Although levels of serum osteocalcin are thought to be an indicator of osteoblastic activity and bone formation, there is little information regarding the acute effects of changes in calcium or PTH levels on circulating osteocalcin concentrations. To study the effect of stepwise decreases in ionized calcium (CaI) on osteocalcin levels, we infused six normal subjects with citrate for four 30-min intervals using two different protocols. One protocol (pulse infusion) used alternating rates of infusion and resulted in rapid stepwise decrements in serum CaI. The second protocol (continuous infusion) used constant intermediate rates of citrate infusion and produced slower decrements in CaI, but with similar changes in magnitude. We monitored serum CaI, intact PTH, and osteocalcin concentrations during the course of these infusions. During each step of the pulse infusion the osteocalcin responses to changes in CaI in general were parallel to the changes in PTH (r = 0.462; P = 0.02) and were inversely correlated to CaI (r = -0.562; P = 0.003). The osteocalcin concentrations at the end of each 30-min period were higher than at the beginning of that period; over the total 120 min, osteocalcin levels rose from 3.46 +/- 0.63 to 6.88 +/- 1.08 micrograms/L (P less than 0.05). In contrast, during the first two periods of the continuous infusion, osteocalcin concentrations changed slightly. Only during the last two periods of the continuous infusion did osteocalcin respond in a manner characteristic of that observed with the pulse infusion. These data indicate that osteocalcin concentrations in the circulation may be acutely regulated by calcium and/or PTH.