Summary. The C1 component of the voltage-and time-dependent inward current activated by hyperpolarizing the membrane of Chara inflata increases exponentially as the external pH, pH,,, is lowered from 7 with the membrane potential difference (PD) kept constant. Lanthanum and anthracene-9-carboxylic acid (A-9-C, a C1-channel blocker) both blocked the CI-component and removed the pHo sensitivity of the inward current. Lanthanum, however, also decreased the K + conductance. The hyperpolarized membrane is depolarized by A-9-C in a manner similar to that caused by the removal of external C1-. Low external concentrations of NH~-stimulated the CI-component of the inward current probably as a result of a change in cytoplasmic pH rather than as a result of a change in cytoplasmic [C1-], since the effect was observed in Cl--free solutions. The results show that the membrane PD, at hyperpolarized levels, is most likely determined by two factors: the proton extrusion pump, provided it has a reversal PD more negative than about -300 mV, and a voltagedependent C1-leak.
Summary. An inward current which increases in magnitude over a period of seconds is activated when the membrane of Chara ir~flata (a green alga) in a K+-conductive state is hyperpolarized by a voltage clamp. The peak current and the half-time of activation are exponentially dependent on membrane potential difference. It was found by using an external CI electrode that the component exponentially dependent on potential was due to an efflux of C1-. The measured current-voltage curves and the kinetics of deactivation of the current showed that other timedependent components contributed to the net inward current. The "punchthrough" theory of Coster (Biophys. J. 5:669-686, 1965) does not adequately explain the inward current since a "punchthrough potential" could not be obtained, and the inward current was distinctly time dependent. The voltage and time dependence of the inward current strongly suggests that the C1-efflux activated by hyperpolarization is through voltage-gated channels which open more frequently as the membrane is hyperpolarized.
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