Elevation of intracellular Ca2+ in the physiologically relevant range does not inhibit voltage-gated K+ channels in human T lymphocytes

Human T lymphocytes express both voltage-gated (K(V)) and Ca 2+ -activated (K(Ca)) potassium channels. The K(Ca) channel is activated by elevations of intracellular Ca 2+ ([Ca 2+ ] i ) at concentrations attained during physiological Ca 2+ signalling. Whether or not the K(V) channel is affected by [Ca 2+ ] i is a matter of controversy. Here, the interaction between the K + channels of lymphocytes and [Ca 2+ ] i was studied using cell-attached and whole-cell patch-clamp recordings, while [Ca 2+ ] i of the same cell was monitored simultaneously by fura-2 imaging or from the activity of the K(Ca) channels. The K + channels in cell-attached patches were measured using a high K + pipette solution. The K(V) conductance was quantified as the integral of the inward current during voltage ramp stimulation, yielding a measure independent of the cell membrane potential. Whereas the open probability of the K(Ca) channel showed an absolute dependence on [Ca 2+ ] i , the K(V) channel was little affected by [Ca 2+ ] i . The K(V) conductance is not reduced by elevations of [Ca 2+ ] i in the range 0–8 μ m . On the contrary, a modest but consistent increase in the K(V) current component in cell-attached currents was observed when [Ca 2+ ] i was elevated. The absence of inhibition of the K(V) current by [Ca 2+ ] i was also apparent from whole-cell measurements with pipette solutions buffered to 1 μ m free Ca 2+ : following break-in to whole-cell configuration, depolarizing voltage ramps were applied at regular intervals to activate the K(V) current while the K(Ca) current was measured from the slope of the ramp current below the activation of the voltage-gated current. During the gradual activation of the K(Ca) current, as the cell interior was perfused with the pipette solution, the K(V) current remained constant in amplitude. In the initial period following break-in to whole-cell configuration, a gradual increase in the rate of K(V) current inactivation was generally observed. However, the time course of this change in kinetics was much slower than the perfusion of the cell interior, as judged from the activation of the K(Ca) conductance, ruling out a direct effect of Ca 2+ , within the physiological range, on K(V) channel kinetics.