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

We have isolated the early Ca current (I-Ca) from the whole cell current that activates upon depolarisations in the marine ciliate Euplotes vannus. The peak of I-Ca 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, I-Ca was not steady-state inactivated; I-Ca half-inactivated at -14.5 mV, and totally inactivated at -5 mV. I-Ca was inhibited by 10 mM extracellular Cd2+. The peptides omega-conotoxin-GVIA (20 mu M), omega-conotoxin-MVIIC (600 nM), omega-agatoxin-IVA (60 nM) and calciseptine (900 nM) did not block I-Ca. The benzothiazepine-derivative diltiazem (100 mu M) and the dihydropiridine nifedipine (100 mu M) inhibited 51% and 33% of I-Ca, respectively. The naphthalene sulfonamide W7 reduced I-Ca with an inhibition coefficient of 33 mu M.