The loop diuretics are weak organic acids which cause ototoxicity in humans as well as in experimental animal models. Unfortunately, the molecular basis for these ototoxic effects has not yet been elucidated. Recent experimental findings have shown that certain organic acid compounds can reduce the cochlear effects of the sulfamoyl benzoic acid loop diuretic furosemide, but not those caused by the phenoxyacetic acid derivative ethacrynic acid. These two chemical classes of loop diuretics interact with renal receptors in different ways. The proposed studies will extend these findings to study additional members of these two loop diuretic chemical classes --piretanide, which is closely related to furosemide, and indacrinone, which is a phenoxyacetic acid derivative. The proposed studies will examine whether organic acid transport inhibitors reduce the ototoxicity or piretanide and indacrinone. The specific eletrophysiologic studies will measure changes in endocochlear potential, eighth nerve action potential and intracellular potentials recorded from marginal cells. Effects on in transport will be assessed by measuring concentrations and changes of ion conductances and permeabilities of the cochlear duct, using ion-sensitive microelectrodes. Correlation of these findings with ultrastructural stereologic studies of the volume density of each cell layer of the stria vascularis to further clarify the sites of action on the stria will be made. The blood levels of loop diuretics will be measured by high pressure liquid chromatography to determine whether the concentrations of diuretics in blood are altered by organic acids which reduce cochlear toxicity of the loop diuretic. The diuretic effects in control and experimental animals will be evaluated to determine whether protection against ototoxicity has an effect on the diuretic response by the kidney. Recent intracellular studies of the stria vascularis have provided startling new data which mandate re-evaluation of models of normal ion transport in the cochlea and the effects of inhibitors such as loop diuretics. The combination of the study of the effects of loop diuretics on the marginal cell potentials will provide new information on the cellular locus of action of loop diuretics. The correlation of the ultrastructural volume density analysis of the cellular components of the stria vascularis with the intracellular studies will provide novel insights into the mechanisms of loop diuretic ototoxicity.