NEUTRAL AMINO ACID TRANSPORT IN SHEEP EXTERNAL INTERCOSTAL MUSCLE

The transport of two amino acid anologs, aminoisobutyric acid (AIB) and N-methylaminoisobutyric acid (MeAIB), was studied in external intercostal muscle bundles taken from 12 wether lambs to examine the significance of these transport systems in sheep muscle tissue. Competition for transport between AIB and other amino acids was evaluated to determine proportions of AIB transported on individual carriers. Rates of AIB transport into EIC muscle in relation to previous liveweight gains of lambs and animal weights were also examined. Aminoisobutyric acid was concentrated to approximately fourfold extracellular AIB concentrations via a sodium gradient-dependent process within 2 h of incubation. However, MeAIB was not concentrated by this tissue. Kinetic analyses of MeAIB transport indicated that its entry was entirely nonsaturable. Since MeAIB is a model substrate for transport system A (Na gradient-dependent, hormone and substrate concentration regulated) and since AIB may be transported by both systems A and ASC (Na gradient-dependent, hormone and substrate concentration insensitive), the active transport of AIB is attributed primarily to a system ASC-like agency. Kinetic analyses of the Na gradient-dependent transport of AIB revealed Jmax and Kt values of 2.48 ± 0.35 μmoles g tissue −1 30 min −1 and 11.4 ± 4.2 mM, respectively. Competition studies between AIB and other analogs provided further support for the existence of an ASC-like transport agency and the possible absence of or a repressed system A. A significant relationship between previous liveweight gain and amino acid transport was not detected; however, AIB transport in muscle taken from more mature animals was significantly lower than in muscle taken from younger animals. Key words: Sheep, muscle, amino acid transport, system A, system ASC