d-Alanine in the islets of Langerhans of rat pancreas

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.