Displacement and translocation of osteoblast-like cells by osteoclasts.

Rabbit osteoclasts and rabbit osteoblast-like stroma cells (OB cells) were placed onto plastic surfaces and the migration patterns of individual osteoclasts and osteoclast-OB interactions were analyzed with time-lapse recording. To induce directed migration, the cultures were exposed to an electrical field of 0.01 or 0.1 V/mm. At 0.1 V/mm, osteoclasts moved directly toward the anode in some cases, clearing OB cells from their path of migration. In other cases, osteoclasts migrated toward the anode for part of the time but then changed direction and moved toward groups of OB cells. Observations were made on osteoclasts interacting with single OB cells or small colonies and on osteoclasts interacting with OB monolayers, at both field strengths; the results were independent of field strength. There were several characteristic behaviors. With single OB cells and small OB colonies, retraction of OB cells upon contact with the osteoclast was the predominant mechanism whereby these cells begin to move out of the path of the osteoclast. A pronounced ruffling or blebbing of the OB cell membrane often followed retraction. When osteoclasts displaced OB cells that were part of a monolayer, extension of an osteoclast lamellipodium underneath the edge of the OB cell layer generally preceded partial retraction of the OB cells involved. It sometimes appeared as if the detached or partially detached OB cells were "pushed" by the osteoclast, which in some cases resulted in OB cells being moved hundreds of microns in a period of a few hours, at rates comparable to the normal speed for osteoclast migration (congruent to 100 microns/h), much faster than the normal speed for OB cells (congruent to 10 microns/h).(ABSTRACT TRUNCATED AT 250 WORDS)

Ba(2+)-induced action potentials in osteoblastic cells.

Trains of long-duration "action potentials" were induced by Ba2+ in osteoblast-like rat osteosarcoma cells (ROS 17/2.8), under current clamp and voltage clamp. Large depolarizing pulses were seen in microelectrode measurements at 37 degrees C following the addition of 10 or 20 mM Ba2+ to physiological bathing medium. Application of BAY K 8644 resulted in the onset of the pulses at earlier times and at more negative potentials. The pulses were blocked by nifedipine and Cd2+, but not by Ni2+. Large inward current pulses were seen in whole-cell patch technique voltage-clamp measurements at 37 degrees C in the presence of from 10 to 110 mM Ba2+ in the bathing medium. The current pulses were not seen at 22 degrees C in the presence of 110 mM Ba2+, but could be induced by BAY K 8644. These pulses were not blocked by TTX, but were blocked by nifedipine, Cd2+, Zn2+, Co2+, and by an increase in bathing [Ca2+]. The shape and frequency of the current pulses were the same as for voltage pulses under current clamp. A model that can explain these observations involves opening of L-type Ca2+ channels in a voltage-independent manner by cytosolic Ba2+ via a screening of Ca2+ from sites that produce either inactivation or a lower probability of opening in the activated state. There would be a closing of these channels at higher [Ba2+] as Ba2+ is forced onto these sites. A refractory period is also required to give repeated pulses of openings.

An experimental test of a model for repeated Ca2+ spikes in osteoblastic cells.

A model for cytosolic Ca2+ spikes is presented that incorporates continual influx of Ca2+, uptake into an intracellular compartment, and Ca(2+)-induced Ca2+ release from the compartment. Two versions are used. In one, release is controlled by explicit thresholds, while in the other, release is a continuous function of cytosolic and compartmental [Ca2+]. Some model predictions are as follows. Starting with low Ca2+ influx and no spikes: (1) induction of spiking when Ca2+ influx is increased. Starting with spikes: (2) increase in magnitude and decrease in frequency when influx is reduced; (3) inhibition of spiking if influx is greatly reduced; (4) decrease in the root-mean-square value when influx is increased; and (5) elimination of spiking if influx is greatly increased. Since there is good evidence that hyperpolarizing spikes reflect cytosolic Ca2+ spikes, we used electrophysiological measurements to test the model. Each model prediction was confirmed by experiments in which Ca2+ influx was manipulated. However, the original spike activity tended to return within 5-30 min, indicating a cellular resetting process.