Cationic inhibition of Ca current and Ca-dependent ciliary responses in Paramecium

The Paramecium cell membrane was voltage-clamped under K current suppression conditions. Ciliary beating was registered using high-speed video microscopy. Depolarizing step pulses activated a transient inward current and induced reversed ciliary beating. Very strong positive steps 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 transition potential. The transition potential rose with increasing external Ca²⁺ showing saturation beyond 1 mM Ca²⁺. Addition of Mg²⁺, Ba²⁺ or K⁺ to the 1 mM Ca²⁺ bathing solution depressed the transition potential in a concentration-dependent manner. The depolarization-activated inward Ca current increased with rising external Ca concentration, and addition of either Mg²⁺, Ba²⁺ or K²⁺ diminished the inward Ca current. The diverging results of Ca²⁺-dependent positive shifts, and Mg²⁺-(Ba²⁺-, K⁺-) dependent negative shifts in transition potential are compared with shifts of V_Imax. It is concluded that external cations bind competitively — in addition to membrane surface charges — to affinity sites of Ca channel, where they specifically modulate permeation of calcium.