Calcium-sensitive Delayed Rectifier Potassium Current in Guinea Pig Ventricular Cells

The calcium sensitivity of the delayed rectifier K+ current (IK) was investigated in guinea pig single ventricular cells using the whole cell configuration of the patch-clamp technique with a cell dialysis method. The concentration-response curve of IK for intracellular Ca2+ indicated that IK started to increase at intracellular Ca2+ concentration [Ca2+]i of 10(-8) M (pCa 8) and it increased threefold at pCa 7. At lower [Ca2+]i than pCa 9, IK remained unchanged. A shift of the activation curve of IK by the elevation of [Ca2+]i was not observed. Although [Ca2+]i had little effect on time constants of the activation and deactivation of IK, it predominantly increased the amplitudes of the fast components in the activation process and the slow component in the deactivation process. In the ensemble noise analysis, the elevation of [Ca2+]i increased the number and open probability of functional IK channels but not the unit amplitude of IK channel. These results suggest that the elevation of [Ca2+]i enhances IK, probably by increasing the number and open probability of functional IK channels. Ca2(+)-sensitive IK in cardiac cells is a class of current different from Ca2(+)-activated K+ current in other tissue because the activation curve of Ca2(+)-sensitive IK was not shifted by the [Ca2+]i elevation, and the single channel conductance of IK was smaller than the one of Ca2(+)-activated K+ current.