LIDOCAINE INHIBITS CHOLINE UPTAKE AND PHOSPHATIDYLCHOLINE BIOSYNTHESIS IN HUMAN LEUKEMIC MONOCYTE-LIKE U937 CELLS

The effect of lidocaine on [H-3]choline uptake and the incorporation o f label into phosphatidylcholine (PC) in human monocyte-like U937 cell s was investigated. Lidocaine inhibited the rate of choline uptake in a dose-dependent manner; at 3.2 mM it resulted in a drastic reduction, by as much as 65 per cent (n = 10; p < 0.0005) or 55 per cent (n = 10 ; p < 0.0006) in a 3- or 6-h incubation, respectively. Lidocaine also decreased the rate of choline incorporation into PC in a dose-dependen t manner. At the highest dose, nearly 70 per cent or 45 per cent reduc tion was seen in a 3- or 6-h incubation, respectively. Analysis of cho line-containing metabolites showed that the major label association wi th phosphocholine and PC was reduced to a similar extent which was als o parallel to the inhibition of choline uptake. At 3.2mM lidocaine, th e reduction of choline uptake was shown to follow a competitive inhibi tion. In the case of [H-3] choline incorporation into PC, the inhibito ry pattern was shown to be of a mixed type. The pulse-chase study diss ecting the effect on choline metabolism from that on total choline upt ake indicated that lidocaine exerted an additionally inhibitory effect on intracellular choline metabolism into PC, In a separate protocol i n which the labelled cells were first allowed to be chased until H-3-i ncorporation into PC reached a steady state, lidocaine no longer showe d any effect. These results seem to exclude the possibility of enhance d PC breakdown and further suggest that the main inhibitory effect is on the CDP-choline pathway for PC biosynthesis. After a 3-h treatment, CTP: cholinephosphate cytidylyltransferase (CYT) in both the cytosoli c and microsomal fractions was inhibited by approximately 20 per cent, while choline kinase (CK) and choline phosphotransferase (CPT) remain relatively unchanged. There was no evidence for translocation of CYT between cytosol and microsomes. Taken together, we have demonstrated a dual inhibitory function of lidocaine which inhibits PC biosynthesis in addition to its ability to block choline uptake profoundly in U937 cells.