Acute and Chronic Ethanol Treatment in Viuo Increases Malate-Aspartate Shuttle Capacity in Perfused Rat Liver*

The effects of acute and chronic treatment with ethanol on transport of reducing equivalents into mitochondria via the malate-aspartate shuttle were studied in perfused rat liver. The shuttle capacity was estimated from the decrease in rates of glucose production from the reduced substrate sorbitol caused by an increase in the NADH/NAD+ ratio in the cytosol due to metabolism of ethanol. The greater the capacity of the malate-aspartate shuttle, the smaller the inhibition of glucose synthesis by ethanol. Glucose synthesis was decreased about P-fold less in livers from fasted rats treated acutely 2.5 h earlier with ethanol than in untreated controls. Chronic treatment with ethanol for 3-5 weeks prevented completely the decrease in glucose synthesis from sorbitol due to ethanol oxidation. Rates of ethanol uptake were elevated significantly from 69 f 7 rmol/g/h in livers from control rats up to 92 & 7 rmol/g/h in livers from SIAM rats. Similarly, rates of ethanol uptake were stimulated by chronic ethanol treatment from 71 & 6 to 222 f 15 pmol/g/h; this increase was largely sensitive to aminooxyacetate. Taken together, these data indicate that flux of reducing equivalents over the malate-aspartate shuttle is increased by both acute and chronic treatment with ethanol and that movement of reducing equivalents from the cytosol into the mitochondria via the malateaspartate shuttle is an important rate determinant in hepatic ethanol oxidation. oxalacetate is converted into aspartate and moves into the cytosol in exchange for glutamate. To complete the cycle, oxalacetate is regenerated in the cytosol from aspartate by transamination. The malate-aspartate shuttle has been difficult to study because essential intermediates are distributed in both the cytosol and mitochondria (2). Recently, a new method was developed to estimate the capacity of the malate-aspartate shuttle mechanism based on inhibition of gluconeogenesis from a reduced substrate such as sorbitol due to ethanol oxidation in the perfused liver (3). By increasing NADH in the cytosol, ethanol increases competition for the movement of reducing equivalents produced by oxidation of sorbitol by sorbitol dehydrogenase into the mitochondria via the malateaspartate shuttle, resulting in inhibition of gluconeogenesis from sorbitol(4). Thus, the greater the capacity of the malateaspartate shuttle, the smaller the inhibition of glucose synthesis by ethanol. For example, in livers from normal rats, ethanol inhibited glucose production from sorbitol about 50% (3); however, if rats were exposed to cold for 3-6 weeks, ethanol failed to inhibit glucose formation. From such experiments it was concluded that the capacity of the malateaspartate shuttle was increased by cold exposure (3).