Presence of insulin-like growth factor I but absence of the binding proteins in the bile of rats

Whereas insulin-like growth factor I (IGF-I) has been found in various body fluids from different species, the presence or absence of IGF and associated binding proteins (IGFBPs) in bile has not been clearly defined. Bile concentration of IGF-I was measured in this study and found to be highest in the neonate and lowest in adult rats (133 ± 15.9, 79.4 ± 10.5, 45.3 ± 12.7 ng/ml (mean ± SE) in 12-day-old, 33-day-old, and adult rats, respectively]. When bile delivery rates of IGF-I (i.e., the product of IGF-I concentration in bile and the biliary flow rate) were calculated, IGF-I delivery was highest in weanling rats (469 pg · h^-1 · g body wt^-1). When expressed as amount of IGF-I in bile delivered per day, however, delivery rates rose from 0.2 μg/day in the suckling and remained constant at 1.6-1.7 μg/day in both weanling and adult animals. Bile samples exposed to a placental membrane IGF receptor preparation showed significant dose-dependent inhibition of binding of native IGF-I. Because no IGF binding proteins were identified by Western ligand blot or by Sephadex gel chromatography, the results suggest the presence of biologically significant quantities of bioactive IGF-I in bile. We speculate that IGF-1 in bile may play an important role in the growth of the gastrointestinal tract, both in the suckling as well as later in life.