Mechanisms of Time-Dependent Potentiation of Insulin Release

Abstract

Time-dependent potentiation (TDP) of insulin release is normally absent in mice. However, we recently demonstrated that TDP occurs in mouse islets under conditions of forced decrease of intracellular pH (pH i ) associated with elevated NADPH؉H ؉ (NADPH) levels. Hence, TDP in mouse islets may be kept suppressed by neuronal nitric oxide (NO) synthase (nNOS), an NADPH-utilizing enzyme with alkaline pH optimum. To determine the role of nNOS in the suppression of TDP in mouse islets, glucose-induced TDP was monitored in mouse islets in which nNOS activity had been genetically removed or chemically inhibited and compared with the TDP response in wild-type mouse islets with and without forced intracellular acidification. Genetic deletion of nNOS was provided by an nNOS knockout (NOS-KO) mouse model, B6 -129S4-Nos1tm1Plh /J. To explore how nNOS inhibits TDP, we compared pH i and NADPH levels in wild-type and NOS-KO islets and monitored TDP with various components of the nNOS reaction added. Glucose normally does not produce TDP in wildtype mouse islets except under forced intracellular acidification. Remarkably, glucose produced strong TDP in NOS-KO islets and in wild-type islets treated with nNOS inhibitors. TDP in NOS-KO islets was not inhibited by the addition of NO, and NOS-KO islets exhibited a lower pH i than wild-type islets. The addition of arginine to wild-type islets also enabled glucose to induce TDP. Our results show that nNOS activity contributes to the absence of TDP in mouse islets putatively through depletion of intracellular arginine.

Dimethyl amiloride improves glucose homeostasis in mouse models of type 2 diabetes

“…Glucose does not induce TDP in normal mouse islets, but it does so in the presence of DMA, nNOS inhibitors, or added arginine (32). However, this may not be the only mechanism, since the magnitude of glucose-induced TDP in the presence of DMA is much greater than that in the presence of nNOS inhibitors or arginine (32). To further explore any additional mechanisms, we tested the ability of selected nonglucose secretagogues to induce TDP in the presence of DMA or (L-NAME), an inhibitor of nNOS.…”

“…We also found remarkable effects of DMA on time-dependent potentiation (TDP) (29,(31)(32)(33), which is defined as the enhancement of the insulin response resulting from a memory induced by a previous exposure to certain secretagogues. Such secretagogues include glucose and other compounds such as glyceraldehydes, leucine, methyl pyruvate, ␣-ketoisocaproate (KIC), and 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), all of which enhance mitochondrial metabolism (22,25,30,43,(72)(73)(74).…”

“…We previously demonstrated that DMA can enable TDP to occur in mouse islets, where this function is normally absent (29,31). We also found that the positive effect of DMA on TDP is mediated, at least in part, through inhibition of neuronal nitric oxide synthase (nNOS), since this inhibition increases the amount of arginine available for other cellular functions (32). Therefore, DMA, nNOS inhibitors, and arginine, all have the potential to correct the secretory defect in type 2 diabetes (T2D) by enabling and/or enhancing TDP.…”

“…Whereas increasing the availability of arginine via nNOS inhibition seems to be a major mechanism of action of DMA (32), the pattern and magnitude of TDP by different secretagogues indicates the presence of other mechanisms. Since TDP is dependent on mitochondrial metabolism (30) but completely independent of Ca 2ϩ and K ϩ (33,67,68), these mechanisms are likely to be enzymatic rather than inotropic.…”

Modes of exocytosis and electrogenesis underlying canine biphasic insulin secretion

“…In the simplest formulation of the granule pool hypothesis, one pool of granules is docked to the plasma membrane and released first in response to stimulation, whereas a second reserve pool of granules in the cytoplasm is recruited to the plasma membrane for release with prolonged stimulation (6). Ongoing work, however, suggests that granules may exist in a wide range of functional states that differ in their relationship to the plasma membrane and to Ca 2+ channels, and vary in fusion competence and Ca 2+ sensitivity (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). A current challenge is to map these functional states defined from powerful reductionist approaches to physiologically relevant phases of insulin release elicited by glucose in the native islets.…”