Mathematical Modelling of the Autonomous Activity of Cultured Neonatal Rat Ventricular Myocytes

Problematic. The biological pacemaker is a new therapeutic approach that could lead to optimized treatment of bradycardia. A possibility is the development of a thin sheet of cardiomyocytes, cultured to obtain a target activation rate. Fundamental research, often conducted with neonatal rat ventricular myocytes (NRVMs), partially revealed two basic coupled mechanisms of automaticity termed Voltage Clock (synergy of membrane currents) and Calcium Clock (internal oscillations of calcium concentration). To date, no ionic model is able to reproduce in silico the autonomous activity found in cultured NRVMs. The present project aims to fill this gap.Methods. A non-automatic NRVM ionic model (Korhonen-Tavi, 2009) is modified according to documented Voltage and Calcium Clocks mathematical formulations. The myocytes are cultured for 48 hours at low density, allowing cells to remain single on dishes. Autonomous action potentials (APs) are measured with patch clamp method, and calcium transients (CTs) with Fluo-4 AM fluorescence imaging. Experimental variables (spontaneous rate, AP amplitude and duration, CT decay time course) are extracted and compared to the output of the model. Results. Insertion of Voltage Clock (upregulation of If and ICaL, and downregulation of IK1 and Ito) into the model showed the presence of automaticity with the characteristic slow late diastole depolarization. The maximum activation frequencies (0,7Hz) reached only the lower frequency range documented experimentally and found in the litterature (0,4 to 6Hz). Conclusion. Limitations in the rate of automaticity found emphasise the importance of exploring the Calcium Clock mechanism whose insertion may lead to higher spontaneous rates. In the future, a bifurcation analysis will give some insights on the dynamical mechanisms behind pacemaker generation in cultured NRVMs.