Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K ؉ -ATP channels are the major site of regulation) or in a high K ؉ medium (when glucose modulates the effectiveness of cytosolic Ca 2؉ on exocytosis). Glucose induced a concentration-dependent (0 -20 mM) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4 -5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 mM glucose in control medium and between 0 and 20 mM glucose in high K ؉ medium. The increases in the GTP: GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.The regulation of pancreatic B cell function differs from that of other secretory cells in that the control of insulin secretion does not depend on the binding of the major physiological stimulator, glucose, to a receptor, but on its metabolism within B cells (reviewed in Refs. 1-5). It is now widely accepted that this metabolism generates several signals that close ATP-sensitive K ϩ channels (K ϩ -ATP channels) 1 in the plasma membrane. This closure (hereafter referred to as the "primary mechanism" of control) leads to membrane depolarization with subsequent opening of voltage-dependent Ca 2ϩ channels, Ca 2ϩ influx, rise in cytoplasmic free Ca 2ϩ concentration ([Ca 2ϩ ] i ), and eventual triggering of insulin secretion (4 -9). A "second mechanism" of control by glucose exists, which also depends on changes in metabolism (10 -12). It does not involve a further change in [Ca 2ϩ ] i but an increase in the effectiveness of Ca 2ϩ on its intracellular targets (12). Despite numerous studies, the nature of the signals that link the acceleration of metabolism to the closure of K ϩ -ATP channels and to the increase in Ca 2ϩ action has only partially been elucidated. Purine nucleotides clearly stand out as potential candidates.The popular hypothesis that variations in the cytosolic concentrations or ratio of adenine nucleotides are involved in the primary mechanism of control by glucose rests primarily on the fact that K ϩ channels, which control the B cell membrane potential, are regulated by intracellular ATP and ADP (6,8,9,13). Although this hypothesis has long been disputed (reviewed in Ref. 14), our demonstration of a correlation between insulin release and the ATP:ADP ratio in islets incubated in the presence of...