Local Re-Utilization of Thymidine In Normal Mouse Tissues As Measured With Iododeoxyuridine

Abstract. Thymidine (TdR) and its analogue, iododeoxyuridine (IdUdr), were used to quantitate nucleoside re-utilisation in vivo. Significantly different results are obtained, however, depending upon what form of isotopically labelled iododeoxyuridine is used. No measurable local thymidine re-utilization was found in mouse thymus, spleen or bone marrow when the retention of ³H]IdUdR was compared with ¹⁴C]TdR. On the other hand, significant differences were found between the retention of ¹²⁵I]IdUdR and ³H]IdUdR, which is attributed to de-iodination of iododeoxyuridine. Some thymidine re-utilization was found in duodenum using both ³H]IdUdR and ¹²⁵I]IdUdR. Information on the in vivo distribution of TdR and the contention that a large degree of TdR re-utilization in the thymus is evidence of extensive cell death must be re-interpreted in the light of these results. In addition, evidence for little or no local re-utilization in some tissues will greatly simplify the use of ¹¹C]TdR as an imaging agent for measuring tissue proliferation in vivo with positron emission tomography (PET).