Cell Position Dependence of Labelling Thymidine Nucleotides Using the De Novo and Salvage Pathways In the Crypt of Small Intestine

About twice as much tritiated thymidine ([3H]TdR) is taken up by cells at the bottom of the crypt of the small intestine as by the rapidly cycling mid-crypt cells. However, the uptake of tritiated deoxyuridine ([3H]UdR) is even throughout the crypt. Exogenous thymidine is incorporated about four times and eight times more efficiently than deoxyuridine by the cells in the mid-crypt and cells at the bottom of the crypt, respectively. However all S phase cells in the crypt appear to be capable of using either precursors, i.e. either the de novo or salvage pathway. Since methotrexate (1 or 5 mg/kg) inhibits (at 5 mg/kg completely) the uptake of [3H]UdR, but has no effect on [3H]TdR uptake, the de novo and salvage pathways appear to be independent. Within the precision of the methods used in the experiments the 3 hr inhibition of the de novo pathway of deoxythymidylic acid (dTMP) synthesis by methotrexate does not produce any increase in utilization of the salvage pathway measured by incorporation of [3H]TdR into DNA. The increased efficiency of thymidine utilization by crypt base cells is not attributable to differences in accessibility of thymidine; differences in the rate of DNA synthesis or the size of the nuclei. It appears that crypt base cells (which include the putative stem cells) are efficient scavengers of [3H]TdR, and this might be related to the level of thymidine kinase activity within the cells, and/or to changes in the availability of endogenous thymidine (break-down products) which compete with exogenous [3H]TdR.(ABSTRACT TRUNCATED AT 250 WORDS)