Activation of active Na transport in sheep Purkinje fibres by external K or Rb ions

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Abstract
The intracellular Na activity (a Na i ) of sheep Purkinje fibres bathed in solutions with and without K (Rb) is studied by means of Na sensitive microelectrodes. Perfusion with K (Rb) free media increasesa Na i . Upon reapplication of K (Rb) containing solutionsa Na i decreases and the cell membrane hyperpolarizes transiently. After an initial delay attributed to K (Rb) equilibration in the extracellular space, the decline ofa Na i and membrane potential towards their respective resting values is approximately monoexponential and displays the same time constant (τ). The τ values anda Na i in the steady state vary with the extracellular K (Rb) concentration ([K]0, [Rb]0). According to a simple model the activation of the Na pump by external K (Rb) can be estimated from the time constants or thea Na i steady state values at various [K]0 ([Rb]0). Half maximal activation occurs at 1.6–3.7 mM K (Rb). External K and Rb ions are equipotent activators of the Na pump in sheep Purkinje fibres.