破骨細胞の細胞膜に発現する空胞型H+-ATPase電流の特性と膜電位およびpH依存性

抄録

The vacuolar-type H⁺-ATPase (V-ATPase) is an electrogenic H⁺ pump that is distributed in living organisms. By carrying out uphill H⁺ transport, V-ATPases acidify lysozomes and energize intracellular membranes. In osteoclasts, the V-ATPases are enriched in the plasma membrane faced to the bone surface (ruffled membrane) and serve as a major acid-secretion pathway required for bone resorption. In this study, we attempted to identify the pump currents of osteoclasts electrophysiologically and investigated their dependence on the membrane potential and/or H⁺ gradient, which may change widely under different functional states. Outward H⁺ currents were increased by intracellular dialysis with ATP up to 10 mM in dose-dependent manner. The V-ATPase current was evaluated by blockers for the V-ATPase, bafilomycin A₁ and N,N'-dicyclohexylcarbodiimide, in the presence of 5 mM ATP. The V-ATPase currents were decreased by hyperpolarization, but were still outward at -80 mV under pH_o/pH_i of 7.3/5.5. The current amplitude was decreased by either intracellular alkalization or extracellular acidification, but did not show current reversal. Significant outward H⁺ currents were seen at 0 mV even under pH_o/pH_i of 5.5/7.3. The data showed that the V-ATPase-medaited currents depends on both voltage- and pH gradients across the plasma membrane. The pump, however, could secrete H⁺ upon exposure to strong acids as far as energy is supplied sufficiently. [J Physiol Sci. 2006;56 Suppl:S113]