AbstractThe rat mammary myoepithelial-like cell line Rama 401 possesses 46,000 high-affinity receptors (Kd52 pM) and 2.8106low-affinity receptors (Kd24 nM) for basic fibroblast growth factor (bFGF) per cell. Heparin or heparinase pretreatment of the cells inhibits the specific binding of [125I]-bFGF by over 70%, and abolishes binding to the low-affinity sites. Dissociation experiments suggest that there are three kinetically distinct low-affinity receptors, with dissociation rate constants of 3.8 s−1, 0.067 s−1and 0.0018 s−1. Consistent with the presence of low-affinity receptors possessing a slow dissociation rate constant, exogenously added bFGF bound to the low-affinity receptor can stimulate DNA synthesis in Rama 401 cells without being released into the bulk of the culture medium. These results suggest that the low-affinity receptors on Rama 401 cells are heparan sulfate glycosaminoglycans (HSGAGs) and that their ability to modulate the action of bFGF may result from their diverse range of dissociation rate constants. A cell line, Rama 401ts, derived from Rama 401 by transformation with a temperature sensitivesrcgene, deposits less extracellular matrix at the permissive temperature of 34 C than at the non-permissive temperature of 41 C. Whilst the binding of [125I]-bFGF to Rama 401ts cells at 41 C is identical to that observed with the parental Rama 401 cells, at 34 C there are fewer low-affinity receptors. These results suggest the (HSGAGs) low-affinity receptors on Rama 401 cells are associated at least in part with the extracellular matrix.