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 HCO(3-)-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 A1 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 approximately 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.