Response of LLC-PK1-F+ cells to metabolic acidosis

The role of extracellular glutamate formation as opposed to cellular glutamate removal in regulating monolayer glutamate content in response to metabolic acidosis was studied in LLC-PK1-F+ cells. Exposure to metabolic acidosis (14 mM bicarbonate; pH 7.1) for 18 h resulted in a 24% fall in monolayer glutamate content. Of this, approximately one-half could be attributed to enhanced glutamate removal via glutamate dehydrogenase, consistent with a rise in ammonium production. The remainder appears due to reduced extracellular glutamate formation as a consequence of diminished gamma-glutamyltranspeptidase (gamma-Gt) activity. Metabolic acidosis, but not respiratory acidosis, resulted in a 33% fall in gamma-Gt activity and a proportional fall in extracellular glutamate formation; glutamate transport into these cells was not rate limiting in acidosis. Overall glutamine utilization decreased 36%, reflecting the fall in gamma-Gt activity as well as a decrease in a pi-I-sensitive glutamine uptake, whereas glutamine transport coupled to the phosphate-dependent glutaminase flux increased. It is noteworthy that the increased ammonium produced in metabolic acidosis was preferentially secreted into the apical compartment; acid secretion, but not production, was similarly increased. Thus reduced cellular glutamate appears to coordinate activation of intracellular glutaminase to the apical membrane exchanger, consistent with the functioning kidney response to metabolic acidosis.