Insulin-like growth factor binding proteins from adult-hamster pancreatic islets: influence of glucose concentration

This study investigated the effect of glucose on insulin-like growth factor binding proteins (IGFBPs) in islets isolated from pancreas of adult hamsters and compared the response pattern with that of their serum IGFBPs. Serum samples and islets were obtained from adult normal male hamsters, and IGF-binding capacity was measured in aliquots of serum, sonicated islets, or conditioned medium using either 125I-hIGF-I or -II. IGFBPs were characterized in these samples by the ligand-blotting technique, and insulin was measured in conditioned medium by radioimmunoassay. Three IGFBP fractions were identified in serum, with relative molecular weights of 38, 30-33, and 24 kDa, while only two fractions of 30-33 and 24 kDa were identified in islets or in their conditioned medium. Islets cultured with 2 or 16 mM glucose for 48 h released more insulin in the presence of the higher glucose concentration. The binding capacity measured in the islet suspension or conditioned medium increased as a function of glucose concentration in the incubation medium. The IGFBPs present both in islets and conditioned medium had a 3- to 4-fold higher apparent affinity for IGF-II than IGF-I. The higher glucose concentration increased the intensity of the two IGFBP bands identified in the islet suspension by 2- to 3-fold. Our data show that two low-molecular-weight IGFBPs were released from adult hamster pancreatic islets, with a different distribution pattern from that of hamster serum, and that the amount of IGFBPs released by islets depended on the glucose concentration in the culture medium. Though not conclusive, these data suggest that IGFBPs may play a regulatory role in B-cell turnover in adult islets as they do in foetal islets.