CHARACTERISTICS OF THIAMIN TRANSPORT IN THE ISOLATED-PERFUSED GUINEA-PIG HEART

The cellular uptake of (C-14)-thiamin hydrochloride was studied in the isolated perfused guinea pig heart, using the rapid single circulatio n, paired-tracer technique, in which D-(H-3)-mannitol serves as an ext racellular marker. Cellular uptake of this vitamin was estimated by di rectly comparing venous dilution profiles of (C-14) and (H-3) radioact ivities in the absence and presence of unlabelled thiamin hydrochlorid e and pyrithiamin hydrobromide. The maximal cellular uptake (U-max) Of thiamin was very low (5.31 +/- 1.79%), while in the presence of 10 mM unlabelled thiamin and 1 mM pyrithiamin, U-max was significantly grea ter (9.71 +/- 1.57% and 12.30 +/- 0.82%, respectively). Our data sugge st that there is a saturable mechanism of sarcolemmal thiamin transpor t out of myocardial cell, while this transport into the cell is unsatu rable.