Secretion and protein metabolism in the rat pancreas in vitro

Abstract

1. 1. Carbamylcholine (> 1 · 10 −6M), pancreozymin (> 0.1 unit/ml) or caerulein (> 2 ng/ml) inhibit the incorporation of L[4,5- 3H 2]leucine intoacid-insoluble proteins of the rat pancreas in vitro. Incorporation of other amino acids, L-[1 3H 2]-methionene and L-[3,5- 3H 2]tyrosine, is also inhibited. 2. 2. The incorporation of labeled leucine into proteins proceeds linearly for at least 2 h in the presence of 0.8 mM leucine but slows down in the absence of the carrier amino acid (26 μM leucine). In both cases, the incorporation is decreased by carbamylcholine immediately after 5 min incubation. 3. 3. During incubation in “leucine-free” (26 μM) medium, the intracellular leucine pool remains constant and its specific activity regularly but rapidly declines. 0.1 mM carbamylcholine increases both the pool size and its specific activity to a maximum after 30 min of incubation. In a medium containing leucine (800 μM), 0.1 mM carbamylcholine also increases the intracellular leucine pool as well as its specific activity. 4. 4. Glutamic acid, aspartic acid and alanine incorporated label from [1- 14C]-leucine; the initial slope of their specific activity curve during the incubation is steeper for the carbamylcholine-treated gland. 5. 5. The following conclusions are made: at low secretion rate there is independence of protein synthesis and the extrusion-secretion process. At high secretion rate there is: (a) an inhibition of protein synthesis at a later step than the availability of the precursor amino acid, and (b) an increase of leucine catabolism.