Organic substrate effects on and heterogeneity of Necturus proximal tubule function

Organic substrate effects on and heterogeneity of Necturus proximal tubule function. The effect of organic substrates such as lactate, alanine, glutamine, glutamate, lysine, and butyrate is compared with that of glucose, and the role of tubular heterogeneity is assessed in Necturus proximal tubule. Fluid absorption (J v ) is enhanced 34% by the presence of all substrates together in the tubular lumen. Addition of glucose to the peritubular perfusion fluid does not increase J v significantly, whereas the other substrates augment J v by 181%. Transepithelial potential difference (V 3 ) and peritubular membrane potential difference (V 1 ) are unaffected by these organic substrates. Transepithelial chloride to sodium transference number ratios (t Cl /t Na ) and transepithelial specific resistance (R m ) are higher in the presence of substrates, possibly because of effects on paracellular conductance. Organic solutes could influence fluid absorption by increasing entry of sodium across the luminal cell membrane, by increasing energy supply to the active transport mechanism, or by decreasing paracellular conductance. Early and late straight segments of the proximal tubule do not differ significantly in V 3 , V 1 , and transepithelial t Cl /t Na . Early convoluted segments have a higher J v than do late straight proximal segments, but they have a lower transepithelial specific resistance. We have concluded that net fluid absorption and transepithelial ionic conductance correlates with the supply of certain organic solutes and with specific localization along the proximal tubule. Effet des substrats organiques sur, et heterogeneite de, la fonction tubulaire proximale du Necturus. L'effet de substrats organiques tels que lactate, alanine, glutamine, glutamate, lysine, et butyrate est compare a celui du glucose, et le role de l'heterogeneite tubulaire est evalue dans le tube proximal du Necturus. L'absorption de liquide (J v ) est augmentee de 34% par la presence simultanee de tous ces substrats dans la lumiere tubulaire. L'addition de glucose au liquide de perfusion peritubulaire n'augmente pas significativement J v alors que les autres substrats augmentent J v de 181%. La difference de potentiel transepitheliale (V 3 ) et la difference de potentiel de la membrane peritubulaire (V 1 ) ne sont pas modifiees par ces substrats organiques. La resistance transepitheliale specifique (R m ) ainsi que t Cl /t Na sont plus eleves en presence de ces substrats, peut-etre du fait des effets sur la conductance paracellulaire. Les substrats organiques peuvent affecter l'absorption de liquide en augmentant l'entree de sodium a travers la membrane luminale ou en diminuant la conductance paracellulaire. Les segments precoce et tardif du tube proximal ne different pas significativement en ce qui concerne V 3 , V 1 et t Cl /t Na transepitheliaux. Les segments contournes precoces ont des J v superieurs a ceux des segments droits tardifs, mais une resistance transepitheliale specifique plus faible. Il est conclu que l'absorption nette de liquide et la conductance ionique transepitheliale sont correlees avec l'apport de certaines substances organiques et avec des localisations specifiques le long du tube proximal.