Electrophysiological measurements on three clonally derived bone cell populations showed a positive correlation between longer-term hyperpolarizing membrane potential responses to parathyroid hormone (PTH) and an intracellular cAMP response to PTH. One clone (RCJ 1.20) had no sustained electrophysiological response and no cAMP response to PTH. Another clone (ROS 17/2.8) had both a sustained hyperpolarizing response and a cAMP response to PTH. The third clone (RCB 2.2) initially had both an electrophysiological response and a cAMP response to PTH, but both responses were lost after prolonged growth in culture. Application of dibutyryl cAMP to RCJ 1.20 and ROS 17/2.8 cells produced both transient and sustained hyperpolarizing responses. Application of isobutylmethylxanthine produced a sustained hyperpolarization. These results suggest that the hyperpolarizing response to PTH is related to a cAMP-mediated increase in Ca2+ conductance, which leads to an increase in Ca2+-activated K+ conductance. The pronounced membrane potential spikes and fluctuations that occur in some of the clonal lines were shown to be unrelated to the hyperpolarizing response to PTH. This was demonstrated by the lack of correlation between the occurrence of the spikes or fluctuations and the occurrence of the hyperpolarizing response to PTH in the various cell lines, by the lack of effect of PTH on the spikes and fluctuations, and by the lack of effect on the hyperpolarizing response to PTH of verapamil and quinine, both of which significantly reduce the spikes and fluctuations.

Electrophysiological measurements were carried out on osteoclasts in vitro. Such isolated osteoclasts are able to resorb bone in vitro and contract in response to calcitonin (CT). Our measurements show that individual osteoclasts respond to CT with a significant transient hyperpolarization of membrane potential. Application of parathyroid hormone (PTH) and dibutyryl cAMP produced a transient hyperpolarization in some osteoclasts. Measurements on an osteoblastlike line (ROS 17/2.8) showed a sustained hyperpolarizing response to CT, which is similar to but smaller than the hyperpolarizing response to PTH and dibutyryl cAMP in this and some other osteoblastlike lines. In contrast to osteoblastlike cells, the osteoclasts have no long term membrane potential response to CT, to PTH, or to dibutyryl cAMP. These results show that there are distinct differences between osteoclasts and osteoblasts in their ion transport responses to hormones.

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