THE UPTAKE OF MN-DPDP BY HEPATOCYTES IS NOT MEDIATED BY THE FACILITATED TRANSPORT OF PYRIDOXINE

Manganese-dipyridoxal diphosphate (Mn-DPDP) is a liver-selective contr ast agent selectively taken up by the hepatocytes. Because of the anal ogy of structure with pyridoxine (vitamin B-6), it was previously sugg ested that this compound can be selectively taken up by the facilitate d transport of vitamers B-6. To understand the uptake mechanism, an in vivo binding study was performed based on a competition between Mn-54 -DPDP and pyridoxine on the one hand, and Mn-DPDP and [H-3]pyridoxine on the other. We found that the [H-3] pyridoxine levels in the liver w ere not significantly different 5 min after intravenous administration of several doses of Mn-DPDP (5 nmol/kg to 50 mu mol/kg): 5.0 +/- 0.3% of the injected dose/g tissue, The content of Mn-54 (administered as Mn-54-DPDP) in the liver was not affected by a saturation dose of pyri doxine (1 mmol/kg) and was found to be constant (+/- 10% of the inject ed dose/g tissue) for 60 min, These experiments showed that the uptake of Mn-DPDP is not mediated by the transporter of pyridoxine. Copyrigh t (C) 1996 Elsevier Science Inc.