Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes.
1. In order to investigate the possible effect of membrane potential on cytoplasmic Na+ binding to the Na+-K+ pump, we studied Na+-K+ pump current-voltage relationships in single guinea-pig ventricular myocytes whole-cell voltage clamped with pipette solutions containing various concentrations of Na+ ([Na+]pip) and either tetraethylammonium (TEA+) or N-methyl-D-glucamine (NMDG+) as the main cation. The experiments were conducted at 30 C under conditions designed to abolish the known voltage dependence of other steps in the pump cycle, i.e. in Na+-free external media containing 20 mM Cs+. 2. Na+-K+ pump current (Ip) was absent in cells dialysed with Na+-free pipette solutions and was almost voltage independent at 50 mM Na+pip (potential range: -100 to +40 mV). By contrast, the activation of Ip by 0.5-5 mM Na+pip was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species. 3. The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+]pip required for half-maximal Ip activation (K0.5 value) amounted to 5.6 mM at -100 mV and to 2.2 mM at +40 mV. 4. The results suggest that cytoplasmic Na+ binding and/or a subsequent partial reaction in the pump cycle prior to Na+ release is voltage dependent. From the voltage dependence of the K0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be approximately 0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation.
Barmashenko
G
G
Arbeitsgruppe Muskelphysiologie, Ruhr-Universitat, D-44780 Bochum, Germany.
Kockskämper
J
J
Glitsch
H G
HG
D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D005260:Female; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology; D006352:Heart Ventricles; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase