OBJECTIVE: Peritoneal mesothelial cells (PMC) are exposed to a hypertonic environment during peritoneal dialysis. When exposed to a hypertonic medium, many types of cells accumulate small osmotically active organic solutes, which are called osmolytes, to match the higher external osmolality. However, no information has been available concerning the osmolytes in PMC. To investigate osmoregulation in rat PMC, the levels of amino acids in the cells and the activity of system A, a major neutral amino acid transport, were measured after switching to a medium made hypertonic by the addition of NaCl. System A was measured by Na+-dependent [14C]-2-methylamino-isobutyric acid (MeAIB) uptake. RESULTS: Total amount of 20 amino acids increased from 306 to 757 nmol/mg protein after 12 hours of hypertonicity. The amount of neutral amino acids accounted for 81% of the increase in total amino acids. Glutamine, alanine, glycine, threonine, and serine were the major neutral amino acids that accumulated in the hypertonic mesothelial cells. The amount of neutral amino acids increased 2.9-fold after 12 hr of hypertonicity, and decreased thereafter. MeAIB uptake increased 36-fold relative to the uptake in isotonic cells after 4-8 hr of hypertonicity. When the culture medium was made hypertonic by adding raffinose or glucose, the activity of system A was also stimulated (raffinose > glucose > NaCl). System A was located on both the apical and basal sides of isotonic PMC, and extracellular hypertonicity stimulated the MeAIB uptake on both sides. CONCLUSIONS: These data indicate that neutral amino acids and system A transport play an important role in early-phase osmoregulation in rat peritoneal mesothelial cells.

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