Further analysis of spontaneous membrane potential activity and the hyperpolarizing response to parathyroid hormone in osteoblastlike cells.

Whole cell voltage clamp measurements using the patch technique on well-attached and well-spread cells of an osteoblastlike line (ROS 17/2.8) show the same spontaneous membrane potential activity as measurements with inserted microelectrodes. Furthermore, membrane potential measurements during the first 80 milliseconds (ms) following microelectrode penetration of the cell membrane usually show no decay. There is also good agreement between values of cell membrane resistance obtained by the microelectrode technique, the whole cell patch clamp technique, and the single channel patch clamp technique. These results indicate that our microelectrode measurements are not dominated by leak-induced artifacts, and that the spontaneous membrane potential activity is not induced by Ca2+ leakage around the microelectrode. The spontaneous membrane potential activity is eliminated in the presence of the Ca2+ ionophore A23187, also in serum-free medium, and by K+ and Ca2+ channel blockers, but it is not affected by the hyperpolarizing responses to parathyroid hormone (PTH) and dibutyryl cAMP, which persist under all of these conditions. These results support the hypothesis that the spontaneous membrane potential activity is related to repeated fluctuations of internal [Ca2+] and that such fluctuations result from a feedback loop involving Ca2+ channels or Ca2+ pumps in the cell membrane.