Whole cell clamp recordings were conducted to examine the general behavior of ion channels in osteoclasts which were generated from bone marrow/stromal cell co-culture. In the resting state, inwardly and outwardly rectifying (IR and OR) currents were observed in the osteoclasts cultured for 5-18 days and the whole cell currents were classified into three types according to the conductance ratio between the OR and IR currents, such as, the IR type (OR/IR conductance < or = 0.2), the IR-OR type (0.2 < OR/IR < or = 5), and the OR type (OR/IR > 5). The IR type was found in 73% of cells (n = 78), the IR-OR type in 26% and the OR type in 1%. No relation was found between the current types and the number of nuclei or cultured days. The conductances varied among cells, but the IR conductance was larger than the OR conductance. The IR current was characterized by voltage-dependent kinetics, a negative conductance region and Ba(2+)-sensitivity. The reversal potential depended on the extracellular K+ concentration, indicating that K+ mediates the IR current. On the other hand, the OR current was reversibly reduced by decreasing extracellular Cl- concentration but not affected by K+, suggesting that the OR current depends on Cl-. High intracellular Ca2+ (1-10 microM) transiently activated a different class of outward current. These electrophysiological features resemble those in freshly isolated osteoclasts. We suggest that the ion channels involved in osteoclastic functions are expressed in the in vitro-generated osteoclasts and that the channels develop early in differentiation and are maintained for at least up to 18 days. Thus the co-culture system provide a useful model to examine roles of ion channels in osteoclastic functions.