A slow outward current and a hypoosmolality induced anion conductance in embryonic chicken osteoclasts

In this paper we report on a hypoosmolality induced current, I-osmo in embr yonic chicken osteoclasts, which could only be studied when blocking a simu ltaneously active, unidentified slow outward current, I-sIo. I-sIo was observed in all of the examined cells when both the intracellular and extracellular solutions contained sodium as the major cation and no po tassium. The current was outwardly rectifying and activated at membrane pot entials more positive than +44 +/- 12 mV (n = 31). The time to half activat ion of the current was also voltage dependent and was 350 ms at V-m= +80 mV , and 78 ms at V-m = +120 mV. The current did not inactivate during periods up to 5 s. Extracellular 4-AP (5 mM), TEA(5 mM) and Ba2+ (1 mM), blockers of K+ conductances in chicken osteoclasts, did not influence I-sIo. However , I-sIo was inhibited by 50 muM extracellular verapamil, which allowed us t o study I-osmo in isolation. Exposure of the osteoclasts to hypotonic solution resulted in the developme nt of a depolarization activated I-osmo. It developed after a 1-min delay a nd reached its maximum within 10 minutes. Half-maximal activation occurred after 4.4 +/- 0.9 min (n = 9). The current activated within a few ms upon d epolarization and did not inactivate during at least 5 sec. I-osmo reversed around the calculated Nernst potential for Cl (E-cl = +7.3 mV and V-rev = +5.4 +/- 3.6 mV, n = 9). The underlying conductance, G(osmo) exhibited mode rate outward rectification around 0 mV in symmetrical Cl solutions. Ion sub stitution experiments showed that G(osmo) is an anion conductance with P-Cl approximate to P-F > P-gluc much greater than P-Na. I-osmo was blocked by 0.5 mM SITS but 50 muM verapamil, 5 mM TEA, 5 mM 4-AP, 1 mM Ba2+, 50 muM cy tochalasin D and 0.5 mM alendronate did not have any effect on the current. Cl currents have been implicated in charge neutralization during osteoclast ic acid secretion for bone resorption. The present results imply that osmol ality may be a factor controlling this charge neutralization.