Abstract
At least seven functionally distinct nucleoside transport processes exist; however, mouse leukemic L1210/MA27.1 cells possess only one subtype, a Na+-dependent transporter termed N1/cif. The capacity of this transporter subtype to release nucleosides from L1210/MA27.1 cells was investigated with the poorly metabolized inosine analog [3H]formycin B. Uptake of [3H]formycin B into these cells was inhibited by replacement of Na+ in the buffer with choline, or by blocking Na+/K+ ATPase with 2 mM ouabain, inhibiting glycolysis with 5 mM iodoacetic acid or inhibiting nucleoside transport with 1 mM phloridzin. Sodium stimulated uptake with an EC50 value of 12 mM. To measure release of [3H]formycin B, cells were loaded with [3H]formycin B (10 μM) then washed and resuspended in buffer. Replacement of Na+ in the buffer with choline enhanced [3H]formycin B release by 20 to 47%, and significant stimulation of release was observed with Na+concentrations of 30 mM or less. Resuspending loaded cells into Na+ buffer containing 2 mM ouabain or 10 μM monensin, a Na+ ionophore, significantly enhanced [3H]formycin B release during 20 min by 39% or 29%, respectively. Release of [3H]formycin B into choline buffer was inhibited 26.5% by 10 mM phloridzin and 39.6% by 10 mM propentofylline, compounds known to inhibit various transporters including Na+-dependent nucleoside transporters. Release was also inhibited significantly by 100 μM concentrations of dilazep, dipyridamole and nitrobenzylthioinosine, inhibitors with selectivity for Na+-independent nucleoside transporters. In the absence of Na+, the permeants adenosine and uridine enhanced [3H]formycin B release by up to 40.9% and 21.4%, respectively. These data indicate that in the absence of an inwardly directed Na+ gradient, Na+-dependent nucleoside transporters can function in the release of nucleosides.
Footnotes
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Send reprint requests to: Dr. F.E. Parkinson, Department of Pharmacology and Therapeutics, University of Manitoba, 770 Bannatyne Avenue, Winnipeg, Canada R3E 0W3.
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↵1 This work was supported by the Medical Research Council of Canada (MRCC). F.E.P. is a Scholar of the MRCC.
- Abbreviations:
- NBMPR
- nitrobenzylthioinosine or nitrobenzylmercaptopurine riboside
- Received August 26, 1996.
- Accepted December 16, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
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