Gating of the Late Na+ Channel in Normal and Failing Human Myocardium

A. I. Undrovinas, V. A. Maltsev, J., W. Kyle, N. S ilverman and H. N. Sabbah. Gating of the Late Na+ Channel in Normal and Failing Human Myocardium. Journal of Molecular and Cellular Cardiology (2002) 34, 1477–1489. We previously reported an ultraslow inactivating late Na+ current (INaL) in left ventricular cardiomyocytes (VC) isolated from normal (NVC) and failing (FVC) human hearts. This current could play a role in heart failure-induced repolarization abnormalities. To identify properties of NaCh contributing to INaL, we examined early and late openings in cell-attached patches of HEK293 cells expressing human cardiac NaCh α-subunit (α-HEK) and in VC of one normal and three failing human hearts. Two types of the late NaCh openings underlay INaL in all three preparations: scattered late (SLO) and bursts (BO). Amplitude analysis revealed that slope conductance for both SLO and BO was the same compared to the main level of early openings (EO) in both VC (21 vs 22.7 pS, NVC; 22.7 vs 22.6 pS, FVC) and α-HEK (23.2 vs 23 pS), respectively. Analysis of SLO latencies revealed voltage-independent ultraslow inactivation in all preparations with tendency to be slower in FVC compared to NCV. EO and SLO render one open voltage-independent state (~0.4 ms) for NVC and FVC. One open (voltage-dependent) and two closed states (one voltage-dependent and another voltage-independent) were found in BO of both specimens. Burst duration tend to be longer in FVC (~50 ms) than in NVC (~30 ms). In FVC we found both modes SLO and BO at membrane potential of −10 mV that is attribute for take-off voltages (from −18 to −2 mV) for early afterdepolarizations (EADʹs) in FVC. In conclusions, we found a novel gating mode SLO that manifest slow (hundreds of ms), voltage-independent inactivation in both NVC and FVC. We were unable to reliably demonstrate any differences in the properties of the late NaCh in failing vs a normal human heart. Accordingly, the late current appears to be generated by a single population of channels in normal and failing human ventricular myocardium. Both SLO and BO could be implicated in EADs in HF.