Substrate and inhibitor specificities of the monocarboxylate transporters of single rat heart cells

We have used the intracellular pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) to characterize the substrate and inhibitor specificity of monocarboxylate transport into isolated rat heart cells. Further evidence was obtained for the presence of two lactate carriers present in heart cells (Wang et al., Biochem. J. 290: 249-258, 1993) both distinct from the recently cloned monocarboxylate transporter isoform 1 (MCT-1) found in many other cell types. Only one isoform was potently inhibited by alpha-cyano-4-hydroxycinnamate [CHC; inhibitor constant (K-i) 190 mu M] and the stilbene disulfonates 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (K-i 79 mu M) and 4,4'-dinitrostilbene-2,2'-disulfonate (K-i of cis- and trans- isomers 38 and 171 mu M, respectively; neither isomer inhibits MCT-1). The second carrier had a K-i of similar to 3 mM for CHC and 0.5-2 mM for the stilbene disulfonates. Thus, unlike in many other tissues, in rat heart cells these inhibitors are not effective at blocking lactate transport totally unless used at very high concentrations. Both carriers were inhibited by 3-isobutyl-1-methylxanthine (K-i 340 mu M) and neither by 5-nitro-2-(3-phenylpropylamino)benzoate (a potent inhibitor of MCT-1). The overall Michaelis constant (K-m) and maximum reaction rate (V-max) for transport of a variety of substituted monocarboxylates (C2-C5) were determined, although it was not possible to elucidate the kinetic parameters of the two isoforms. Of physiological interest, the ketone bodies D-beta-hydroxybutyrate and acetoacetate had K-m values of 10 and 5.4 mM, respectively. V-max values were similar to those of L-lactate and pyruvate and indicate that transport could limit rates of utilization of ketone bodies. No stereoselectivity for Lover D-isomers of 2-chloro or 2-hydroxy acids was observed.