| Abstract: | Propionate extraction by liver is generally in the range of 95%, which could depend on a transport process across the cell membrane. The study reports conditions in which 14C]propionate uptake can be measured with minimal interferences from metabolism. Propionate uptake by isolated hepatocytes was mediated by two components: a low-affinity component of limited physiological interest and a high-affinity (apparent Km about 0.15 mM) component. This last component displayed a high capacity but was not Na+-dependent nor concentrative. Propionate transport was not markedly affected by acetate, butyrate or other C3 glucogenic compounds; it was inhibited by halogenated monocarboxylates, monochloroacetate and 2-chloropropionate being the most potent. Classical inhibitors of anion transport and of functional-SH groups were ineffective. Propionate uptake was responsive to external pH: stimulated by acidic and depressed by alkaline pH. Hepatic uptake of propionate in vivo was practically quantitative up to 0.8-1.0 mM in afferent plasma, in keeping with the measured capacity of the high-affinity component. It is suggested that propionate uptake is essentially carrier mediated but this process should not be rate limiting for hepatic utilization in physiological conditions. |
