GLUCOSE-INDUCED INSULIN-SECRETION IN INS-1 CELLS DEPENDS ON FACTORS PRESENT IN FETAL CALF SERUM AND RAT ISLET-CONDITIONED MEDIUM

To study the regulation of growth and differentiated function of insul in-secreting cells, the rat insulinoma cell Line INS-1 was cultured in a defined serum-free medium containing prolactin, IGF-I, and triiodot hyronine, which was originally reported to maintain insulin secretion of islet cells. Growth and viability, as well as cellular insulin cont ent of INS-1 cells in the defined medium, were comparable to the contr ol cells cultured in the complete medium containing 10% fetal calf ser um. However, after a 3-day culture in this medium, insulin secretion i n response to glucose, pyruvate, and leucine was markedly blunted comp ared with the control cells (-78, -68, and -56%, respectively), wherea s the response to 30 mmol/l K+ was only slightly decreased. In these c ells: 1) nutrient metabolism assessed by tetrazolium salt reduction wa s reduced in response to pyruvate and leucine, which are mainly metabo lized in the mitochondria; 2) oxidation of both [3,4-C-14]glucose and [1-C-14]pyruvate was decreased (-22 and -32%, respectively); 3) glucos e failed to depolarize the membrane potential, whereas tolbutamide was fully active; 4) video imaging analysis of cytosolic Ca2+ showed a de crease in the population of glucose-responsive cells, while the respon se to 30 mmol/l K+ was preserved; 5) serum replenishment for 3 days re stored glucose-induced insulin secretion. Interestingly, conditioned s erum-free medium from rat islets maintained the insulin secretory func tion of INS-1 cells, although glucagon, somatostatin, and some other f actors failed to restore the function. In contrast, conditioned media from HepG2, PC12, and human umbilical vein endothelial cells did not s ubstitute for serum. Thus, the impaired insulin secretion of the cells cultured in the defined medium is best explained by defective mitocho ndrial metabolism. Islet cells, but not INS-1 cells, produce factors r equired for normal signal generation by nutrient secretagogues.