K efflux through inward rectifying K channels in voltage clamped Purkinje fibers

The 42K efflux was measured in voltage clamped sheep Purkinje fibers. The voltage dependence of the K efflux can be described as N-shaped, showing a negative slope region. At potentials negative to -30mV, the K efflux is largely due to K flowing through a channel which rectifies in the inward direction and which is blocked by external application of 20mM Cs+. At potentials positive to -30mV an outward rectifier dominates the shape of the K efflux-voltage relationship. This component is insensitive to short external application of Cs+. Both components were also found when Na+ was replaced by tetramethylammonium. When the steady-state current-voltage relationship is compared with the K efflux one can conclude that the outward rectifying K flux largely determines the shape of this curve at positive membrane potentials, while the negative slope region of the K efflux correlates with the negative slope of the steady-state current-voltage relation. The K efflux is only slightly enhanced by stimulation of the preparation, corroborating the finding of inward-going rectification of the K channel. A clamp program repetitively activating the positive dynamic current e.g. by alternating the membrane potential between -70 and +10mV, increases the K efflux by about 50% as compared to the efflux measured in steady-state at this positive membrane potential. 4-Aminopyridine suppresses both this extra K efflux and the positive dynamic current. It is concluded that K ions contribute to the positive dynamic current.