High ascorbic acid content in the rat endocrine pancreas.

The peptidyl alpha-amidation of biologically active peptides (a number of which are found in the endocrine pancreas) requires several co-factors, including ascorbic acid. In the present study, tissue contents and developmental changes of ascorbic acid in the rat endocrine pancreas were measured using a highly sensitive HPLC system. High concentrations were found in neonatal rats, with the highest value, 42.5 nmol/mg protein, in 2-day-old rats. The concentration decreased gradually with age to 19.4 nmol/mg in 5-week-old rats. The exocrine pancreas had a lower concentration, but a peak was also observed in 2-day-old rats. In freshly isolated islet cells, an intracellular concentration of 7.5 mmol/l was estimated in 5-7-day-old rats. Secretory granules isolated from 4-6-day-old rat islets contained 10.6 nmol/mg protein. Culturing islets or cells in ascorbic acid free medium resulted in a marked decrease in their contents. Ascorbic acid in secretory granules from such islets decreased at a relatively lower rate. Addition of ascorbic acid to cultured cells or islets reduced the loss markedly. Increasing the glucose concentration in islet culture medium in the presence of 100 mumols/l ascorbic acid increased the islet ascorbic acid concentration.