Role of microtubules in the synthesis, conversion, and release of (pro)insulin. A biochemical and radioautographic study in rat islets.

In the pancreatic B cell, microtubules are thought to be involved in the process of insulin release. Their possible participation in the sequence of events leading from the biosynthesis and conversion of proinsulin to the release of newly synthesized insulin was investigated in rat isolated islets exposed to colchicine (0.1 mM). When the islets were preincubated for 30 min with colchicine and [3H]-leucine and, thereafter, incubated for two successive periods of 90 min each, still in the presence of colchicine, the release of preformed insulin was progressively inhibited and that of newly synthesized hormone delayed. When the islets were preincubated for 120 min with colchicine, subsequently pulse-labeled with [3H]leucine, and eventually examined by ultrastructural autoradiography, the export of newly synthesized proinsulin out of the rough endoplasmic reticulum, its transit through the Golgi complex, and its eventual packaging in secretory granules were all retarded. This situation was associated with a delayed conversion of proinsulin to insulin. Under the same experimental conditions, colchicine failed to affect the oxidation of glucose and adenylate charge in the islets. The effect of colchicine upon the release of preformed and newly synthesized insulin was not reproduced by lumicolchicine. It is concluded that colchicine interferes with the system controlling the intracellular transfer of secretory material from site of synthesis to site of release. This interference is likely to be linked to the effect of colchicine on microtubules.