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.

Dietary betaine supplementation has energy-sparing effects in feedlot cattle during summer, particularly in those without access to shade

“…This supports the findings of Bock et al (2004), who demonstrated that 40 g head/day of betaine supplemented to beef steers consuming pasture for 7 days pre-slaughter improved carcass weight, although this was not the case for steers supplemented with dietary betaine while in the feedlot. Betaine can reduce maintenance energy requirements (Schrama et al 2003) by reducing ion pumping associated with maintaining cellular osmolarity (Nakanishi et al 1990;Lohr et al 1991;Peterson et al 1992), and in cattle betaine is lipotropic and may increase fat thickness (Bock et al 2004;Loest et al 2002), as somewhat supported by the results of the present experiment in the P8 backfat responses to the 10 g dose. Thus, the trend for carcass weight to be increased in steers supplemented with betaine may be the result of increased fat thickness.…”

Organic osmolytes as compatible, metabolic and counteracting cytoprotectants in high osmolarity and other stresses

“…and trophosome (Troph., location of sulfide-oxidizing microbial symbionts), which also has high levels of thiotaurine; (6) mammalian renal cells (inner medulla) have varying levels of sorbitol, myo-inositol, GPC, betaine and taurine (along with urea). Data from Peterson et al (1992); Yin et al (2000); Yancey et al (2002); Fiess et al (2002); Rosenberg et al (2003).…”

“…As another example, the mammalian renal medulla appears to regulate one of its methylamine osmolytes, GPC, to maintain a constant ratio to urea, rather than for osmotic stress alone (Peterson et al, 1992). Again, if this methylamine is a simple compatible solute, why not use it at high levels under all water-stress conditions?…”

Co-evolution of proteins and solutions: protein adaptation versus cytoprotective micromolecules and their roles in marine organisms

“…Medullary cells, which often have high urea content as a result of the urinary concentrating mechanism, have close to a 2:1 ratio of urea to the methylamines glycerophosphorylcholine (GPC) and betaine ( Fig. 1), with GPC regulated in parallel with urea (Peterson et al, 1992). These methylamines can counteract the effects of urea on proteins (Burg et al, 1996).…”