Methylamine and polyol responses to salt loading in renal inner medulla

Methylamines and polyhydric alcohols (polyols) are major organic osmolytes of the mammalian renal inner medulla and have generally been noted to change in parallel with urine osmolality. In the present study, responses of inner medullary methylamines and polyols to 5 days of salt loading were investigated. Salt loading increased plasma sodium concentration and induced a saline diuresis that resulted in a significantly lower urine osmolality (Uosmol) in salt-loaded rats (1,246 mosmol) compared with controls (2,147 mosmol). Analysis of inner medullary organic osmolytes using 1H-NMR spectroscopy and biochemical assays indicated no significant change in total methylamines, total polyols, or total osmolytes with salt loading. However, there were marked changes in individual organic osmolytes. Renal inner medullary glycerophosphorylcholine (GPC) was 41% lower in salt-loaded rats, and was the only organic osmolyte that changed in parallel with Uosmol, which was 42% lower in this group. In contrast, glycine betaine (betaine) and sorbitol contents were elevated by 286% and 33%, respectively, with salt loading, and myo-inositol (inositol) was unchanged. These findings indicate selective renal inner medullary osmolyte responses to salt loading with only GPC varying directly with changes in urine osmolality.