Adhesive substrates influence acid-productive activities of cultured rabbit osteoclasts: cultured osteoclasts with large vacuoles have enhanced acid-productive activities.

Since acid secretion of polarized osteoclasts on the bone surface has an important role in bone resorption, we examined the acid-productive activities of osteoclasts by measuring the pH of acidic organelles such as endosomes, lysosomes, and vacuoles in cultured rabbit osteoclasts with FITC-dextran as a fluorescent pH probe. The average pH value of acidic organelles of osteoclasts cultured on plain glass coverslips was 5.3 +/- 0.2. The pH of the acidic organelles correlated well with the size and amount of the vacuoles in the cells. Using this FITC-dextran method, we also examined the effects of adhesive substrates, such as type I collagen, vitronectin, and dentine, on the acid-productive activities of osteoclasts and found that the pH value of acidic organelles of osteoclasts cultured on dentine slices was significantly lower than that of osteoclasts cultured on plain glass coverslips. Likewise, in the case of the osteoclasts cultured on type I collagen- or vitronectin-coated glass coverslips, the pH values of acidic organelles were slightly lower and the proportion of osteoclasts having large vacuoles was increased compared with the cells cultured on the plain glass coverslips. These results indicate that osteoclasts containing large vacuoles have high acid-productive activities, and adhesive substrates such as type I collagen and vitronectin influence the formation of large vacuoles in cultured osteoclasts.