Na(+)-independent L-alanine uptake by trout cells. Evidence for the existence of at least two functionally different acs systems.

Abstract

The Na(+)-independent uptake of L-alanine has been studied in trout red blood cells, isolated hepatocytes and peripheral blood lymphocytes. The present study shows the existence of two functionally different Na(+)-independent systems for short chain neutral amino acids in these cells. They are designated as asc systems because of their resemblance to systems described in other cell types. Besides their independence of sodium and a rough similarity in substrate preference, the most important property shared by the two carriers is a lack of trans-stimulation, allowing further differentiation from system L. One of them is an unusually stereospecific carrier present in red blood cells, the other is less restrictive and present in hepatocytes and peripheral blood lymphocytes. Extracellular acid pH increases the incorporation to red blood cells, while it slightly depresses the uptake in the other cells. From the data presented, it is not possible, at first, to classify these carriers as asc1 or asc2 systems. Moreover, the system present in red cells resembles that found in the nonerythroid cells, BSC-1, while there is no clear parallelism between the system found in hepatocytes/lymphocytes and any of those described previously.