Characterisation of the voltage-activated calcium current in the marine ciliate Euplotes vannus

We have isolated the early Ca current (ICa) from the whole cell current that activates upon depolarisations in the marine ciliate Euplotes vannus. The peak of ICa activated within 4.2 ms at depolarisations to 5 mV with an amplitude of 2.5 +/- 0.35 nA and was reduced to 1.0 +/- 0.14 nA (n = 5) when the extracellular Ca concentration was changed from 10 to 1 mM. The voltage-dependent activation curve was steeper and shifted to more negative values when external Ca2+ was replaced by Ba2+. The early inward current inactivated with a double-exponential time course including a fast and a slow component, and no inactivation was recorded with Ba2+. The time constants for the recovery from inactivation varied between 44 and 153 ms according to the depolarisation-dependent Ca influx. At the common resting potential of -25 mV, ICa was not steady-state inactivated; ICa half-inactivated at -14.5 mV, and totally inactivated at -5 mV. ICa was inhibited by 10 mM extracellular Cd2+. The peptides omega-conotoxin-GVIA (20 microM), omega-conotoxin-MVIIC (600 nM), omega-agatoxin-IVA (60 nM) and calciseptine (900 nM) did not block ICa. The benzothiazepine-derivative diltiazem (100 microM) and the dihydropiridine nifedipine (100 microM) inhibited 51% and 33% of ICa, respectively. The naphthalene sulfonamide W7 reduced ICa with an inhibition coefficient of 33 microM.