COMPARISON OF THE BUFFER CAPACITY OF ENDOCYTOTIC VESICLES, LYSOSOMES AND CYTOPLASM IN CELLS DERIVED FROM THE PROXIMAL TUBULE OF THE KIDNEY (OPOSSUM KIDNEY-CELLS)

During transepithelial acid-base transport cells of the proximal tubule of the kidney have to maintain a relative constant intracellular pH. Herein cellular buffer capacity plays an important role. We measured vesicular buffer capacity in proximal tubule-derived opossum kidney cells and compared it with cytoplasmic buffer capacity to determine the possible importance of vesicular buffer capacity for cellular pH homeostasis. Under HCO3--free conditions endocytotic vesicular buffer capacity was 43+/-4 mmol.l(-1).Delta pH-unit(-1) (n = 7) and exceeded cytoplasmic buffer capacity (19+/-3 mmol.l(-1) .Delta pH-unit(-1); n = 7) significantly. Lysosomal buffer capacity was 19+/-6 mmol.l(-1).Delta pH-unit(-1); (n = 5). Inhibition of vesicular H+-ATPase using bafilomycin A(1) led to a dramatic increase of vesicular pH but to a decrease of cytoplasmic pH indicating the importance of organellar buffer systems. We estimated that endocytotic buffer capacity accounts for similar to 23% of cellular buffer capacity under our experimental condition and thus, impairment of endosomal acidification may affect cytoplasmic pH indeed. From our results we conclude that endocytotic vesicles have a large buffer capacity and might play a role in cellular pH homeostasis.