Transfer and metabolism of citrate, succinate,α-ketoglutarate and pyruvate by hamster small intestine

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Abstract
The transfer of citrate, succinate, $\alpha $-ketoglutarate and pyruvate was studied with everted sacs of hamster small intestine and evidence presented for specific transfer mechanisms for each of these. All four intermediates studied were rapidly removed from the mucosal fluid and appeared in the serosal fluid in the presence of metabolites. Only in the case of citrate, where consistently high recoveries of the unchanged intermediate were obtained, has evidence been presented for transfer against a concentration gradient. Several carboxylates compete at the luminal membrane for the mechanisms involved in the transfer of citrate, succinate and $\alpha $-ketoglutarate. The transfer processes for citrate and succinate were saturable, and more than one pathway may be involved in the transfer of succinate. Pyruvate showed little interaction with the other three intermediates. A saturable component of transfer was demonstrated although diffusion contributed a considerable proportion of the total transfer. The importance of a downhill sodium gradient for transfer of citrate, succinate, $\alpha $-ketoglutarate and pyruvate across the luminal membrane, has been established. A number of carboxylates stimulated the p.d. across the intestine although this was different from the p.d. produced by sugars and amino acids.
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