α-Aminoisobutyric acid transport in isolated rat submandibular salivary acinar cells

Abstract

Na +-dependent α-aminoisobutyric acid (AIB) transport by isolated submandibular cell aggregates (pmol min −1 mg protein −1) was greater in the presence than in the absence of insulin, V max (5220 compared with 2900). K m (1.78 and 1.40 mM, respectively) was unaffected by insulin. Na +-dependent methyl-aminoisobutyric acid (MeAIB) transport was also greater in the presence of insulin ( V max, 3120 compared with 2010 pmol min −1 mg protein −1; K m, 1.03 and 0.93 mM). In the presence of 10 mM MeAIB, Na +-dependent AIB transport was reduced to 76 pmol min −1 mg protein −1 in both control and insulin-treated cells. The remaining Na +-dependent uptake of AIB was inhibited by 10 mM serine. Na +-independent AIB transport was unaffected by insulin, and in the presence of 5mM 2-aminobicyclo-[2,2,1]-heptane-carboxylic acid (BCH) AIB uptake was reduced to 10% of that observed under Na +-replete conditions. In the absence of insulin, the rate of Na +-dependent AIB uptake rapidly decayed; however, following the addition of hormone the rate of transport was maintained. Thus in the rat submandibular gland AIB uptake is mediated by at least three transport systems (A, L and ASC), and maintenance of normal system A activity requires insulin.