Comparison of the buffer capacity of endocytotic vesicles, lysosomes and cytoplasm in cells derived from the proximal tubule of the kidney (opossum kidney cells)

Abstract

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·ΔpH-unit⁻¹ (n=7) and exceeded cytoplasmic buffer capacity (19±3 mmol·l⁻¹ ·ΔpH-unit⁻¹; n=7) significantly. Lysosomal buffer capacity was 19±6 mmol·l⁻¹·ΔpH-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 ∼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.