The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre.

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Abstract
Intracellular Na activity .**GRAPHIC**. was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibers. Tension was measured simultaneously. .**GRAPHIC**. was increased reversibly either by exposing the preparation to K-free, Rb-free solution or by adding the cardioactive steroid strophanthidin. An increase of .**GRAPHIC**. produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of .**GRAPHIC**. (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of .**GRAPHIC**. and a depolarization are required to increase tonic tension. Either a low level of .**GRAPHIC**. or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of .**GRAPHIC**. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = .**GRAPHIC**. where y is a constant and b depends on membrane potential. In 5 experiments y was found to be 3.7 .+-. 0.7 (mean .+-. SEM [standard error of the mean]) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given .**GRAPHIC**. the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on .**GRAPHIC**. In 5 experiments a was found to be 0.006 .+-. 0.01 mV-1 (mean .+-. SEM). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of .**GRAPHIC**. 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 .**GRAPHIC**. The relationship between tonic tension and .**GRAPHIC**. is similar to that seen when .**GRAPHIC**. is increased by inhibiting the Na pump. Apparently, the fall of .**GRAPHIC**. is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of .**GRAPHIC**. 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.