Nervous tissue thiamine metabolism in vivo. I. Transport of thiamine and thiamine monophosphate from plasma to different brain regions of the rat.

The transport of thiamine (T) and thiamine monophosphate (TMP) across the blood-brain barrier was measured in vivo in the rat. Different doses of [14C]T (15-550 nmol) and [14C]TMP (11-110 nmol) were injected into the femoral vein. The content of T and its phosphoesters in blood and brain tissue (cerebellum, pons, medulla and cerebral cortex) 20 s after the injection was determined radiometrically after electrophoretic separation. Blood flow and blood volume in the same regions of the brain was also determined. Both T and TMP entered rapidly the cerebral tissue, where they were found chemically unmodified. The cerebral tissue extracted less than 7% of plasma T. At physiological plasma T concentrations, the rate of transport ranged from 0.43 to 0.65 nmol X g-1 X h-1 with only minor differences among the various regions. T was transported into the nervous tissue by two separate mechanisms: one saturable, that at physiological plasma T levels accounted for 95% (cerebellum) to 91% (cerebral cortex) of the total T taken up, and one non-saturable, that was most efficient in the cerebral cortex. The Km (half-saturation constant) of the former transport mechanism ranged from 1.95 to 2.75 nmol X ml-1 in the 4 areas investigated. Vmax (maximal transport rate) values ranged from 6 to 9 nmol X g-1 X h-1, the highest value being found in the cerebellum. The overall transport rate of TMP was on average 5-10 times as low as that of T and also showed a saturable and a non-saturable component. Both components were slower than those observed for T.