitting Γ₀ as a separate parameter. These results show the value of making Γ₀ explicit in model formulations, as it forces modellers and experimenters to consider the effects of uncertainty and potential discrepancy in initial concentrations upon model predictions.

Increased intracellular potassium and water contents in rat heart after alpha-1-adrenoceptor stimulation

Potassium accumulation in rat heart after alpha-1-adrenoceptor stimulation has previously been reported from indirect measurements. Here we present data on intracellular potassium content measured directly in the heart. Isolated rat hearts perfused in a non-recirculating system were exposed to alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine in the presence of 10(-6) mol/l timolol). 14C-Sucrose was used to estimate the extracellular space. From heart homogenates intracellular potassium, magnesium and cellular water contents were determined and the ion concentrations calculated accordingly. The intracellular magnesium content remained unchanged during all experimental conditions. alpha-1-Adrenoceptor stimulation evoked an increase in potassium content by 9% (4, 14; 95% confidence interval (CI), P = 0.0006). Due to an observed increase in intracellular water by 17% (9, 26; 95% CI, P = 0.0006), the potassium concentration apparently decreased by 8% (0.3, 15; 95% CI, P = 0.04). During partial inhibition of the Na+/K(+)-ATPase by 10(-5) mol/l ouabain, there was an increase in potassium content by 5% (1, 9; 95% CI, P = 0.008). There was, however, no significant increase in intracellular water in this situation. Calculated intracellular potassium concentration showed accordingly a slight increase. The effects upon potassium and water both in the absence and presence of ouabain were eliminated by the alpha-1-adrenoceptor blocker prazosin (10(-6) mol/l). alpha-1-Adrenoceptor stimulation apparently increased cellular dry weight by 10% (2, 18; 95% CI, P = 0.02). Changes in translocation of potassium and water must be considered as part of the alpha-1-adrenergic heart effects.