INHIBITION OF RNA-SYNTHESIS BY ADIPOKINETIC HORMONES AND BRAIN FACTOR(S) IN ADULT FAT-BODY OF LOCUSTA-MIGRATORIA

Extracts of brains and corpora cardiaca from adult Locusta migratoria inhibit total RNA synthesis in vitro in the fat body of Locusta migrat oria. Adipokinetic hormones (AKHs) appear to be responsible for the in hibitory activity in extracts of corpora cardiaca, but these peptides are not the active factor(s) in the brain. Locusta adipokinetic hormon es-I, -II and -III (AKH-I, AKH-II and AKH-III) inhibit RNA synthesis i n vitro in both females and in males. In males, the responses are dose -dependent with their potencies decreasing in the order: AKH-III > AKH -II > AKH-I. All three AKHs are equally efficacious, suppressing RNA s ynthesis to levels similar to those measured when actinomycin D is add ed to the incubations of fat body tissue. Mature adults (25 days old o f either sex) are the most sensitive, while younger insects show pract ically no inhibition of RNA synthesis in response to injected adipokin etic peptides, At supramaximal doses, however, the peptides cease to b e effective in females, while they retain their efficacy in males. The chemical nature of the active material in the extracts of brain remai ns to be determined, but its mechanism of action in inhibiting RNA syn thesis differs from that of the AKHs: the latter require extracellular Ca2+ for their action, while extracts of brain do not, Neither mechan ism appears to involve cAMP.