| Abstract | When isolated thymic lymphocytes were incubated with known mitotically stimulating concentrations (1.6 × 10−7 M and 1.6 × 10−6 M) of a mixture of cyclic AMP labeled with 32phosphorus and tritium, 0.5% of the phosphorus label and 3 to 6% of the tritium label entered the cells. None of the radioactivity from either isotope appearing in the cells was present as cyclic AMP. This difference between the entry of the two isotopes was explained by the observation that the cyclic AMP in the medium surrounding the cells was metabolized, and it was breakdown products of the cyclic nucleotide which entered the cells. It is concluded that cyclic AMP does not enter cells as such, and may exert its action on mitosis at the cell surface. |
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