Action of bPTH and bPTH fragments on embryonic bone in vitro: Dissociation of the cyclic AMP and bone resorbing response

Summary The effects of bPTH-(1-84), bPTH-(1-34), [Nle-8, Nle-18, Tyr-34] bPTH-(1-34), bPTH-(1-34) amide (NTA 1-34, desamino bPTH-(1-34), bPTH-(2-34), bPTH-(3-34), and [Nle-8, Nle-18, Tyr-34] bPTH-(3-34) amide (NTA 3-34) were tested in cultured bone cells, isolated from the osteoblast layers of fetal chicken calvaria (cyclic AMP) and in fetal rat calvaria (cyclic AMP, Ca release, and lactate production). Only bPTH-(1-84), bPTH-(1-34), and NTA 1-34 increased cyclic AMP production in a doserelated manner, both in calvaria and in bone cells, whereas all fragments (except NTA 3-34) stimulated bone resorption, the order of decreasing potency being bPTH-(1-84), NTA 1-34, bPTH-(1-34), desamino bPTH-(1-34), bPTH-(2-34), bPTH-(3-34). As in human cells, the antagonist NTA 3-34 inhibited specifically and in a dose-dependent way the cyclic AMP response of maximal concentrations of both bPTH-(1-84) and bPTH-(1-34) in rat calvaria and in chicken bone cells, when measured after short (15 min) and longer (1 1/2–16 h) incubation periods. In addition, measured after 4 h of incubation, NTA 3-34 completely inhibited bPTH-(1-84)-stimulated Ca release using maximal and submaximal concentrations. However, after 6–24 h of incubation, NTA 3-34 had no effect on bPTH-(1-84)-stimulated Ca and lactate release, even at an antagonist/agonist ratio up to 12.5 M, perhaps due to its lower affinity for the PTH receptor. From these findings we propose that (a) in bone there are two types of receptors, one governing demineralization via regulation of the calcium influx and one governing adenylate cyclase activity, and (b) the receptors are different from each other with respect to their affinities toward the agonists and the antagonist.