Methionine-induced acidosis in the weanling rat

Whole body balances of non-metabolizable base (NB) were studied in weanling rats fed a cereal-based diet with or without added L-methionine. In response to L-methionine loading (2.5 mmol day-1) the rats exhibited a significant reduction in rates of food intake (109 vs. 160 g per 8 days) and body growth (3 vs. 52 g per 8 days); fractional oxidation of absorbed dietary amino acid sulphur increased from 0.28 to 0.64; and mean urinary sulphate excretion increased from 2.3 to 14.8 mmol per 8 days. Mean rates of renal excretion of NB and filtered titratable organic acid decreased from 20 to -11 mmol per 8 days and from 22 to 8 mmol per 8 days. Balances of calcium and NB remained at reference values despite a decrease in whole blood 'base excess' from -1.0 to -6.4 mmol l-1. The concentration of NB in plasma rose but slightly. It is concluded that L-methionine loading in the weanling rat leads to extracellular non-carbonic acidosis subject to renal compensation. This acidosis is due not to retention of H+ released by ionization of endogenous sulphuric acid but possibly to accumulation of (acid) organic metabolites of methionine which are efficiently conserved by the kidneys. The rise in sulphuric acid production leads to an adaptive decrease in fractional reabsorption of filtered sulphate. Even during inhibited growth, absorbed dietary NB is retained and deposited in the skeleton, probably as calcium carbonate.