Exploring mechanisms for reversal of flow in tunicate hearts

Ascidians, a class of tunicates, are marine invertebrates with valveless, tubular hearts. Recent experiments suggest that the hearts of Ciona robusta have two distinct pacemakers, one at each end of the heart tube. Variations in the pacing intervals of these two pacemakers are implicated as being responsible for an unusual phenomenon: after minutes of sustained pumping of blood in one direction, the heart can suddenly reverse the direction of blood flow, pumping blood in the opposite direction for several minutes. In this article, we explore possible electrophysiological bases for these reversals and the roles that different ionic currents might play. We model the heart tube as an excitable, one-dimensional medium using the standard cable equation. For cells within the interior of the cable, we use a three-current model for the membrane potential, the three currents representing the total transmembrane currents associated with sodium, potassium, and calcium. As the two pacemakers consist of cells that exhibit automaticity, cells at the cable boundaries are simulated using an adaptation of a sinoatrial node model.