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Hyperpolarization of Paramecium tetraurelia under conditions where K + currents are suppressed elicits an inward current that activates rapidly toward a peak at 25-80 ms and decays thereafter. This peak current (Ihyp) is not affected by removing CI ions from the microelectrodes used to clamp membrane potential, or by changing extracellular CI-concentration, but is lost upon removing extracellular Ca z+. lhyp is also lost upon replacing extracellular Ca ~+ with equimolar concentrations of Ba 2+, Co 2+, Mg 2+, Mn 2+, or Sr ~+, suggesting that the permeability mechanism that mediates /hyp is highly selective for Ca ~+. Divalent cations also inhibit lhyp when introduced extracellularly, in a concentration-and voltagedependent manner. Ba 2+ inhibits lhyp with an apparent dissociation constant of 81 ~M at -110 mV, and with an effective valence of 0.42. Ihyp is also inhibited reversibly by amiloride, with a dissociation constant of 0.4 raM. lhyp is not affected significantly by changes in extracellular Na +, K ÷, or H + concentration, or by EGTA injection. Also, it is unaffected by manipulations or mutations that suppress the depolarization-activated Ca 2+ current or the various Ca2+-dependent currents of Paramecium. We suggest that lhyp is mediated by a novel, hyperpolarization-activated calcium conductance that is distinct from the one activated by depolarization.

Recent reappraisals of the role of ionized magnesium in cell function suggest that many cells maintain intracellular free Mg2+ at low concentrations (0.1 to 0.7 mM) and that external agents can influence cell function via changes in intracellular Mg2+ concentration. Depolarization and hyperpolarization of voltage-clamped Paramecium elicited a Mg2(+)-specific current, IMg. Both Co2+ and Mn2+ were able to substitute for Mg2+ as charge carriers, but the resultant currents were reduced compared with Mg2+ currents. Intracellular free Mg2+ concentrations were estimated from the reversal potential of IMg to be about 0.39 mM. The IMg was inhibited when external Ca2+ was removed or a Ca2+ chelator was injected, suggesting that its activation was Ca2(+)-dependent.

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