Key Points
- To determine the electrophysiological effects of low concentrations of ouabain on potassium gradients and membrane currents in cardiac Purkinje fibres.
- Exposed isolated sheep cardiac Purkinje fibres to low concentrations of ouabain (5 × 10⁻⁸ to 5 × 10⁻⁷ M) across different extracellular potassium concentrations ([K]o).
- Recorded total steady-state current-voltage relations and measured changes in the reversal potential and activation curve of the potassium-specific pacemaker current (iK2).
- Low ouabain concentrations shifted the iK2 reversal potential in a negative direction at normal [K]o, matching the effect of reduced extracellular potassium and demonstrating stimulation of the Na+-K+ exchange pump.
- Lowering [K]o switched the ouabain effect from stimulation to inhibition, shifting the reversal potential in a positive direction without modifying the activation curve (s infinity) of iK2.
- Net pump stimulation coincided with ouabain concentrations that produce positive inotropy, showing that cardiac contraction enhancement does not require net Na+-K+ pump inhibition.
Structured PICO
PPopulationSheep cardiac Purkinje fibres
IInterventionLow concentrations of ouabain (5 X 10(-8) to 5 X 10(-7) M)
CComparatorAbsence of ouabain or reduced external potassium concentration ([K]o)
OOutcomeChanges in the reversal potential for a K specific current iK2 and total steady-state current-voltage relationssurrogate
Low concentrations of ouabain stimulate the Na+-K+ exchange pump in sheep Purkinje fibres, suggesting its positive inotropic action is not causally related to net pump inhibition.