Potencies of naturally-occurring AKH/RPCH peptides in Locusta migratoria in the acetate uptake assay in vitro and comparison with their potencies in the lipid mobilisation assay in vivo

The biological potencies of a number of naturally-occurring octa- and decap eptides of the large AKH/RPCH family of peptides were determined in Locusta migratoria using the lipid-mobilising assay in vivo and the acetate uptake assay in vitro. The most potent of the newly-tested peptides in the in vit ro assay, Ph1-CC, differs from the endogenous major locust peptide, Lom-AKH -I, only by an exchange of serine versus threonine at position 10. However, the most active peptide in the ill vitro assay remains Lom-AKH-III. At the other extreme is the peptide Mem-CC which contains a tyrosine residue at p osition 4 rather than the more typical phenylalanine. This peptide is over 20000 times less potent than LomAKH-III in the ill vitro assay, and also re sults in an unusual dose-response curve in the in vivo assay. Only a few pe ptides are approximately equipotent in both assays, but mostly the bioanalo gues have a higher potency in vitro. The majority of them are 2- to 10-fold more potent in vitro, but Ani-AKH and LomAKH-III are 19- and 48-fold more potent. The results are discussed in relation to either the actions of prot eases or of possible preferential binding of different receptors involved i n the different assays.