Voltage Sensitive Ca-Channels and the Transient Inward Current in Paramecium Tetraurelia

ABSTRACT Transient inward currents across the membrane of P. tetraurelia are recorded upon step depolarizations with a voltage clamp in solutions where Ca2+ is the only added inorganic cation. It is shown that the current is normally carried by Ca2+ through the Ca-channels which activate and inactivate in time. The transient inward current is dependent on both the size of the depolarizing step and the holding level before the step. Maximum inward current (Imax) occurs when the membrane is first held at the resting level (– 30 mV), then stepped to o mV in a solution containing 0·91 mm-Ca2+. The Imax is smaller when the membrane is first held at depolarized level. This is due to the depolarization-sensitive inactivation of the Ca-channels. The is also smaller when the membrane is first held at a hyperpolarized level. This may be explained by the activation of hyperpolarization-sensitive K-channels known to exist in the Paramecium membrane. I max increases with concentration of Ca2+ up to 0·9 mm. Further increase in the Ca2+ concentration does not affect /max-This apparent saturation at 0·9 mm-Ca2+ may reflect a rate-limiting step of Ca2+ permeation. The increase in Ca2+ concentration shifts the V−Ipeak curve in the direction of less sensitivity. This result is best explained as the effect of bound Ca2+ on the surface potential of the Paramecium membrane. These results provide the first detailed description of the properties of the action current through the Ca-channel in Paramecium. They also define the conditions under which future voltage-clamp studies of wild-type and mutant membranes of P. tetraurelia should be performed, i.e. to maximize the resolution of the Ca-channel activity, the membrane should be held at or near the resting potential and there should be over 0·9 mm-Ca2+ in the test solutions. The behaviour of the Paramecium Ca-channel and small Imax in the presence of K+ are discussed.