The endolymph in the endolymphatic sac (ES) is acidic (pH 6.6-7). Maintaining this acidic lumen is believed to be important for the normal function of the ES. The acid-base regulation mechanisms of the ES are unknown. Here we investigated the expression patterns of acid-base regulators, including vacuolar (v)H+-ATPase (proton pump), carbonic anhydrase (CA) II, and pendrin in the murine ES epithelium by immunohistochemistry (IHC) and compared their expression patterns by double immunostaining. We found that pendrin and vH+-ATPase were co-localized in the apical membrane of a specific type of ES epithelial cell. Pendrin- and vH+-ATPase-positive cells also expressed cytoplasmic CA II. Co-expression of pendrin, vH+-ATPase, and CA II in the same subgroup of ES cells suggests that this specific type of ES cell is responsible for the acid-base balance processes in the ES and pendrin, vH+-ATPase, and CA II are involved in these processes.

1. Animal kidneys are exquisitely sensitive to the vasoconstrictor and antinatriuretic effects of the endogenous vascular peptide endothelin. Animal studies have implicated endothelin in cyclosporin A and ischaemia-mediated renal damage. 2. In man, endothelin levels are raised in various disorders. Orally active endothelin antagonists are now being developed, but little was known of endothelin's role as a renal peptide in humans. These studies therefore aimed to characterize endothelin peptides and receptors ETA and ETB in human kidney, to direct potential therapeutic endeavours. 3. Ligand binding, immunocytochemical, radioimmunoassay and molecular biological studies were used to establish endothelin as a renal peptide in man. 4. The identification of species differences between man and rat directed further development of quantitative molecular biological methodology to permit analysis of endothelin receptors in human renal biopsies, and demonstrated perturbation of the system in the context of cyclosporin A therapy in renal transplantation.

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