X. Wang et al., SUBSTRATE AND INHIBITOR SPECIFICITIES OF THE MONOCARBOXYLATE TRANSPORTERS OF SINGLE-RAT HEART-CELLS, American journal of physiology. Heart and circulatory physiology, 39(2), 1996, pp. 476-484
We have used the intracellular pH-sensitive fluorescent dye 2',7'-bis(
carboxyethyl)-5(6)-carboxyfluorescein (BCECF) to characterize the subs
trate and inhibitor specificity of monocarboxylate transport into isol
ated rat heart cells. Further evidence was obtained for the presence o
f two lactate carriers present in heart cells (Wang et al., Biochem. J
. 290: 249-258, 1993) both distinct from the recently cloned monocarbo
xylate transporter isoform 1 (MCT-1) found in many other cell types. O
nly one isoform was potently inhibited by alpha-cyano-4-hydroxycinnama
te [CHC; inhibitor constant (K-i) 190 mu M] and the stilbene disulfona
tes 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 se
cond 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 hea
rt cells these inhibitors are not effective at blocking lactate transp
ort totally unless used at very high concentrations. Both carriers wer
e 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 kinet
ic parameters of the two isoforms. Of physiological interest, the keto
ne 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-lac
tate and pyruvate and indicate that transport could limit rates of uti
lization of ketone bodies. No stereoselectivity for Lover D-isomers of
2-chloro or 2-hydroxy acids was observed.