Predominance of poorly reopening single Na+ channels and lack of slow Na+ inactivation in neonatal cardiocytes.
Elementary Na+ currents through single cardiac Na+ channels were recorded at -50 mV in cell-attached patches from neonatal rat cardiocytes kept at holding potentials between -100 and -120 mV. Na+ channel activity may occur as burst-like, closely-timed repetitive openings with shut times close to 0.5-0.6 msec, indicating that an individual Na+ channel may reopen several times during step depolarization. A systematic quantitative analysis in 19 cell-attached patches showed that reopening may be quite differently pronounced. The majority, namely 16 patches, contained Na+ channels with a low tendency to reopen. This was evidenced from the average value for the mean number of openings per sequence, 2.5. Strikingly different results were obtained in a second group of three patches. Here, a mean number of openings per sequence of 3.42, 3.72, and 5.68 was found. Ensemble averages from the latter group of patches revealed macroscopic Na+ currents with a biexponential decay phase. Reconstructed Na+ currents from patches with poorly reopening Na+ channels were devoid of a slow decay component. This strongly suggests that reopening may be causally related to slow Na+ inactivation. Poorly pronounced reopening and, consequently, the lack of slow Na+ inactivation could be characteristic features of neonatal cardiac Na+ channels.
Kohlhardt
M
M
Physiological Institute, University of Freiburg, West Germany.
Fichtner
H
H
Fröbe
U
U
D000818:Animals; D000831:Animals, Newborn; D002478:Cells, Cultured; D007700:Kinetics; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000378:metabolism*
D015222:Sodium Channels; D012964:Sodium