title: Cell position dependence of labelling thymidine nucleotides using the de novo and salvage pathways in the crypt of small intestine.
abstract: 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)
mesh_terms: D000818:Animals; D003856:Deoxyuracil Nucleotides / Q000378:metabolism; D007413:Intestinal Mucosa / Q000166:cytology* / Q000378:metabolism; D007421:Intestine, Small / Q000166:cytology / Q000378:metabolism*; D008297:Male; D008727:Methotrexate / Q000494:pharmacology; D051379:Mice; D013936:Thymidine / Q000378:metabolism*; D013937:Thymidine Kinase / Q000378:metabolism; D013939:Thymidine Phosphorylase / Q000378:metabolism
chemical_list: D003856:Deoxyuracil Nucleotides; D013939:Thymidine Phosphorylase; D013937:Thymidine Kinase; D013936:Thymidine; D008727:Methotrexate