Identical unitary current amplitude and Ca(2+) block of cardiac Na channel before and during beta-adrenergic stimulation.
We examined the possibility of Ca(2+) permeation through cardiac Na channels ("slip mode conductance") by an analysis of the voltage-dependent block of Na channels by Ca(2+). A Ca(2+) block of Na channels was evident in rat and guinea pig ventricular myocytes during cell-attached single channel recordings with a physiological ionic environment (140 mM Na(+) and 1 to 10 mM Ca(2+) in the pipette solution). Increasing external Ca(2+) concentration ([Ca(2+)](o)) in the pipette solution reduced the unitary current amplitude predominantly at negative potentials. With [Ca(2+)](o) > 1 mM, unitary current amplitude did not increase at potentials negative to -40 mV in spite of augmented driving forces. The application of 5 microM isoproterenol potentiated the single channel activity elicited by depolarizing pulses from the holding potential of -120 mV, indicating that the channels in the patch under examination were modified by protein kinase A (PKA) stimulation. Increased activity was also confirmed with veratridine-modified Na channels, where channel openings were markedly prolonged. In either case, isoproterenol-induced potentiation neither reduced nor altered the properties of Ca(2+) block of cardiac Na channels, as evidenced by the stable unitary current amplitudes at potential levels from -60 to -20 mV. These results indicate that interactions among Na(+), Ca(2+), and the channel molecule were not modified with respect to permeation properties. They therefore argue against the "slip mode" concept of classical cardiac Na channel if a general concept of ion permeation through "multi-ion pores" is applicable to determine the ionic selectivity of Na channels.
Hirano
Y
Y
Department of Cardiovascular Diseases, Medical Research Institute, Tokyo Medical and Dental University, Bunkyo-ku, Tokyo, 113-8510 Japan. hirano.card@mri.tmd.ac.jp
Hiraoka
M
M
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004553:Electric Conductivity; D006168:Guinea Pigs; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007545:Isoproterenol / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009206:Myocardium / Q000166:cytology*; D018408:Patch-Clamp Techniques; D051381:Rats; D015222:Sodium Channels / Q000378:metabolism*; D014701:Veratridine / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D015222:Sodium Channels; D014701:Veratridine; D017868:Cyclic AMP-Dependent Protein Kinases; D007545:Isoproterenol