In the perfused rat liver, proteolysis is inhibited by cell swelling in response to hypo-osmotic media, glutamine and insulin. Colchicine, an inhibitor of microtubules, did not affect cell swelling in response to these agonists. However, the antiproteolytic action of these effectors was largely blunted in the presence of colchicine or the microtubule inhibitors colcemid and taxol. On the other hand, inhibition of proteolysis by phenylalanine, asparagine or NH4Cl, i.e. compounds which exert their antiproteolytic effects by mechanisms distinct from cell swelling, was not sensitive to colchicine. Swelling-induced inhibition of proteolysis was not affected by cytochalasin B. The anti-proteolytic effect of hypo-osmotic cell swelling and insulin was largely abolished in freshly isolated rat hepatocytes; however, it reappeared upon cultivation of the hepatocytes for 6-10 h. The restoration of the sensitivity of proteolysis to cell volume changes was accompanied by a progressive reorganization of microtubule structures, as shown by immunohistochemical staining for tubulin. It is concluded that intact microtubules are required for the control of proteolysis by cell volume, but not for the control of proteolysis by phenylalanine, asparagine or NH4Cl. These findings may explain why others [Meijer, Gustafson, Luiken, Blommaart, Caro, Van Woerkom, Spronk and Boon (1993) Eur. J. Biochem. 215, 449-454] failed to detect an antiproteolytic effect of hypo-osmotic exposure of freshly isolated hepatocytes. This effect, however, which is consistently found in the intact perfused rat liver, also reappeared in isolated hepatocytes when they were allowed to reorganize their microtubular structures in culture.
NH4+ metabolism and the intracellular pH in isolated perfused rat liver
