We have compared the effects of long-term exposure to L-glutamine or D-glucose on nutrient-induced insulin release from pancreatic islets of the rat. After 3 days of culture islets were finally tested in 1 h incubations for insulin responses to 16.7 mM of glucose, glutamine, leucine or a combination of leucine and glutamine. After culture at 11 mM glucose + 2 mM glutamine (A), glucose, leucine and glutamine stimulated release to a similar extent from islets. After culture at 1.7 mM glucose + 10 mM glutamine (B), only leucine stimulated insulin release. After culture at 11 mM glucose + 10 mM glutamine (C), both leucine and glutamine increased the insulin response. After culture at 1.7 mM glucose and 2 mM glutamine (D), only glutamine slightly stimulated release. After culture in high glutamine (B or C), a combination of leucine and glutamine significantly inhibited release as compared to leucine alone. A switch in culture media from B to A for 1 h prior to final incubations revived insulin release in response to glucose but not to glutamine. The reverse switch (A to B) abolished both subsequent glucose-and glutamine-induced insulin release. A switch from D to B revived an insulin response to leucine. Exposure of B-cells to 11 mM glucose during 30 min in another experimental system (perfused pancreas) induced a significant insulin response to subsequent stimulation with glutamine; this response was, however, only 17% of that to glucose per se observed in the same experiments.(ABSTRACT TRUNCATED AT 250 WORDS)