Uptake and release of [3H]formycin B via sodium-dependent nucleoside transporters in mouse leukemic L1210/MA27.1 cells.
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 microM) 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 microM 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 microM 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.
Borgland
S L
SL
Department of Pharmacology and Therapeutics, University of Manitoba, Winnipeg, Canada.
Parkinson
F E
FE
D000255:Adenosine Triphosphate / Q000032:analysis; D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000493:pharmacokinetics*; D002352:Carrier Proteins / Q000502:physiology*; D005573:Formycins / Q000493:pharmacokinetics*; D007939:Leukemia L1210 / Q000378:metabolism; D008565:Membrane Proteins / Q000502:physiology*; D051379:Mice; D033703:Nucleoside Transport Proteins; D012964:Sodium / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology; D014407:Tumor Cells, Cultured
D000903:Antibiotics, Antineoplastic; D002352:Carrier Proteins; D005573:Formycins; D008565:Membrane Proteins; D033703:Nucleoside Transport Proteins; C034561:formycin B; D000255:Adenosine Triphosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase