Implication of altered levels of plasma alpha(1)-acid glycoprotein and its derived sialic acid on plasma protein binding of trimethoprim in pigs in physiological and pathological states.

In growing pigs (0 to 158 days after birth), pregnant sows (0 to 90 days), and in pigs suffering from respiratory disease (Actinobacillus pleuropneumoniae), the plasma levels of alpha(1)-acid glycoprotein (AGP) and its derived sialic acid were determined, as was the meaning of their altered levels on the plasma protein binding of trimethoprim. The AGP level was very high immediately after birth (more than 10,000 mu g/ml), decreased markedly during 2 weeks after birth (about 700 mu g/ml) and thereafter stayed at a constant level (about 400 mu g/ml). Pregnant sows had a low AGP level with a narrow variation throughout pregnancy (about 190 to 260 mu g/ml). Pigs infected with A. pleuropneumoniae showed an increased AGP level (mean value; 732 mu g/ml) with a wide variation (range: 170-1,840 mu g/ml). N-acetylneuraminic acid (NANA) and N-glycolylneuraminic acid (NGNA) were sialic acid subtypes detected in porcine plasma. In growing pigs, the time course of changes in NANA concentrations was consistent with that of AGP, whereas that of NGNA was different, implying that NANA is a sialic acid subtype derived from porcine AGP, in contrast to NGNA. The relationship between AGP and NANA levels in growing pigs could be expressed by the following equation: NANA=0.14 x AGP + 159 mu g/ml, whereas that in pigs with respiratory disease could be expressed by NANA=0.O67 x AGP + 357 mu g/ml, indicating a low fraction of NANA in AGP in diseased pigs. The regression lines between the AGP level and the plasma protein binding of trimethoprim < or = 2,000 mu g of AGP/ml were similar as follows: binding(%)=0.O23 x GP + 34 in growing pigs and binding(%)=0.O22 x GP+29 in diseased pigs, implying a minor role of sialic acid residues in the binding of basic drugs to AGP. In conclusion, the wide change in plasma AGP levels in diseased pigs as well as during the initial growth phase can alter the plasma protein binding of basic drugs such as trimethoprim, probably leading to a change in drug disposition. The low sialylation of AGP in diseased pigs may not have a great influence on the binding of basic drugs to AGP, implying the quantitative importance of AGP.