Starvation-induced increase of hepatic alanine uptake is related to changes in sensitivity to SH-group reagents

Changes in L-alanine transport in plasma membrane vesicles from livers of control and 24- and 48-h starved adult rats and the sensitivity of alanine uptake to sulfhydryl group reagents [N-ethylmaleimide (NEM) and p-chloromercuribenzenesulfonate (p-CMBS)] were studied. The portal concentration of certain amino acids was measured, and the relationship between L-alanine transport kinetic parameters and amino acid levels was analyzed. Starvation only induced a decrease in portal concentration of these amino acids that are mainly carried by Na(+)-dependent systems (85 and 61% for 24- and 48-h starved rats, respectively). Portal alanine concentration was lower in 24-h starved animals than in control rats (370 vs. 587 microM) and further decreased after 48 h of fasting (228 microM). Starvation induced an increase in maximum velocity (Vmax) values of Na(+)-dependent L-alanine transport (7.19, 8.97, and 12.38 pmol.U 5'-nucleotidase-1.10 s-1 for control and 24- and 48-h starved rats, respectively) with slight, but not significant, changes in the apparent Michaelis constant (Km) values (3.35, 2.63, and 2.20 mM for control and 24- and 48-h starved rats, respectively). Portal alanine showed a directly close correlation with Km values and inverse with Vmax values. The mean affinity constant values for the effects of NEM and p-CMBS on Na(+)-dependent L-alanine transport were lower in 48- (2.57 and 0.13 mM, respectively) and 24-h starved rats (3.59 and 0.32 mM, respectively) than in control rats (8.56 and 0.59 mM, respectively) and showed a directly strong correlation with kinetic characteristics of L-alanine transport and portal alanine concentration.(ABSTRACT TRUNCATED AT 250 WORDS)