J. Membrane Biol. 140, 163-171 (1994) The Journal of Membrane Biology 9 Springer-Verlag New York Inc. 1994 Effects of Extracellular Calcium and Protons on Osteoclast Potassium Currents S.A. Arkett, 1 S.J. Dixon, 1,2 S.M. Sims 1 IDepartment of Physiology, The University of Western Ontario, London, Ontario Canada N6A 5C1 2Division of Oral Biology, Faculty of Dentistry, The University of Western Ontario, London, Ontario Canada N6A 5C1 Received: 30 November 1993/Revised: 25 February 1994 Abstract. During resorption of mineralized tissues, os- teoclasts are exposed to marked changes in the con- centration of extracellular Ca 2+ and H +. We examined the effects of these cations on two types of K + currents previously described in these cells. Whole-cell patch clamp recordings of membrane currents were made from osteoclasts freshly isolated from neonatal rats. In control saline (1 mM Ca 2+, pH 7.4), the voltage-gated, outwardly rectifying K + current activates at approxi- mately -45 mV and the conductance is half-maximal- ly activated at -29 mV (V0.s). Increasing [Ca2+]out rapidly and reversibly shifted the current-voltage (I-V) relation to more positive potentials. Current at -29 mV decreased to 28 and 9% of control current at 5 and 10 mM [Ca2+]out , respectively. This effect of elevating [Ca:+]out was due to a positive shift of the K + channel voltage activation range. Zn 2+ or Ni 2+ (5 to 500 ~tM) also shifted the I-V relation to more positive potentials and had additional effects consistent with blockade of the K + channel. Based on the extent to which these di- valent cations affected the voltage activation range of the outwardly rectifying K + current, the potency se- quence was Zn 2+ > Ni 2+ > Ca 2+. Lowering or rais- ing extracellular pH also caused shifts of the voltage ac- tivation range to more positive or negative potentials, respectively. In contrast to their effects on the out- wardly rectifying K + current, changes in the concen- tration of extracellular H + or Ca 2+ did not shift the voltage activation range of the inwardly rectifying K + current. These findings are consistent with Ca 2+ and other cations affecting voltage-dependent gating of the osteoclast outwardly rectifying K + channel through changes in surface charge. Correspondence to: S.A. Arkett Key words: Kvl.3 -- Inwardly rectifying K + channel -- Surface charge-- Zn 2+ -- pH Introduction Changes in the ionic milieu of cells affect membrane ex- citability and thus cell activity. Since the seminal work by Frankenhaeuser and Hodgkin (1957), changes in the concentration of extracellular divalent cations have been shown to alter the properties of voltage-gated channels in many different cell types (see Green & Andersen, 1991; Hille, 1992). By altering the charge on the mem- brane surface, variations in the concentration of Ca 2+ or other divalent cations shift the voltage dependence of channel gating and affect the rate of activation and in- activation (DeCoursey et al., 1985; Armstrong & Lopez- Barneo, 1987; Mayer & Sugiyama, 1988; Grissmer & Cahalan, 1989; Spires & Begenisich, 1992). Similarly, changes in extracellular pH alter membrane surface charge, with resultant shifts in the threshold of K + channel activation (Deutsch & Lee, 1989). Osteoclasts are exposed to marked changes in the concentration of extracellular Ca :+ and H + during the resorption of bone and other mineralized tissues. Dis- solution of mineral is accomplished by acidification of a confined extracellular compartment (the resorption lacuna), which osteoclasts make by forming a seal to the substrate (reviewed by Blair & Schlesinger, 1992). A vacuolar-type H + ATPase, located in the specialized plasma membrane ("ruffled border") apposed to the bone surface, is responsible for this acidification (Blair et al., 1989; V~i~in~nen et al., 1990; Chatterjee et al., 1992). During resorption, the mineral component of bone, which is largely hydroxyapatite, dissolves to yield calcium and inorganic phosphate (Delaiss6 & Vaes, 1992). Ion-sensitive microelectrode measurements have revealed a pH of 5 and a Ca 2+ concentration of 20 to 40