ABOLITION OF THE INHIBITORY EFFECT OF ETHANOL OXIDATION ON GLUCONEOGENESIS FROM LACTATE BY ASPARAGINE OR LOW CONCENTRATIONS OF AMMONIA

When isolated hepatocytes from fasted rats were incubated with 10 mM l actate, the [lactate]/[pyruvate] ratio measured at the beginning of th e incubation was raised above 70:1 but declined to a steady level of a bout 8:1 within 30 min. The rate of gluconeogenesis from lactate was i nitially slow but gradually increased over the incubation period becom ing maximal by 30 min. The simultaneous addition of lactate and ethano l resulted in an initial [lactate]/[pyruvate] ratio above 250:1 which by 60 min had declined to a new steady-state level of approx. 60:1. Th e lactate, ethanol combination also brought about a prolongation of th e lag phase before glucose synthesis became maximal; however, by 40 mi n the rate of gluconeogenesis was independent of the presence of ethan ol. Thus the inhibitory effect of ethanol an glucose synthesis was man ifest only over the early portion of the incubation period. When aspar agine, a precursor of malate/aspartate components, was added to the in cubation mixture, the lag before maximal rates of glucose formation fr om lactate in the absence or presence of ethanol was almost abolished. The presence of asparagine also rapidly lowered the [lactate]/[pyruva te] ratio of hepatocytes incubated with lactate plus ethanol establish ing a steady-state level of 15:1 within 10-15 min. Asparagine enhanced the rate of lactate-stimulated ethanol oxidation, particularly during the early part of the incubation. In endeavouring to elucidate which of the products of asparagine catabolism (i.e. ammonia and aspartate) were responsible for these effects, we found that a small and constant level of ammonia, formed by the degradation of urea by urease, almost reproduced the effects of asparagine on the [lactate]/[pyruvate] rati o, glucose synthesis and ethanol oxidation. A bolus addition of 10 mM aspartate or 4 mM ammonia to cells metabolising lactate and ethanol we re less effective than a steady-state low ammonia concentration, gener ated from urea/urease. Our studies suggest that asparagine or a low co ncentration of ammonia, by providing components of the malate/aspartat e shuttle, can ameliorate some of the metabolic effects of ethanol on the liver.