Parathyroid hormone (PTH) receptors and the biological response to PTH in osteoblasts have been shown to be influenced by glucocorticoids, growth factors, cytokines or PTH itself. Furthermore, components of extracellular matrix (ECM) appear to regulate the response to PTH as well. We investigated the effects of osteoblast-deposited ECM on PTH-related protein (PTHrP)-stimulated cAMP production, PTHrP binding and PTH/PTHrP receptor mRNA in the human osteoblast-like cell line SaOS-2. ECM was laid down by the human osteoblastic cell line MG-63. At confluence, maximal cAMP stimulation induced by 100 nmol/l PTHrP (1 -34) was decreased in SaOS-2 cells grown on ECM as compared with cultures on plastic dishes, without any change in PTHrP concentration producing half-maximal stimulation. In contrast, cAMP production stimulated by PGE2 was increased in cells on ECM. Saturable '"I-PTHrP binding (as evaluated by Scatchard plot analysis) was markedly diminished in cells grown on ECM (5,600 5 2,010 vs. 20,700 t 1,710 binding sites/cell, x 5 S.E.M., P < 0.01, n = 4 experiments), without any significant change in affinity (1.3 -t 0.4 vs. 2.5 ? 0.5 nmol/ 1 (NS), in cells on ECM and plastic, respectively). This apparent decrease in membrane receptor density was associated with markedly lower steady state PTH/PTHrP receptor mRNA levels as assessed by Northern blot analysis (ECM/ control: 0.4 2 0.1). A difference in PTH/PTHrP receptor mRNA levels between cells on ECM or on plastic dishes was detectable by 8 hours, but not by 4 hours, after seeding the cells at high density. By 24 hours after plating, PTH/PTHrP receptor mRNA levels were maximally decreased in cells on ECM. These results in the human osteoblast-like cell line SaOS-2 indicate that PTH/PTHrP receptors are down-regulated by growth on ECM. Thus, attachment of bone cells to bone surface could influence differentiation and function of osteoblasts. tj 1995 WIIPY-LIS, Inr