Osteoclasts are the cells responsible for resorption of bone and other mineralized tissues, and participate in physiological processes such as bone remodelling and tooth eruption. These multinucleated cells arise from fusion of mononucleated precursors of the monocyte-macrophage lineage that originate in the bone marrow (Roodman, 1996). Osteoclasts exhibit different phases of activity, with cells alternating between motile and resorbing phases. Motile osteoclasts typically exhibit extensive pseudopods and are flattened in appearance (spread morphology). In contrast, resorbing osteoclasts lack extensive pseudopodia and are more dome shaped (rounded morphology). Resorption is accomplished by acidification of a compartment referred to as the resorption lacuna. Transport of H¤ by an electrogenic H¤-ATPase causes dissolution of the mineral phase of bone, while secreted hydrolytic enzymes digest the organic matrix (Roodman, 1996). Since H¤ transport is electrogenic, ion channels are required to provide pathways to dissipate charge. Previous studies have revealed that mammalian osteoclasts express a number of channel types, including an inwardly rectifying K¤ channel (IRK1), a transient outwardly rectifying K¤ channel (Kv1.3), H¤ channels and Cl¦ channels (Arkett, Dixon & Sims, 1992;Kelly, Dixon & Sims, 1992;Arkett, Dixon, Yang, Sakai, Minkin & Sims, 1994;Yamashita, Ishii, Ogata & Matsumoto, 1994;Kelly, Dixon & Sims, 1994; N ordstrom et al. 1995). The expression of channels is related to the morphology of the osteoclast, with spread osteoclasts exhibiting IRK1 and Cl¦ current, were recorded in isolated rat osteoclasts using patch clamp and fluorescence techniques. 2. At negative membrane potentials, ATP (1-100 ìÒ) activated an inward current that peaked rapidly and then declined. A later current was outward at potentials positive to the equilibrium potential for K¤ (EK) and showed oscillations. 3. The initial inward current, studied in isolation using Cs¤ in the electrode solution, showed rapid activation, inward rectification and reversal at +3 ± 4 mV. Reduction of [Na¤]ï to 10 mÒ shifted the reversal potential to −21 ± 3 mV, indicating that ATP activates a nonselective cation current, consistent with involvement of P2X receptors. 4. The later current activated by ATP, studied with K¤ in the electrode solution, exhibited a linear I-V relationship, and reversed at −71 ± 4 mV.
1. Cholinergic regulation of L-type Ca2P channels was investigated in freshly dissociated guinea-pig gastric and tracheal smooth muscle cells. Acetylcholine (ACh, 50 uM)
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