A series of 3H-leucine pulse-labeling experiments was carried out with dispersed cells freshly isolated from transplanted rat insulinomas. After secreted fractions were separated, insulin was purified and specific activities were determined for both secreted and average cellular insulins. Labeling patterns in this line of tumor cells were compared with those previously established for isolated rat islets. With both tumors and islets, conversion of labeled proinsulin to insulin occurred to the same extent by 2.5 h, suggesting similar onset and half-time of proteolysis in these cells. However, total cellular insulin in tumors attained a threefold higher specific activity than in islets. Because total B-cell mass in these tumors was unknown, either a more rapid proinsulin biosynthesis or diminished cellular storage (or both) could lead to this faster fractional replacement of total stored insulin. Insulin secretion in these tumor cells was insensitive to high glucose but responded, albeit poorly, to leucine plus 3-isobutyl-1-methylxanthine (IBMX). Under all secretory conditions tested, tumor cells continuously secreted insulin at elevated fractional rates, which were slightly higher than fractional insulin secretory rates in maximally glucose-stimulated islets. In contrast with normal islets, newly synthesized insulin was stored homogeneously in tumor cells, and compartmental storage characteristics were not generated by incubation with either 20 mM glucose or leucine plus IBMX in the marking period. Thus, preferential secretion of insulin was never observed in tumor cells. These altered labeling patterns in tumor cells suggested: (1) continuous, rapid proinsulin biosynthesis; (2) normal proinsulin-to-insulin proteolytic rates; (3) absence of marking by either glucose or a secretagogue of these tumor cells (leucine + IBMX); (4) loss of heterogeneous insulin storage characteristics; (5) diminished time of insulin storage before secretion, which probably reflects both loss of compartmental storage characteristics and diminished cellular storage capacity; and (6) continuous, elevated fractional rates of insulin secretion even with only low glucose concentrations present.

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