A range of short-chain aliphatic monocarboxylates, both unsubstituted and substituted with hydroxy, chloro and keto groups, were shown to inhibit transport of L-lactate and pyruvate into both guinea-pig cardiac myocytes and rat erythrocytes. The carrier of heart cells exhibited a higher affinity (approx. 10-fold) for most of the monocarboxylates than did the erythrocyte carrier. A notable exception was L-lactate, whose Km for both carriers was similar. The K1 values of the two carriers for inhibitors such as phenylpyruvate and alpha-cyanocinnamate derivatives were also different. The high affinity of the heart cell carrier for ketone bodies and acetate may be physiologically important, since these substrates are used as fuels by the heart.

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