Effects of dietary protein and salt on rat renal osmolytes: covariation in urea and GPC contents.

Abstract

Renal medullary cells contain high levels of (glycine) betaine, glycerophosphorylcholine (GPC), myo-inositol, and sorbitol. Two functions of these have been proposed: 1) that they are compatible osmolytes which regulate cell volume (against high external NaCl) without inhibiting proteins and 2) that methylamines (GPC and betaine) are counteracting osmolytes which stabilize proteins against perturbation from high renal urea. As a test of the latter, osmolyte contents in kidney medullas were measured in rats subjected to three types of dietary manipulation: 1) diets with protein and NaCl contents varied oppositely, 2) diets with a constant low NaCl and varied protein content, and 3) a low-calorie diet. With low-protein and low-calorie diets, only renal contents of urea, GPC, and inositol decreased; betaine and sorbitol contents increased such that contents of total nonurea organic osmolytes remained constant. With high-protein diets, only renal contents of sodium, urea, and GPC increased, with the latter giving total organic osmolytes a consistent correlation to sodium. Across all diets, the only consistent (linear) correlations were 1) between urea and GPC contents, supporting previous suggestions that GPC is the major counteractant to urea, and 2) between total organic osmolytes and sodium (but not urea) contents, as predicted by the compatible osmolytes hypothesis.