Activation properties of the inward-rectifying potassium channel on mammalian heart cells

The early phase of activation of the inward-rectifying potassium channel is studied on single cells from guinea-pig heart. The current is quasi-instantaneous when it is outward, but activates with time when it is inward. This relaxation is exponential and its time-constant decreases with hyperpolarization. The I/V curve reflects a strong inward rectification and has a negative slope conductance on depolarization. Similar results were recorded in the absence of sodium, calcium, chloride ions and in isotonic potassium. Cesium slows down the phase of activation, and eventually appears to block the channels by suppression of the activation. Barium, conversely, does not affect the activation, but promotes an 'inactivation' of this current, which blocks it. These results are independent on the cells' dissociation method. They suggest that this current is the inward rectifier, called IK1 on heart. Its activation curve suggests that the inward and outward currents are flowing through the same channels. The inward rectifier is time- and voltage-dependent on heart as on other tissues. The effects of cesium and barium are also similar. The importance of its negative slope conductance is discussed.