STIMULATION OF ECDYSTEROIDOGENESIS BY SMALL PROTHORACICOTROPIC HORMONE - ROLE OF CYCLIC-AMP

Prothoracicotropic hormones (PTTHs) stimulate synthesis and secretion of ecdysteroids by insect prothoracic glands. In Manduca sexta, PTTH e xists as two size variants, small and big PTTH. Experiments were perfo rmed to assess the possible role of cyclic AMP in small PTTH signal tr ansduction. cAMP analogs, or agents that increase intracellular cAMP, stimulated ecdysteroidogenesis. Small PTTH enhanced glandular cAMP lev els; the rise in cAMP preceded an increase in ecdysteroid secretion. P rothoracic glands accumulated less cAMP when treated with small PTTH t han when treated with big PTTH. A phosphodiesterase inhibitor (1-methy l-3-isobutylxanthine) (MIX) increased the amount of cAMP in glands tre ated with small but not big PTTH, suggesting that glandular phosphodie sterase activity may be elevated in the presence of small PTTH. PTTH-s timulated ecdysteroid secretion was suppressed by a cAMP antagonist (R p-cAMPS). The effects of small and big PTTH on ecdysteroidogenesis wer e non-additive. The combined results suggest that cAMP is employed as a second messenger by both prothoracicotropins, and that there may be subtle differences in their respective mechanisms of action.