Serum albumin is the major soluble protein constituent of blood serum. The biosynthsis and secretion of albumin is, in terms of mass, the principal endocrine protein secretory event in the bodies of almost all animals. To date there is no evidence to suggest that either the processes of synthesis or secretion are regulated other than by gene transcription.

Male Sprague-Dawley rats (approximately 250 g) were anaesthetised with halothane and then killed humanely (in accordance with both local and national guidelines). After cannulation of the hepatic portal vein the liver was perfused with Ringer solution containing 2 mM EGTA. The tissue was cut into small pieces passed though a sieve and the hepatocytes recovered on a discontinuous Percol gradient. Live cells (> 95 % viable by dye exclusion) were cultured overnight in Williams E medium supplemented with insulin and serum in collagen-coated plastic dishes.

For the experiments albumin was radiolabelled with methionine using a ‘pulse-chase’ protocol. Secretion of the newly synthesised albumin was allowed to proceed for 30 min at 37 °C. Albumin and its immediate metabolic precursor proalbumin, were recovered for the supernatant and the cells by immuno-precipitation and quantitated by phosphoimaging. All data were analysed with paired two-tailed t tests, allowing for unequal variance, and are shown with S.E.M. Data are expressed as the percentage of total newly synthesised albumin secreted in 30 min.

Firstly, we found that normally 51 ± 2 % (n = 25) of the newly synthesised albumin was secreted. Secondly, the treatment of the cells with several drugs caused severe disruption of albumin secretion. The inhibitors of Golgi body function, Brefeldin A (10 µg ml^-1) and monensin (3 µg ml^-1), were used as positive controls and showed very strong inhibition of albumin secretion (reduced to 5 ± 4 and 7 ± 5 %, respectively, n = 5 each). This is as expected. In addition, broad-spectrum inhibitors of protein tyrosine kinases (genistein, 100 µM) and phosphatases (vanadium peroxide, 100 µM) both caused strong inhibition (reduced to 21 ± 8 and 20 ± 5 %, respectively, n = 4 each). Lastly, microscopic examination of the Golgi indicated that the same drugs caused clearly observable changes in the subcellular distribution of Golgi-resident proteins.

We conclude that at least one protein tyrosine kinase and phosphatase play significant roles in the secretion of albumin from rat hepatocytes.