Influence of the assay conditions on the lipolytic potency in vitro of different adrenocorticotrophins.

The lipolytic activity of ACTH-(1-39) and ACTH-(1-24) in vitro has been compared on a molar basis in assays performed with both adipose-tissue slices and isolated fat cells. In the tissue-slice assay ACTH-(1-24) displayed 29% of the potency of ACTH-(1-39), whereas in the isolated-cell assay, ACTH-(1-24) was 3.9 times more potent than the longer peptide. This discrepancy seems to be due to the fact that ACTH-peptides are inactivated in the adipose-tissue-slice assay, but remain stable in the isolated-fat-cell assay. ACTH-(1-24) is inactivated more rapidly than ACTH-(1-39). Pre-incubation and washing of the tissue slices removes the inactivating principle, which most likely consists of peptidases, and their responsiveness is increased, so that both ACTH-peptides display similar lipolytic activity to that seen in the isolated-cell system. Comparison of the responsiveness of isolated fat cells and isolated adrenal cells indicates that the affinity of ACTH for the receptor of adrenal cells is approximately 100 times greater than its affinity for the fat-cell receptor.