Inhibition of RNA synthesis by adipokinetic hormones and brain factor(s) in adult fat body of Locusta migratoria

Abstract

Extracts of brains and corpora cardiaca from adult Locusta migratoria inhibit total RNA synthesis in vitro in the fat body of Locusta migratoria. Adipokinetic hormones (AKHs) appear to be responsible for the inhibitory activity in extracts of corpora cardiaca, but these peptides are not the active factor(s) in the brain. Locusta adipokinetic hormones-I, -II and -III (AKH-I, AKH-II and AKH-III) inhibit RNA synthesis in 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 synthesis to levels similar to those measured when actinomycin D is added to the incubations of fat body tissue. Mature adults (25 days old of either sex) are the most sensitive, while younger insects show practically no inhibition of RNA synthesis in response to injected adipokinetic peptides. At supramaximal doses, however, the peptides cease to be effective in females, while they retain their efficacy in males. The chemical nature of the active material in the extracts of brain remains to be determined, but its mechanism of action in inhibiting RNA synthesis differs from that of the AKHs: the latter require extracellular Ca 2+ for their action, while extracts of brain do not. Neither mechanism appears to involve cAMP.