Relatively high levels of D-alanine (D-Ala), an endogenous D-amino acid, have been found in the endocrine systems of several animals, especially in the anterior pituitary; however, its functional importance remains largely unknown. We observed D-Ala in islets of Langerhans isolated from rat pancreas in significantly higher levels than in the anterior/intermediate pituitary; specifically, 180 +/- 60 fmol D-Ala per islet (300 +/- 100 nmol/g islet), and 10 +/- 2.5 nmol/g of wet tissue in pituitary. Additionally, 12 +/- 5% of the free Ala in the islets was in the D form, almost an order of magnitude higher than the percentage of D-Ala found in the pituitary. Surprisingly, glucose stimulation of the islets resulted in D-Ala release of 0.6 +/- 0.5 fmol per islet. As D-Ala is stored in islets and released in response to changes in extracellular glucose, D-Ala may have a hormonal role. (C) 2014 Elsevier Inc. All rights reserved.