Regulation of thymocyte proliferation: effects of L-alanine, adenosine and cyclic AMP in vitro.

Thymocyte proliferation in culture was studied by measuring the mitotic activity at hourly intervals during two periods, 0-13 and 18-30 h after the start of incubation. In particular, we investigated the effect of increased levels of cyclic AMP which are generated during preparation of the cells due to influence of endogeneously released adenosine. This cycle AMP surge, which could be blocked by combined treatment with adenosine deaminase and theophylline, caused a transient inhibition of proliferation at a stage 4 h prior to mitosis, suggesting a cyclic AMP-sensitive step in that part of the cell cycle. The temporary rise of cyclic AMP did not trigger any resting cells to proliferate. Addition of adenosine deaminase plus theophylline stimulated mitotic activity in the 18-30 h interval, possibly by blocking the influence of continuously released adenosine. The growth of thymocytes was partly synchronous, which was independent of the early cyclic AMP peak. About half of the cycling cell population was shown to be critically alanine dependent. L-alanine deprivation inhibited these cells at a stage in the cell preparation, release of prostaglandins and changes in cell viability could not explain the synchronous growth. The selective elimination of late S- and G2-phase cells during preparation of thymocytes is offered as a possible explanation instead.