The mechanism by which leucine and arginine stimulate insulin release in vitro

Amino acid transporters in the regulation of insulin secretion and signalling

Amino acids are increasingly recognised as modulators of nutrient disposal, including their role in regulating blood glucose through interactions with insulin signalling. More recently, cellular membrane transporters of amino acids have been shown to form a pivotal part of this regulation as they are primarily responsible for controlling cellular and circulating amino acid concentrations. The availability of amino acids regulated by transporters can amplify insulin secretion and modulate insulin signalling in various tissues. In addition, insulin itself can regulate the expression of numerous amino acid transporters. This review focuses on amino acid transporters linked to the regulation of insulin secretion and signalling with a focus on those of the small intestine, pancreatic β-islet cells and insulin-responsive tissues, liver and skeletal muscle. We summarise the role of the amino acid transporter B⁰AT1 (SLC6A19) and peptide transporter PEPT1 (SLC15A1) in the modulation of global insulin signalling via the liver-secreted hormone fibroblast growth factor 21 (FGF21). The role of vesicular vGLUT (SLC17) and mitochondrial SLC25 transporters in providing glutamate for the potentiation of insulin secretion is covered. We also survey the roles SNAT (SLC38) family and LAT1 (SLC7A5) amino acid transporters play in the regulation of and by insulin in numerous affective tissues. We hypothesise the small intestine amino acid transporter B⁰AT1 represents a crucial nexus between insulin, FGF21 and incretin hormone signalling pathways. The aim is to give an integrated overview of the important role amino acid transporters have been found to play in insulin-regulated nutrient signalling.

Depression of leucine and isoproterenol induced insulin secretions in the spontaneously diabetic New Zealand white rabbit

Rabbits were studied from a closed colony of NZW rabbits which exhibits a 19% occurrance of spontaneous diabetes mellitus. Six overtly diabetic rabbits and eight rabbits with normal glucose disposal were tested with intravenous glucose challenge (500 mg/kg), L-leucine administration (125 mg/kg), and 30 minute infusions with isoproterenol (10 microgram/kg/min.). These agents were shown to be ineffective insulin secretogogues in the overtly diabetic group when compared to the highly significant IRI response observed in colony rabbits with normal glucose disposal. The data indicate that the defect in IRI secretion observed in the spontaneously diabetic NZW rabbits is not confined to stimulation by glucose, but represents an abnormal IRI release mechanism which appears to lack secretogogue specificity.

Insulin secretion during acid-base alterations

Insulin secretion under extracellular acid-base alterations (metabolic acidosis or alkalosis) was studied, by challenging in vitro perfused sodium pentobarbital-anesthetized-rat pancreases with glucose, arginine, and tolbutamide. Under our experimental conditions, the amount of insulin released was lower at pH 7.8 than the amount corresponding to the pH 7.4 control, in spite of the agent used to stimulate the pancreas. The effect of pH 7.0 on insulin secretion, however, depends on the type and concentration of the stimulus used. It enhances the secretion elicited by glucose (6.6 mM) and glucose plus arginine (6.6 and 10 mM, respectively). On the other hand, it reduces the beta cell response to glucose plus tolbutamide (3.3 mM and 400 microgram/ml, respectively), whereas the response to high glucose (16.6 mM) is reduced in the first phase and not affected in the second. According to these results, modifications of the extracellular pH, mainly at high levels, may interfere with a common process involved in insulin secretion, namely beta cell emiocytosis.

Urinary metabolites characteristic of urea-cycle amino acid deficiency

Experiments with 45-75-g male rats gave conclusive evidence that the simultaneous absence from the diet of arginine, ornithine, or citrulline caused an immediate and persistent elevation of orotic and citric acids in the urine. The experiments also demonstrated that a deficiency of no other individual amino acid increased urinary citrate and orotate. Elevated urinary excretion of orotic and citric acid occurred independently of the form of nonessential nitrogen. Replacement of arginine isonitrogenously with ornithine or citrulline prevented the rise in urinary orotic acid, but had different effects on growth, urinary citrate, and urinary urea. These differences were probably due to differential uptake of arginine, ornithine, and citrulline by tissues; In the reported experiments employing L-amino acids as sources of dietary nitrogen, a deficiency of any amino acid indispensable for growth and nitrogen balance or a deficiency of arginine, ornithine, or citrulline retarded growth, increased urinary urea, and decreased urinary ammonia. It is concluded that the severe loss of orotic acid during urea-cycle amino acid deficiency arises from a decreased capacity of the urea cycle to detoxify ammonia, thereby causing increased shunting of intramitochondrial carbamyl phosphate into pyrimidine synthesis. The similarities in metabolism during arginine deficiency and ammonia intoxication are discussed. The evidence shows that urinary orotic acid may be a valuable measure of arginine nutrition in mammals.

Paradoxical glucagon response after stimulation with glucose and arginine in isolated pancreatic sand rat islets

Isolated pancreatic islets of normoglycemic sand rats do not respond to 2.5 mM glucose with an enhanced glucagon secretion, which could be observed in normal Wistar rats. Arginine stimulates glucagon release in the presence of 2.5 mM glucose in Wistar rats as well as in sand rats. The secretion pattern is not caused by insulin deficiency since sand rat islets are characterized by an increased insulin secretion rate in vitro. This paradoxical glucagon secretion is not caused by a changed glucagon content but might be related to this species which is able to develop a diabetic syndrome spontaneously.