Cytosolic calcium regulates a potassium current in corn (Zea mays) protoplasts

The voltage- and time-dependent K+ current, IK+ out, elicited by depolarization of corn protoplasts, was inhibited by the addition of calcium channel antagonists (nitrendipine, nifedipine, verapamil, methoxyverapamil, bepridil, but not La3+) to the extracellular medium. These results suggested that the influx of external Ca2+ was necessary for K+ current activation. The IC50, concentration of inhibitor that caused 50% reduction of the current, for nitrendipine was 1 microM at a test potential of +60 mV following a 20-min incubation period. In order to test whether intracellular Ca2+ actuated the K+ current, we altered either the Ca2+ buffering capacity or the free Ca2+ concentration of the intracellular medium (pipette filling solution). By these means, IK+out could be varied over a 10-fold range. Increasing the free Ca2+ concentration from 40 to 400 nM also shifted the activation of the K+ current toward more negative potentials. Maintaining cytoplasmic Ca2+ at 500 nM with 40 nM EGTA resulted in a more rapid activation of the K+ current. Thus the normal rate of activation of this current may reflect changes in cytoplasmic Ca2+ on depolarization. Increasing intracellular Ca2+ to 500 nM or 1 microM also led to inactivation of the K+ current within a few minutes. It is concluded that IK+out is regulated by cytosolic Ca2+, which is in turn controlled by Ca2+ influx through dihydropyridine-, and phenylalkylamine-sensitive channels.