Effect of human plasma on the bioactivity of adrenocorticotrophin

The effect of plasma on the bioactivity of ACTH was investigated with the cytochemical section bioassay. Levels of ACTH in normal human plasma were underestimated if the plasma concentration in the incubation medium exceeded 2% and if the plasma ACTH level was less than 2 ng/l. A retrospective analysis of 250 plasma ACTH assays revealed that the ACTH concentration was consistently underestimated in the 1:100 dilution compared with the 1:1000 dilution, although not to the extent that any assay would be rejected for non-parallelism. The extent of the underestimation increased as the ACTH concentration increased. Guinea-pig adrenal sections responded faster to 1:10 or 1:50 dilutions of human plasma (containing 2 ng ACTH/l) than to either 1:100 or 1:1000 dilutions of that same plasma, or to ACTH (over the range 0.005-5 ng/l)in the absence of plasma. This faster response could also be seen in the absence of plasma by increasing the ACTH concentration above 5 ng/l. It seems that plasma contains factors which potentiate the biological effect of ACTH in the cytochemical assay system.U