Cardiac Purkinje fibers: Cesium as a tool to block inward rectifying potassium currents

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Abstract
When a cardiac Purkinje fiber is exposed to 20 mM Cs the membrane potential falls to about −60 mV within 1 min. In voltage clamp experiments, exposure to Cs blocks both the pacemaker currenti K2 and the instantaneous outward currenti K2 , while the delayed outward rectifying potassium currenti x is not affected. In the presence of 20 mM Cs, the steady state currents are related linearly to the clamp potential and are insensitive to alterations in [K] o . The Cs sensitive current was defined as the difference between control and membrane currents measured in the presence of 20 mM Cs. This current displays inward-going rectification and its reversal potential follows log [K] o with a slope of 60 mV per decade. Besides the blockage of potassium conductance, 20 mM Cs affects the rate of sodium pumping. The pump generates an outward currenti p which can be blocked by 10−5 M dihydro-ouabain. In the presence of 20 mM Cs,i p is independent of [K] o and has a value similar to that in Tyrodes containing no Cs, 5.4 mM K. In Cs free solutions,i p decreases with [K] o , thus addition of 20 mM Cs increasesi p when [K] o is lower than 5.4 mM. Effects of 20 mM Cs on other membrane currents were not observed. It is concluded that Cs can be used as a tool to separate inward rectifying potassium currents from the net membrane current of the cardiac Purkinje fiber.

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