The Control of Tonic Tension by Membrane Potential and Intracellular Sodium Activity in the Sheep Cardiac Purkinje Fibre

Intracellular Na activity (aiNa) was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibres. Tension was measured simultaneously. aiNa was increased reversibly either by exposing the preparation to K-free, Rb-free solution of by adding the cardioactive steroid strophanthidin. An increase of aiNa produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of aiNa (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of aiNa and a depolarization are required to increase tonic tension. It is concluded that either a low level of aiNa or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of aiNa. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = b(aiNa)y, where y is a constant and b depends on membrane potential. In five experiments y was found to be 3.7 +/- 0.7 (mean +/- S.E.M.) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given aiNa the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on aiNa. In five experiments a was found to be 0.06 +/- 0.01 mV-1 (mean +/- S.E.M.). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of aiNa that is equivalent to a 3.7 mV change of the Na equilibrium potential, ENa. Hence ENa is nearly 3 times more effective than membrane potential in controlling tonic tension. During a prolonged depolarization (several minutes) the initial increase of tonic tension decays gradually. This is associated with a fall of aiNa. The relationship between tonic tension and aiNa is similar to that seen when aiNa is increased by inhibiting the Na pump. It is concluded that the fall of aiNa is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of aiNa and membrane potential on a voltage-dependent Na-Ca exchange. The possibility that a voltage-dependent Ca channel contributes to tonic tension is also discussed.