Inward-rectifier channels in cardiac cells (IK1) stabilize the resting membrane potential near the K equilibrium potential. Here we investigate the role of IK1 in the regulation of action potentials and link this to the influx of Ca during beating. Inward Ca current alters the open-channel probability of outward IK1 current. Thus Ca ions depolarize cells not only by carrying an inward current but also by blocking an outward current.