Studies on the factors involved in the secretion of enzymic and non-enzymic contents of rat liver lysosomes
The aim of this present study has been to investigate the role of microfilaments in the secretion of lysosomal enzymes from rat liver cells and the effect, if any, on this process of changes in the intralysosomal environment. Using the systems of short-term rat hepatocyte cultures and isolated rat liver prefusions, the effect of the microfilament poison, cytochalasin B, on the secretion of the lysosomal enzymes aryIsulphatase A + B, B-galactosidase and, in some instances, beta-N-acetyl-glucos-aminidase has been examined. In addition, the release of the cytosol enzyme lactate dehydrogenase was monitored to determine non-specific enzyme release due to cell damage. The intralysosomal environment was modified by pre-loading the liver lysosomes in vivo with a variety of macromolecular materials, i.e. 125I-polyvinylpyrrolidone,H-dextran and 125I-Triton WR-1339. The effect of this lysosomal modificationon the release of the lysosomal enzymes and its modulation of the effect of microfilament disruption, as induced by cytochalasin B, as well as the effect of this disruption on the release of the preloaded materials, was determined. In addition, the intracellular distribution of these preloaded materials in the rat liver was investigated before, and in some cases after, the livers had been perfused. Attempts were made to correlate the heterogeneity of the density-gradient-centrifugation profiles of the preloaded, intralysosomal materials and those of the various lysosomal enzymes with their heterogeneous secretion patterns during perfusion in the presence and the absence of cytochalasin B. Preliminary attempts were made to delineate the contributions of secretion (exocytcsis) and reuptake (endo-cytosis) in the net "appearance" of both the enzymic and non-enzymic lysosomal contents in the perfusate. From the results obtained a hypothesis was formulated which suggested that the intralysosomal presence of non-enzymatic material, does indeed modulate not only the "base-line" secretion of lysosomal enzymes, but also the effect of microfilament disruption on this process. This modulation could involve modifications to the lysosomal membrane, such that it might fuse more readily with the plasma membrane.