A study of the outward background current conductance g K1 , the pacemaker current conductance g f , and the gap junction conductance g j as determinants of biological pacing in single cells and in a two-cell syncytium using the dynamic clamp

We previously demonstrated that a two-cell syncytium, composed of a ventricular myocyte and an mHCN2 expressing cell, recapitulated most properties of in vivo biological pacing induced by mHCN2-transfected hMSCs in the canine ventricle. Here, we use the two-cell syncytium, employing dynamic clamp, to study the roles of g f (pacemaker conductance), g K1 (background K + conductance), and g j (intercellular coupling conductance) in biological pacing. We studied g f and g K1 in single HEK293 cells expressing cardiac sodium current channel Na v 1.5 (SCN5A). At fixed g f , increasing g K1 hyperpolarized the cell and initiated pacing. As g K1 increased, rate increased, then decreased, finally ceasing at membrane potentials near E K . At fixed g K1 , increasing g f depolarized the cell and initiated pacing. With increasing g f , rate increased reaching a plateau, then decreased, ceasing at a depolarized membrane potential. We studied g j via virtual coupling with two non-adjacent cells, a driver (HEK293 cell) in which g K1 and g f were injected without SCN5A and a follower (HEK293 cell), expressing SCN5A. At the chosen values of g K1 and g f oscillations initiated in the driver, when g j was increased synchronized pacing began, which then decreased by about 35% as g j approached 20 nS. Virtual uncoupling yielded similar insights into g j . We also studied subthreshold oscillations in physically and virtually coupled cells. When coupling was insufficient to induce pacing, passive spread of the oscillations occurred in the follower. These results show a non-monotonic relationship between g K1 , g f , g j , and pacing. Further, oscillations can be generated by g K1 and g f in the absence of SCN5A.