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

Abstract

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.

A transient outward-rectifying K + channel current down-regulated by cytosolic Ca 2+ in Arabidopsis thaliana guard cells

“…Both the instantaneous and timedependent currents were enhanced by decreasing [K ϩ ] o via a negative shift of activation potentials (6), which is opposite and therefore distinct from the time-dependent I AP . Finally, cytosolic Ca 2ϩ in general does not regulate K ϩ out currents in guard cells (35,42,43), and in some cells [Ca 2ϩ ] i up-regulates K ϩ out currents (10,18,22). I AP showed the unique characteristics of being down-regulated by physiological [Ca 2ϩ ] i increases (Fig.…”

“…The primary structure of KCO1 contains two pore-forming domains within one subunit, showing structural similarities to two poredomain K ϩ channels isolated recently in yeast, Drosophila, and human (19)(20)(21). In maize suspension cells and Mimosa pulvinar cells K ϩ out have been demonstrated to be activated by cytosolic Ca 2ϩ (10,22).…”

Substrate Regulation of Single Potassium and Chloride Ion Channels in Arabidopsis Plasma Membrane

“…Prior to the experiment, a 500 giL aliquot of cells was added to about 2. (8,11,15). Typical gains were 10 mV/pA for monitoring seal formation and 100 mV/pA for channel measurements.…”

Pharmacology of the Ca2+‐dependent K+ channel in corn protoplasts

“…Nevertheless, when compared to all other reagents tested in plant systems this is the most potent K + channel blocker identified to date (Table I). CTX inhibition is not adequately explained as a non-specific interaction with the channel protein since other polycationic reagents, namely the mitochondrial signal peptide and apamin, or La 3 + [11], do not elicit the same response. Minor changes in the sequence of the peptide toxins (e.g.…”

“…The range in unitary conductance, 3 to 65 pS [3][4][5][8][9][10] recorded for plant plasmalemma K ÷ channels, and the observation that multiple regulatory mechanisms (e.g. cytosolic Ca 2÷ [10,11] or abscisic acid [8,12]) can modulate K ÷ currents in some protoplasts, suggest that more than one type of K ÷ channel may be responsible for IK+out in these diverse cell systems.…”