Interleukin-6 (IL-6) is a multifunctional cytokine which is made by osteoblasts and has diverse effects on bone metabolism. We studied the interaction of IL-6 with the Ca2" and cAMP signaling systems in the osteoblastic cell line UMR-106 and in primary osteoblastic cultures derived from neonatal rat calvariae. IL-6 did not alter basal intracellular calcium concentration (ICa2+J1) but inhibited Ca2+ transients induced by parathyroid hormone (PTH), prostaglandin E2 (PGE2), and endothelin-1 in both dose-(100-400 U/ml) and time-(4-48 h) dependent manners. The effect of the cytokine was abolished by the tyrosine kinase inhibitor, herbimycin A (50 ng/ml). The IL-6 effect on the Ca2+ message system was related to suppressed production of hormonally induced inositol 1,4,5-triphosphate and inhibition of Ca2+ release from intracellular stores. Hormonally induced calcium entry pathways (estimated by using Mn2+ as a surrogate for Ca2+) were not, however, altered by the cytokine. IL-6 did not modulate cAMP generation in osteoblasts. With respect to osteoblast function, IL-6, although having no effect on cell proliferation by itself, greatly enhanced the antiproliferative effect of PGE2 and PTH. Because the production of IL-6 in osteoblasts is stimulated by calciotropic hormones (e.g., PIH and PGE2), the suppressive effect of the cytokine on hormonally induced Ca2+ transients may serve as an autocrine/paracrine mechanism for modulating the effect of hormones on bone metabolism. (J. Clin. Invest.