The Involvement of Intracellular Calcium in the MCT-Mediated Uptake of Lactic Acid by HeLa Cells

The main object of this study was to evaluate the role of intracellular free calcium ion [Ca2+](in), in monocarboxylate transporter (MCT) mediated drug uptake by HeLa cells. It was hypothesized that alterations in the [Ca2+](in) levels affect Na+-H+ exchanger (NHE) regulated pH(in) and thereby produce the proton-motivated driving force for monocarboxylate mediated substrate transport. The changes in intracellular pH (pH(in)) and MCT mediated uptake rates of L-lactic acid by HeLa cells, a human cervical adenocarcinoma cell line, were evaluated under the conditions, whose [Ca2+](in) concentrations were altered by various calcium modulators, such as EGTA-AM (a chelator), nifedipine (a Ca2+ channel antagonist) and A23187 (an ionophore). For the purpose of comparison, the L-lactic acid uptake by HeLa cells was also evaluated under various pH(in) conditions induced by dexamethasone. The effects of the extracellular sodium concentration on the L-lactic acid uptake by HeLa cells were evaluated to determine the involvement of NHE-regulated pH changes in the MCT mediated drug uptake process. The [Ca2+](in) concentrations and pH(in) in HeLa were assessed using fluorescent probes fura-2 and 2',7'-bis[2-carboxyethyl-5-carboxyfluorescein] (BCECF), respectively. The treatment of HeLa cells with A23187 at concentrations of 50 and 100 mu M enhanced [Ca2+](in) by 100% and 200% of the control, respectively. EGTA/AM (50 mu M) or nifedipine (100 mu M) did not cause any significant changes in the [Ca2+](in) levels, whereas EGTA/AM (100 mu M) and nifedipine (200 mu M) reduced the [Ca2+](in) levels by 30% and 25%, respectively, as compared with the control. A23187 at a concentration of 100 mu M in the incubation medium lowered pH(in) (pH 5) and subsequently the uptake rate of lactic acid by 50% (0.47 +/- 0.03 mu mol/mg protein/min) of the control. In contrast, nifedipine (200 mu M) and EGTA-AM (100 mu M), the calcium modulators that lowered the [Ca2+](in) levels and maintained the higher pH(in) (pH > 6) of HeLa cells, enhanced the uptake rate of lactic acid by 60% and 130% of the control, respectively. The results of this study demonstrated that there was a close correlation between the [Ca2+](in) level and pH(in) and that NHEs were involved with the MCT mediated uptake process in HeLa cells. An understanding of the role of [Ca2+](in) in the MCT mediated transport process could provide an efficient strategy to improve the systemic delivery of monocarboxylate substrates through the cervical mucosa.