ABSTRACT
HF-1 b, an SP1 -related transcription factor, is preferentially expressed in the cardiac conduction system and ventricular myocytes in the heart. Mice deficient for HF-1 b survive to term and exhibit normal cardiac structure and function but display sudden cardiac death and a complete penetrance of conduction system defects, including spontaneous ventricular tachycardia and a high incidence of AV block. Continuous electrocardiographic recordings clearly documented cardiac arrhythmogenesis as the cause of death. Single-cell analysis revealed an anatomic substrate for arrhythmogenesis, including a decrease and mislocalization of connexins and a marked increase in action potential heterogeneity. Two independent markers reveal defects in the formation of ventricular Purkinje fibers. These studies identify a novel genetic pathway for sudden cardiac death via defects in the transition between ventricular and conduction system cell lineages.
Subject(s)
DNA-Binding Proteins/physiology , Death, Sudden, Cardiac/pathology , Gene Deletion , Heart Conduction System/pathology , Heart Conduction System/physiopathology , Heart Ventricles/pathology , Potassium Channels, Voltage-Gated , Action Potentials , Alleles , Animals , Cell Count , Cell Lineage , Connexins/analysis , DNA-Binding Proteins/analysis , DNA-Binding Proteins/genetics , Electric Conductivity , Electrocardiography , Female , Heart Block/metabolism , Heart Block/pathology , Heart Block/physiopathology , Heart Conduction System/metabolism , Heart Ventricles/embryology , Heart Ventricles/metabolism , Heart Ventricles/physiopathology , Male , Mice , Mice, Knockout , Penetrance , Potassium/metabolism , Potassium Channels/analysis , Potassium Channels/metabolism , Purkinje Fibers/metabolism , Purkinje Fibers/pathology , Purkinje Fibers/physiopathology , RNA, Messenger/analysis , RNA, Messenger/genetics , Radio , Sp4 Transcription Factor , Tachycardia, Ventricular/metabolism , Tachycardia, Ventricular/pathology , Tachycardia, Ventricular/physiopathology , Telemetry , Gap Junction alpha-5 ProteinABSTRACT
OBJECTIVE: Decay kinetics of the voltage-gated L-type Ca(2+) current (I(CaL)) control the magnitude of Ca(2+) influx during the cardiac action potential. We investigated the influence of changes in diastolic membrane potential on I(CaL) decay kinetics in cardiac cells. METHODS: Cells were isolated enzymatically from rat ventricles, human right atrial appendages obtained during corrective heart surgery and left ventricles from end-stage failing hearts of transplant recipients. The whole-cell patch-clamp technique was used to evoke I(CaL) by a 100-ms depolarizing test pulse to -10 mV. Conditioning potentials between -80 and 0 mV were applied for 5 s prior to the test pulse. RESULTS: Depolarizing the cells between -80 and -50 mV prior to the test pulse slowed the early inactivation of I(CaL) both in rat ventricular and human atrial cells. This slowing resulted in a significant increase of Ca(2+) influx. This type of facilitation was not observed when the sarcoplasmic reticulum (SR) Ca(2+) content was depleted using ryanodine which reduced the rate of inactivation of I(CaL), or when Ba(2+) replaced Ca(2+) as the permeating ion. Facilitation was favored by intracellular cAMP-promoting agents that, in addition to increasing current peak amplitude, enhanced the fast Ca(2+)-dependent inactivation of I(CaL). Facilitation was impaired in atrial and ventricular human failing hearts. CONCLUSION: Decay kinetics of I(CaL) are regulated by the diastolic membrane potential in rat and human cardiomyocytes. This regulation, which associates slowing of I(CaL) inactivation with reduced SR Ca(2+) release and underlies facilitation of Ca(2+) channels activity, may have profound physiological relevance for catecholamines enhancement of Ca(2+) influx. It is impaired in failing hearts, possibly due to lowered SR Ca(2+) release.
