Patterns and putative regulatory mechanisms of high-affinity glutamate transporter expression by ruminants

High-affinity, highly concentrative, L-glutamate uptake is integral to support nitrogen and carbon homeostasis of many cell-types and tissue-level metabolic "cycles". Given the physiological costs associated with forestomach fermentation (high ammonia loads and/or low dietary-derived carbohydrates), knowledge about how system XAG transport capacity is achieved and regulated is of particular interest to ruminant physiologists. Accordingly, this review discusses glutamate transport activities (systems); system XAG transport proteins; the importance of system XAG transport capacity in support of liver, white adipose, and striated muscle tissue function; identified and putative regulatory mechanisms that control system XAG transporter expression and function, including whole-body and tissue regulation, transcriptional regulation of GLT-1 expression, coordinated expression and function relationships between GLT-1 and glutamine synthetase, posttranscriptional regulation of EAAC1 functional capacity; and modulation of cattle carcass quality and expression of system XAG transporters and glutamine synthetase by chlortetracycline, a compositional-gain altering "antibiotic." Research that has characterized the patterns of system XAG transporter expression in sheep and/or cattle, and alteration of basal patterns to support altered metabolic demands in response to growth, physiological development under a typical commercial regimen, and alterations to finished cattle in response to subtherapeutic feeding of chlortetracycline are presented and the ramifications discussed.