Determination of imidazole acetic acid and its conjugate(s) levels in urine, serum and tissues of rats: studies on changes in their levels under various conditions.

A convenient and reproducible method for assay of imidazole acetic acid (ImAA) was developed as a modification of that described previously (Watanabe et al., 1983). ImAA conjugate(s) (ImAA-C), mainly consisting of imidazole acetic acid riboside, could be measured by this method after its hydrolysis to ImAA. The ImAA and ImAA-C levels in various tissues of rats were measured and the effects of various agents on these levels were studied. The renal clearance values of ImAA-C in rats and man were similar to the creatinine clearance values, but those of ImAA were 1/40 of those of ImAA-C, suggesting that the latter is readily excreted in the urine. Consistent with this idea, the urinary excretion of ImAA-C was found to increase much more than that of other histamine metabolites during late pregnancy, when the foetus produces much histamine.