Enhancement of fetal rat limb bone resorption by phorbol ester (PMA) and ionophore A-23187

The present investigation was undertaken to determine if an interaction affecting 45Ca release from prelabeled fetal rat long bones could be elicited by the Ca2+ ionophore, A-23187, and the phorbol ester, 12-myristate 13-acetate (PMA). Treatment with either A-23187 at a concentration of 0.3 microM or PMA at concentrations of 10(-6) M, 10(-7) M, and 10(-8) M produced significant 45Ca mobilization. When A-23187 and PMA were combined, enhanced 45Ca release was observed on days 1 and 2 of culture. The stimulation of calcium mobilization noted on day 1 occurred when neither ionophore nor 10(-6) M PMA treatments alone produced significant 45Ca release. On day 2, cumulative 45Ca release elicited by the combination of A-23187 plus 10(-6) M PMA was slightly more than additive (15.9% for combination treatment vs. 13.7% for the sum of the individual treatments). Moreover, when A-23187 was combined with 10(-7) M PMA on day 2, an enhancement of 45Ca release was observed which was clearly more than additive (14.5% for combination treatment vs. 8.8% for the individual treatments), suggesting the possibility of a synergistic interaction between the two agents. These results were in marked contrast to those obtained with the inactive phorbol ester analog, phorbol 13-monoacetate. No stimulation of 45Ca release was observed with 10(-6) M and 10(-7) M phorbol 13-monoacetate alone nor was enhanced 45Ca release noted when the analog was combined with 0.3 microM A-23187.(ABSTRACT TRUNCATED AT 250 WORDS)