Characterization of Ca(2+)-activated K+ channels in excised patches of human T lymphocytes

Ca(2+)-activated K+ [K(Ca)] channels were studied in excised patches of resting and activated human peripheral blood T lymphocytes. The K(Ca) channel had a single-channel conductance of 50 +/- 6 pS in symmetrical high-K+ solutions in the potential range of -100 to -10 mV and was inwardly rectifying at more depolarized potentials. The channel was sensitive to block by charybdotoxin (10 nM) and insensitive to apamin (3 nM). Half-maximum activation occurred at an internal free Ca2+ concentration of 360 +/- 110 nM. The concentration-effect curve had a slope factor of 0.83 +/- 0.12, suggesting a 1:1 interaction of Ca2+ ions with the channel. Ca2+ affects the open time probability of the K(Ca) channels, mainly by modulating the frequency of channel opening. The open probability did not show voltage dependence. The kinetics of the channel could be described assuming one open state and two closed states. The time constant of the exponential describing the open time distribution amounted to 2.8 +/- 1.2 ms, whereas the closed time distribution could be described with two exponentials with time constants of 0.2 +/- 0.05 ms and 8.0 +/- 2.1 ms, respectively. Resting T lymphocytes expressed a low number of channels but the density of channels increased dramatically during chronic phytohaemagglutinin stimulation.