ACIDOSIS INHIBITS OSTEOBLASTIC AND STIMULATES OSTEOCLASTIC ACTIVITY INVITRO

Metabolic acidosis induces net calcium flux (J(Ca)) from cultured neonatal mouse calvariae through physicochemical and cell-mediated mechanisms. To determine the role of osteoblasts in acid-induced J(Ca), collagen synthesis and alkaline phosphatase activity were assessed in calvariae incubated in reduced pH and bicarbonate medium, a model of metabolic acidosis (Met), and compared with controls (Ctl). Collagen synthesis fell from 30.5 +/- 1.1 in Ctl to 25.1 +/- 0.4% with Met, and alkaline phosphatase decreased from 403 +/- 25 in Ctl to 298 +/- 21 nmol P(i).min-1.mg protein-1 with Met. During acidosis J(Ca) was correlated inversely with percent collagen synthesis (r = -0.743, n = 11, P = 0.009) and with alkaline phosphatase activity (r = -0.453, n = 22, P = 0.034). To determine the role of osteoclasts in acid-induced J(Ca), osteoclastic beta-glucuronidase activity was determined in Ctl and Met in the absence or presence of the osteoclastic inhibitor calcitonin (CT, 3 x 10(-9) M). Met increased beta-glucuronidase (5.9 +/- 0.2) compared with Ctl (4.6 +/- 0.3-mu-g phenolphthalein released.bone-1.h-1), whereas CT inhibited beta-glucuronidase in both Ctl and Met (3.1 +/- 0.2 and 3.5 +/- 0.3, respectively). During acidosis J(Ca) was correlated directly with beta-glucuronidase activity (r = 0.683, n = 42, P < 0.001). Thus the cell-mediated component of J(Ca) during acidosis in vitro appears to result from a combination of inhibited osteoblastic and stimulated osteoclastic activity.