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 an d associated binding proteins (IGFBPs) in bile has not been clearly de fined. Bile concentration of IGF-I was measured in this study and foun d 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-da y-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 fl ow 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 bil e delivered per day, however, delivery rates rose from 0.2 mu g/day in the suckling and remained constant at 1.6-1.7 mu g/day in both weanli ng 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 identifi ed by Western ligand blot or by Sephadex gel chromatography, the resul ts suggest the presence of biologically significant quantities of bioa ctive IGF-I in bile. We speculate that IGF-I in bile may play an impor tant role in the growth of the gastrointestinal tract, both in the suc kling as well as later in life.