LACTATE TRANSPORT IN INSULIN-SECRETING BETA-CELLS - CONTRAST BETWEEN RAT ISLETS AND HIT-T15 INSULINOMA CELLS

The transport of L- and D-lactate into rat pancreatic islets and HIT-T15 insulinoma cells was studied by measuring uptake of C-14-labelled substrate at room temperature and by following changes in intracellular pH (pHi) in islets and HIT-T15 cells loaded with 2',7,-bis(carboxyethyl)-5'(6')-carboxyfluorescein (BCECF). Uptake of L-lactate into HIT-T15 cells was rapid, reaching equilibrium after 5 min with an apparent K(m) value of 4.8 mM. Transport was markedly inhibited by alpha-cyano-4-hydroxycinnamate, alpha-fluorocinnamate, quercetin and p-chloromercuribenzenesulphonate (pCMBS), and was enhanced in citrate medium. Uptake Of D-lactate was less rapid, apparent eqilibrium not being reached within 10 min. In contrast to HIT-T15 cells, rat pancreatic islets showed greatly reduced rates of transport Of L- and D-lactate together with a correspondingly lower degree of inhibition by alpha-cyano-4-hydroxycinnamate. The addition of L- or D-lactate to HIT-T15 cells, but not dispersed islet cells, resulted in a marked and rapid intracellular acidification followed by a gradual recovery. In both HIT-T15 cells and isolated islets, the rates of transport of both L- and D-lactate in the presence of alpha-cyano-4-hydroxycinnamate were significantly greater in a depolarising K+ medium compared to the normal Na+ medium. These observations suggest that native rat islet cells have considerably reduced activity of the lactate-/H+ transport system compared to HIT-T15 insulinoma cells. There is evidence in both cell types of an additional electrogenic pathway for lactate which might play a role in coupling lactate efflux to beta-cell depolarisation.