Bursting dynamics in the normal and failing hearts

A failing heart differs from healthy hearts by an array of symptomatic characteristics, including impaired Ca²⁺ transients, upregulation of Na⁺/Ca²⁺ exchanger function, reduction of Ca²⁺ uptake to sarcoplasmic reticulum, reduced K⁺ currents, and increased propensity to arrhythmias. While significant efforts have been made in both experimental studies and model development to display the causes of heart failure, the full process of deterioration from a healthy to a failing heart yet remains deficiently understood. In this paper, we analyze a highly detailed mathematical model of mouse ventricular myocytes to disclose the key mechanisms underlying the continual transition towards a state of heart failure. We argue that such a transition can be described in mathematical terms as a sequence of bifurcations that the healthy cells undergo while transforming into failing cells. They include normal action potentials and [Ca²⁺]_i transients, action potential and [Ca²⁺]_i alternans, and bursting behaviors. These behaviors where supported by experimental studies of heart failure. The analysis of this model allowed us to identify that the slow component of the fast Na⁺ current is a key determining factor for the onset of bursting activity in mouse ventricular myocytes.