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

摘要

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.