Abstract
Gallium(III) is a new therapeutic agent for hypercalcemia. Ga^3+^ reduces osteoclast action, but how it inhibits the cell's physiology is unknown. In vivo, 7–12 μM Ga(III) reduces calcium release from bone, but surprisingly, 10–100 μM Ga^3+^; added to isolated avian osteoclasts did not reduce their degradation of L‐(5‐^3^H)‐proline bone. ^3^H‐proline labels bone collagen specifically, and collagenolysis is an excellent indicator of bone dissolution because collagen is the least soluble component of bone. Ga(III) > 100 μM inhibited osteoclasts in vitro, but also killed the cells. To resolve this apparent conflict, we measured ^67^ Ga distribution between bone, cells, and media. Gallium binds avidly but slowly to bone fragments. One hundred micrograms of bone clears 60% of 1 μM gallium from 500 μI of tissue culture medium, with steady state at > 24 h. Osteoclasts on bone inhibited gallium binding capacity ∼ 40%, indicating a difference in available binding area and suggesting that osteoclasts protect their substrate from Ga binding. Less gallium binds to bone in serum‐containing medium than in phosphate‐buffered saline; 30% reduction of the affinity constant suggests that the serum containing medium competes with bone binding. Consequently, the effect of [Ga] on bone degradation was studied using accurately controlled amounts of Ga(III) pre‐bound to the bone. Under these conditions, gallium sensitivity of osteoclasts is striking. At 2 days, 100 μg of bone pre‐incubated with 1 ml of 1 μM Ga^3+^, with 10 pmoles Ga^3+^/μg bone, was degraded at 50% the rate of control bone; over 50 pM Ga^3+^/μg bone, resorption was essentially zero. In contrast, pre‐treatment of bone with [Ga^3+^] as high as 15 μM had no significant effect on bone resorption rate beyond 3 days, indicating that gallium below ∼150 pg/μg bone acts for a limited time and does not permanently damage the cells. We conclude that bone‐bound Ga(III) from medium concentrations < 15 μM inhibits osteoclasts reversibly, while irreversible toxicity occurs at solution [Ga^3+^] > 50 μM.