Contribution of Sodium Channel Mutations to Bradycardia and Sinus Node Dysfunction in LQT3 Families

Abstract

One variant of the long-QT syndrome (LQT3) is caused by mutations in the human cardiac sodium channel gene. In addition to the characteristic QT prolongation, LQT3 carriers regularly present with bradycardia and sinus pauses. Therefore, we studied the effect of the 1795insD Na + channel mutation on sinoatrial (SA) pacemaking. The 1795insD channel was previously characterized by the presence of a persistent inward current ( I pst ) at −20 mV and a negative shift in voltage dependence of inactivation. In the present study, we first additionally characterized I pst over the complete voltage range of the SA node action potential (AP) by measuring whole-cell Na + currents ( I Na ) in HEK-293 cells expressing either wild-type or 1795insD channels. I pst for 1795insD channels varied between 0.8±0.2% and 1.9±0.8% of peak I Na . Activity of 1795insD channels during SA node pacemaking was confirmed by AP clamp experiments. Next, I pst and the negative shift were implemented into SA node AP models. The −10-mV shift decreased sinus rate by decreasing diastolic depolarization rate, whereas I pst decreased sinus rate by AP prolongation, despite a concomitant increase in diastolic depolarization rate. In combination, moderate I pst (1% to 2%) and the shift reduced sinus rate by ≈10%. An additional increase in I pst could result in plateau oscillations and failure to repolarize completely. Thus, Na + channel mutations displaying an I pst or a negative shift in inactivation may account for the bradycardia seen in LQT3 patients, whereas SA node pauses or arrest may result from failure of SA node cells to repolarize under conditions of extra net inward current.