We have investigated the ability of various divalent cations to carry current through membrane channels in ciliates activated by mechanical stimulation and by membrane depolarization. Both types of channels are identified as Ca2+ channels. Whereas Ba2+ ions and Sr2+ ions can replace Ca2+ ions as charge carriers during either kind of current, Mg2+ ions can carry the current evoked by mechanical stimulation but not the current elicited by membrane depolarization. Mn2+ ions did not carry either of these currents, and they reduced the currents carried by Ca2+ions. Our experiments suggest that the mechano-sensitive and the voltage-senstive channels exhibit different selectivities for divalent cations.

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