Glutamine metabolism in the kidney during induction of, and recovery from, metabolic acidosis in the rat

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Abstract
Experiments were carried out on rats to evaluate the possible regulatory roles of renal glutaminase activity, mitochondrial permeability to glutamine, phosphoenolpyruvate carboxykinase activity and systemic acid-base changes in the control of renal ammonia (NH3 plus NH4+) production. Acidosis was induced by drinking NH4Cl solution ad libitum. A pronounced metabolic acidosis without respiratory compensation [pH = 7.25; HCO3- = 16.9 meq/l; pCO2 = 40.7 mmHg (5.41 kPa)] was evident for the first 2 days, but thereafter acid-base status returned towards normal. This improvement in acid-base status was accompanied by the attainment of maximal rates of ammonia excretion (onset phase) after about 2 days. A steady rate of ammonia excretion was then maintained (plateau phase) until the rats were supplied with tap water in place of the NH4Cl solution, whereupon pCO2 and HCO3- became elevated [55.4 mmHg (7.37 kPa) and 35.5 meq/l] and renal ammonia excretion returned to control values within 1 day (recovery phase). Renal arteriovenous differences for glutamine always paralleled rates of ammonia excretion. Phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase activities and the rate of glutamine metabolism (NH3 production and O2 consumption) by isolated kidney mitochondria all increased during the onset phase. The increases in glutaminase and in mitochondrial metabolism continued into the plateau phase, whereas the increase in the carboxykinase reached a plateau at the same time as ammonia excretion. During the recovery phase a rapid decrease in carboxykinase activity accompanied the decrease in ammonia excretion, whereas glutaminase and mitochondrial glutamine metabolism in vitro remained elevated. The metabolism of glutamine by kidney-cortex slices (ammonia, glutamate and glucose production) paralleled the metabolism of glutamine in vivo during recovery, i.e., it returned to control values. The adaptations in mitochondrial glutamine metabolism must be regulated by extra-mitochondrial factors, since glutamine metabolism in vivo and in slices returned to control values during recovery, whereas the mitochondrial metabolism of glutamine remains elevated.