Coordinate induction of Na +-dependent transport systems and Na +,K +-ATPase in the liver of obese Zucker rats

Abstract

Solute uptake into liver plasma membrane vesicles from either lean or obese Zucker rats was monitored. d-Glucose and l-leucine uptakes at physiological concentrations of the substrate were not different in lean and obese Zucker rats. In agreement with a previous report (Ruiz et al. (1991) Biochem. J. 280, 367–372) l-alanine uptake was significantly enhanced in those preparations from obese animals. Na +-coupled uridine transport was markedly enhanced also in obese rats. The effect was due to an increase in V max ( 5.5 ± 0.6 vs. 2.1 ± 0.2 pmol/mg protein per 3 s, P < 0.01) without any significant change in K m ( 11.0 ± 2.8 vs. 9.0 ± 2.7 μM for obese and lean rats, respectively). Na +,K +-ATPase activity was also higher in liver plasma membrane vesicles from rat liver and it correlated with a higher amount of α 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 occurs and, in order to sustain the metabolic pressure associated with this 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.