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

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 current iK2 and the instantaneous outward current iK1, while the delayed outward rectifying potassium current ix 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]0. 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 E1K]0 with a slope of 60 mV per decade.