Glutamate transport regulation of renal glutaminase flux in vivo

We proposed that glutamate transport into cultured kidney cells represses cellular glutaminase activity and hence regulates glutamine utilization. To test this putative regulatory mechanism in vivo, glutamine uptake and conversion to glutamate as well as ammonium production were measured in the intact functioning rat kidney. Glutamine uptake was determined as net removal, arteriovenous concentration difference times renal plasma flow, and also as unidirectional uptake from the fractional extraction of tracer L-[C-14]glutamine. Ammonium production was measured as that released into the renal vein plus that excreted, and intracellular glutamine conversion to glutamate was assessed from the rise in cortical glutamate radiolabel specific activity. Cellular glutamate content was reduced 50-60% by infusing D-aspartate (a high-affinity glutamate transporter inhibitor) over 30 min, consistent with interdiction of glutamate uptake. This reduction in the glutaminase repressor was associated with a three-to fivefold increase in glutamine uptake and intracellular conversion to glutamate and ammonium. These results are consistent with and predictable from our previous in vitro model and point to an important role for this regulatory mechanism in the intact functioning organ.