COORDINATE INDUCTION OF NA-DEPENDENT TRANSPORT-SYSTEMS AND NA+,K+-ATPASE IN THE LIVER OF OBESE ZUCKER RATS()

Solute uptake into liver plasma membrane vesicles from either lean or obese Zucker rats was monitored. D-Glucose and L-leucine uptakes at ph ysiological concentrations of the substrate were not different in lean and obese Zucker rats. In agreement with a previous report (Ruiz et a l. (1991) Biochem. J. 280, 367-372) L-alanine uptake was significantly enhanced in those preparations from obese animals. Na+-coupled uridin e transport was markedly enhanced also in obese rats. The effect was d ue to an increase in V-max (5.5 +/- 0.6 vs. 2.1 +/- 0.2 pmol/mg protei n per 3 s, P < 0.01) without any significant change in K-m (11.0 +/- 2 .8 vs. 9.0 +/- 2.7 mu M for obese and lean rats, respectively). Na+,K-ATPase activity was also higher in liver plasma membrane vesicles fro m rat liver and it correlated with a higher amount of alpha(1)-subunit protein in both, plasma membrane vesicles and homogenates from obese rat livers. In summary, in the hypertrophic liver of obese Zucker rats a coordinate induction of several Na+-dependent transport systems occ urs and, in order to sustain the metabolic pressure associated with th is adaptation, a significant induction of the Na+,K+-ATPase expression is also found. These data also provide new evidence for regulation of the recently characterized Na+-dependent nucleoside transporter.