Residual calcium ions depress activation of calcium-dependent current.

Calcium ions enter and accumulate during depolarization of some cells, activating a potassium current, IK(Ca), that depends on the cytoplasmic concentration of calcium ions, [Ca]i. However, elevation of [Ca]i can depress IK(Ca) elicited by a subsequent membrane depolarization. The depression of IK(Ca) is ascribed here to a [Ca]i-mediated inactivation of the voltage-gated calcium conductance, which causes a net reduction in calcium ions available for the activation of IK(Ca). This suggests that other processes dependent on gated calcium entry may also be depressed by small background elevations in cytosolic free calcium ions.