1. The kinetics of disappearance from the blood of three organic anions were studied in patients with acute intermittent porphyria. Indocyanine green (ICG) and [14C]bilirubin were cleared normally in this disease, whereas bromsulphthalein (BSP) was cleared at less than the normal rate. The first exponential component of the BSP clearance curve was normal in acute intermittent porphyria, but the second exponential component declined less rapidly than in normals. The abnormality was greater in symptomatic than in asymptomatic patients, but both groups were significantly different from normals.

2. The data are discussed in terms of a previously published model of BSP metabolism. The findings suggest normal BSP uptake initially by the liver with increased regurgitation of dye into the blood and decreased excretion of BSP from the liver cell into the bile.

3. In contrast to human acute intermittent porphyria, plasma clearance of BSP was greatly increased in experimental porphyria. This was associated with an increased level of BSP conjugating enzyme in the liver in experimental porphyria.

4. The BSP retention in acute intermittent porphyria, along with certain other findings, raises the question of the existence of an ‘oestrogen effect’ in this disease, but further studies will be required to determine whether the mechanism of BSP retention is identical to that produced by oestrogen.

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