Reabsorption of dicarboxylic acids from the proximal convolution of rat kidney.

Abstract

The transport of dicarboxylic acid in the late proximal convolution was investigated by measuring the 3.5 s efflux of 2-oxoglutarate from the tubular lumen of rats starved for 3 days. The 3.5 s efflux of 2-oxoglutarate comprises two components, one due to movement across the brush border, obeying Michaelis Menten kinetics with an apparent Km of 0.13 mmol/l and a Jmax of 0.41 pmol cm-1 s-1 and the other due to diffusional movement presumably via the paracellular pathway with a permeability of 23.0 microns2 s-1. Omission of sodium from the perfusion fluid reduced the transcellular efflux of 2-oxoglutarate by 76%, indicating a sodium-dependent transport system. Addition of 5 mmol/l lithium to the liminal and capillary perfusate reduced it by 56% indicating a specific inhibitory effect of lithium on dicarboxylic acid transport. Addition of 5 mmol/l H2DIDS to the luminal perfusate reduced 3.5 s transcellular 2-oxoglutarate efflux by 35%. The molecular specificity of the system was assessed by studying the inhibitory effects of a series of dicarboxylates, both aliphatic and aromatic, on the 3.5 s efflux of 2-oxoglutarate. Inhibitory constants (apparent Ki) were calculated for comparative purposes assuming competitive inhibition. From this, the system was found to have optimal affinity for dicarboxylates in the trans-configuration with a four or five carbon chain (i.e. succinate and glutarate). Substitution on the 2-carbon atom with CH3-, OH-, SH-, and O = resulted in little reduction in inhibitory potency as compared to succinate itself. However, 2-substitution with NH3+ (not with N-acetyl) as well as 2,3-disubstitution with CH3-, OH- or SH-, strongly reduced or abolished the inhibitory potency. Only with the exception of pyruvate all monocarboxylates tested, did not inhibit 2-oxoglutarate transport, but all tricarboxylates tested, i.e. citrate, isocitrate and tricarballate had an inhibitory effect. Citrate inhibition was higher at acidic than alkaline pH. A number of aromatic compounds was also tested. In most cases the inhibitory potency of the aromatic compounds was considerably weaker than that of the effective 4-5 carbon chain aliphatic compounds. Only benzene-1,4-dicarboxylate, benzene-2-nitro-1,4-dicarboxylate, and benzene-1,2-diacetate had a high inhibitory potency. In the case of the aromatic dicarboxylates the most important feature relating molecular structure to transport was the distance between the two carboxyl-groups in the molecule.(ABSTRACT TRUNCATED AT 400 WORDS)