Protein-energy malnutrition, which is common in elderly patients with osteoporotic hip fractures, is associated with reduced plasma levels of insulin-like growth factor-I (IGF-I). IGF-I is an important regulator of bone metabolism, particularly of osteoblastic bone formation both in vivo and in vitro. Pharmacological doses of arginine (Arg) increase growth hormone (GH) and IGF-I serum levels. Whether amino acids, particularly Arg, can directly modulate the production of IGF-I by osteoblasts is not known. We investigated the effects of increasing concentrations of Arg on IGF-I expression and production, alpha1(I) collagen expression and collagen synthesis, and cell proliferation and cell differentiation, as assessed by alkaline phosphatase (ALP) activity and osteocalcin (OC) release, in confluent mouse osteoblast-like MC3T3-E1 cells. The addition of Arg (7.5-7500 micromol/L, equivalent to 0.1- to 100-fold human plasma concentration) for 48 h increased IGF-I production (adjusted for cell number) in a concentration-dependent manner with a maximum of 2.3 +/- 0.3-fold at 7500 micromol/L Arg [x +/- standard error of the mean (SEM), n = 3 experiments, p < 0.01]. Arg (7.5-7500 micromol/L) increased the percentage of de novo collagen synthesis in a concentration-dependent manner (2.1 +/- 0.4-fold with 7500 micromol/L Arg, p < 0.001) and ALP activity with a maximal stimulation of 144% +/- 13% plateauing at 750 micromol/l Arg (p = 0.002). The steady state level of IGF-I messenger ribonucleic acid (mRNA) and alpha1(I) collagen mRNA (both normalized to cyclophilin mRNA) of cells incubated with Arg at high (100-fold) or low (0.1-fold) human plasma concentrations, was 1.4 +/- 0.2, 1.2 +/- 0.2, and 1.1 +/- 0.2 after 24 h for the 7.5, 1.8, and 0.9 kb IGF-I mRNA transcripts, respectively (n = 3 experiments) and 1.5 +/- 0.2 and 3.1 +/- 0.7 after 24 and 48 h, respectively, for the combined analysis of the 5.6 and 4.7 kb alpha1(I) collagen mRNA transcripts (n = 3 experiments). A maximal mitogenic effect (cell number) of +21% +/- 3% (p < 0.01) was obtained with 1000 micromol/L Arg. In contrast, Arg (7.5-7500 micromol/L) induced a reduction of OC production, which reached 30% +/- 3% with 7500 micromol/L Arg (p = 0.02). In conclusion, Arg stimulated IGF-I production and collagen synthesis in osteoblast-like cells. Thus, Arg may influence bone formation by enhancing local IGF-I production.