Substrate oxidation by isolated single nephron segments of the rat.

Substrate oxidation by isolated single nephron segments of the rat. Substrate oxidation was assessed by measuring 14 CO 2 production from 14 C-labeled substrates in proximal convoluted tubules (PCT), medullary (MTAL), and cortical (CTAL) thick ascending limb of Henle, nephron segments rich in mitochondria and characterized by active solute transport. PCT, MTAL, and CTAL were dissected from the outer cortex, outer medulla, and the medullary rays of the cortex, respectively, of collagenase-treated rat kidney slices. Tubules were incubated at 37° C in 150 µl of Krebs-Ringer-bicarbonate buffer (pH, 7.4) with 14 C-labeled substrate. 14 CO 2 production was linear up to 4 and 2 hours in PCT and MTAL, respectively. Freeze-thawing of the tubules markedly decreased 14 CO 2 production, and the addition of cyanide completely abolished it. The PCT demonstrated marked 14 CO 2 production from labeled succinate, 2-oxoglutarate, glutamate, glutamine, and malate (approximately 10 to 45 pmoles/mm/hr) and moderate 14 CO 2 production from citrate (approximately 3 pmoles/mm/hr). Little 14 CO 2 was released from labeled glucose and lactate in PCT. These results are consistent with the existence of gluconeogenesis in this nephron segment. By contrast, MTAL and CTAL oxidized glucose, 2-oxoglutarate, lactate, glutamate, and glutamine, but not malate, succinate, and citrate. The pentose shunt pathway accounted for approximately half of the 14 CO 2 produced from 1- 14 C glucose in MTAL and CTAL. Palmitate oxidation occurred in MTAL and CTAL but minimally in PCT. The results demonstrate a distinct pattern of substrate oxidation in PCT, MTAL, and CTAL where oxidative metabolism is critical to support active solute transport. Oxydation de substrats par des segments uniques isoles de nephron du rat. L'oxydation du substrat a ete mesuree par la production de 14 CO 2 a partir de substrats marques par le 14 C dans le tube contourne proximal (PCT), la branche ascendante large de l'anse de Henle medullaire (MTAL) et corticale (CTAL), c'est a dire des segments riches en mitochondries et sieges de transports actifs. PCT, MTAL, et CTAL ont ete disseques a partir du cortex superficiel de la medullaire superficielle et des axes medullaires du cortex, respectivement, dans des reins de rat traites par la collagenase. Les tubules ont ete incubes a 37° C dans 150 µl de Krebs-Ringer-bicarbonate (pH, 7,4) avec des substrats marques par le 14 C. La production de 14 CO 2 a ete lineaire jusqu'a 4 et 2 heures respectivement dans PCT et MTAL. La congelation-decongelation des tubules a diminue de facon importante la production de 14 CO 2 qui a ete completement abolie par l'addition de cyanure. Le PCT a une production importante de 14 CO 2 a partir du succinate, du 2-oxoglutarate, du glutamate, de la glutamine et du malate (approx., 10–45 pmoles/mm/hr) et une production modeste de 14 CO 2 a partir du citrate (approx. 3 pmoles/mm/hr). Peu de 14 CO 2 a ete libere dans le PCT a partir du glucose et du lactate marques. Ces resultats sont en accord avec la mise en evidence de la gluconeogenese dans ce segment. Au contraire MTAL et CTAL oxydent le glucose, le 2-oxoglutarate, le lactate, le glutamate, et la glutamine mais pas le malate, le succinate ou le citrate. La voie du shunt des pentoses rend compte d'approximativement la moitie du 14 CO 2 produit a partir du 1- 14 C glucose dans MTAL et CTAL. L'oxydation du palmitate est observee dans MTAL et CTAL mais peudans le PCT. Ces resultats mettent en evidence les modalites differentes d'oxydation des substrats dans le PCT, MTAL, et CTAL ou le metabolisme oxydatif est un support critique du transport actif de substances dissoutes.