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Journal of Experimental Biology 119,321-334 (1985)
Published by Company of Biologists 1985

Determination of Pure Voltage-Dependent Ca²⁺ Current in Paramecium Caudatum and its Inhibition by Divalent Cations

FRANK WEHNER ¹ and EILO HILDEBRAND ²

¹ Institut für Neurobiologie, Kernforschungsanlage JülichD-5170 Jülich, Federal Republic of Germany; Abteilung Pharmakologie der RWTH Aachen, D-5100 Aachen, Federal Rupublic of Germany
² Institut für Neurobiologie, Kernforschungsanlage Jülich, D-5170 Jülich, Federal Republic of Germany

Voltage-dependent Ca²⁺ currents in Paramecium caudatum were studied under voltage clamp conditions. To separate Ca²⁺ inward currents from concomitant K⁺ outward currents, the voltage-dependent Ca²⁺ conductance was temporarily inactivated by a preceding depolarization. The remaining currents were then subtracted from the overall currents measured in the absence of a prepulse. In this way pure Ca²⁺ currents could be obtained up to a depolarization of 100 mV, which is about 50 mV below the theoretical Ca2+ equilibrium potential (E_ca). Ca²⁺ currents were maximal at a depolarization of 35 mV and declined with further approach to E_ca, but they did not reverse sign in the voltage range tested.

In the presence of Mg²⁺, Co²⁺, Mn²⁺ or Ni²⁺, the Ca²⁺ inward currents decreased to a different extent. From experiments where these cations were added at different concentrations and from measurements at different Ca²⁺ concentrations in the absence of other divalent cations the following ratio of apparent dissociation constants could be derived: k_Ni: k_co: k_ca: k_Mg = 1:3:4.3-4.7:5:6.5. With a confidence of 95% the absolute value of k_ca lies between 40 and 130µmol l^-1. These results indicate that Ca²⁺ and other divalent cations compete for binding sites at the Ca-channel and thus determine excitability. Indirect effects due to changes of the surface potential are discussed.

Key words: Protozoa, Ciliata, cell membrane, excitability, Ca²⁺ current, divalent cations, cation competition

Accepted on April 16, 1985

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