Real importance of alanine in renal metabolism: in vitro studies in rat and dog.

In vitro studies were performed on cortical renal tubules to clarify possible differences between dog and rat with regard to alanine production and to define more precisely the role of alanine on ammonia and glucose production by the kidney. It was established that glutamate-pyruvate transaminase has an activity that is seven times lower in the rat than in the dog kidney. At the same time, alanine production from lactate, pyruvate, and glutamate is three times lower in the rat than in the dog kidney. The enzymatic reaction could be completely inhibited in a competitive fashion with aminooxyacetate. O2 consumption and CO2 production by the renal tubules were lower than that observed with glutamine. CO2 production in the rat was lowest. Production of ammonia and glucose by the kidney from alanine during acidosis averaged less than 20% of that produced with L-glutamine. Furthermore, during metabolic acidosis the production of ammonia and glucose from alanine was not augmented and failed to be influenced by increasing the concentration of alanine in the incubation medium.

Net tubular secretion of bicarbonate by the alligator kidney. Antimammalian response to acetazolamide.

Net tubular secretion of bicarbonate by the alligator kidney was demonstrated during acute clearance experiments where the animals were infused with an isosmotic solution of one-half 5% mannitol and one-half 0.9% NaCl. Tubular secretion of bicarbonate averaged 3.38 mumoles/min in animals with a mean wt of 1.0 kg. During these experiments, mean tubular secretion of urate was 0.51 mumole/min and urinary ammonia excretion was 4.3 mumoles/min. Urinary pNH3 was high and ranged from 22,231 to 41,223 mm Hg X 10(-6). The administration of acetazolamide 25 mg/kg resulted in abolition of bicarbonate secretion, which was replaced by bicarbonate reabsorption. At the same time, the tubular secretion of urate fell by 70% and the excretion of ammonia fell by 77%. This is the first time that net tubular secretion of bicarbonate is demonstrated in a living animal. Acetazolamide has an antimammalian effect. It is proposed that the alligator that lives with a low plasma bicarbonate concentration (10 mM) possesses a kidney in which the renal tubular cells secrete bicarbonate in the tubular lumen and hydrogen at the peritubular site, in contrast to that which takes place in mammalia and other animal species.

Metabolic machinery of the alligator kidney.

Crocodilians such as caimans and alligators are uricotelic and ammoniotelic animals. They are carnivorous but they excrete ammonium ions in an alkaline urine. The metabolic organization of the kidney of the Mississippi alligator was studied by measuring the renal metabolite profile, the activities of enzymes, and the behavior of kidney tubules in vitro. The liver and tail muscle were also studied. Both awake and anesthetized animals were in a state of low plasma bicarbonate and low blood pH with high plasma lactate concentration. This did not prevent the excretion of an alkaline urine (pH 7.76). alpha-Ketoglutarate was low in all three tissues and lactate was high. Glutamate concentration and glutamate dehydrogenase activity were highest in the kidney with a low equilibrium constant for alanine aminotransferase (KGPT). Glutaminase I was found only in the kidney. It could not be detected in liver or muscle. Glutamine synthetase was found only in the liver. Phosphoenolpyruvate carboxykinase (PEPCK) was present in both liver and kidney. Alanine aminotransferase and malic enzyme showed high activity in the kidney but were inconspicuous in liver and muscle. Malate dehydrogenase and lactate dehydrogenase were present in all three tissues. Renal tubules incubated with glutamine and alanine were ammoniagenic and gluconeogenic. Lactate was gluconeogenic. Enzyme activities were measured at both 30 and 37 degrees C. The studies on renal tubules were also performed at these two temperatures. Temperature had little effect on the data including acid-base values in the blood. Our findings demonstrate that the kidney of the alligator is perfectly equipped for various metabolic functions and especially for ammoniagenesis and gluconeogenesis.