CATIONIC INHIBITION OF CA CURRENT AND CA-DEPENDENT CILIARY RESPONSES IN PARAMECIUM

The Paramecium cell membrane was voltage-clamped under K current suppr ession conditions. Ciliary beating was registered using high-speed vid eo microscopy. Depolarizing step pulses activated a transient inward c urrent and induced reversed ciliary beating. Very strong positive step s inhibited ciliary reversal during the pulse suggesting inhibition of the Ca influx. We call the potential, which is sufficiently positive to induce transition from reversed to normal ciliary beating, the tran sition potential. The transition potential rose with increasing extern al Ca2+ showing saturation beyond 1 mM Ca2+. Addition of Mg2+, Ba2+ or K+ to the 1 mM Ca2+ bathing solution depressed the transition potenti al in a concentration-dependent manner. The depolarization-activated i nward Ca current increased with rising external Ca concentration, and addition of either Mg2+, Ba2+ or K+ diminished the inward Ca current. The diverging results of Ca2+-dependent positive shifts, and Mg2+ (Ba2 +-, K+-) dependent negative shifts in transition potential are compare d with shifts of V-Imax. It is concluded that external cations bind co mpetitively - in addition to membrane surface charges - to affinity si tes of Ca channel, where they specifically modulate permeation of calc ium.