The value of lactulose treatment in hepatic encephalopathy is widely recognised but its mode of action remains controversial. Much evidence supports a role for gamma-aminobutyric acid in hepatic encephalopathy, and lactulose could alter its bacterial production in the gut. Using the rat synaptic membrane assay and gas chromatography mass spectrometry, the production of gamma-aminobutyric acid by faecal Escherichia coli, with and without the addition of albumin, haemoglobin, whole blood, and lactulose under aerobic and anaerobic conditions was determined. Using an inorganic medium, maximal gamma-aminobutyric acid production occurred after a culture period of between 25 and 50 hours. The concentration after 30 hours of aerobic culture at 37 degrees C by a single strain was mean (SEM), 101 (5) mumol/l (99% confidence intervals 87-114 mumol/l; n = 8; interassay coefficient of variation 14.7%). gamma-aminobutyric acid production was significantly increased by the addition of albumin and haemoglobin. Under anerobic conditions, it was one fifth of that produced aerobically, but the addition of albumin and haemoglobin increased production by greater than 700%. Lactulose did not significantly attenuate gamma-aminobutyric acid production under aerobic or anaerobic conditions. gamma-aminobutyric acid determined by the rat synaptic membrane assay showed a highly significant correlation (r = 0.99) with that detected by gas chromatography mass spectrometry. These data confirm that gamma-aminobutyric acid is produced by faecal E coli and that protein enhances its production considerably, and suggest that lactulose does not exert its therapeutic effect by attenuating gamma-aminobutyric acid production.
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