INHIBITION OF ECDYSTEROID SECRETION FROM MANDUCA PROTHORACIC GLANDS INVITRO BY DESTRUXINS - CYCLIC DEPSIPEPTIDE TOXINS FROM THE INSECT PATHOGENIC FUNGUS METARHIZIUM-ANISOPLIAE

Incubation of prothoracic glands (PG) from Manduca sexta with destruxins (DTX) (principally destruxin-A) inhibited ecdysteroid secretion in vitro in a dose-dependent manner over a 6 h test period. A concentration of 0.5 mug ml-1 DTX gave ca 50% inhibition of the basal rate of secretion. The effect of a 1 h preincubation of PG with 1 mug ml-1 DTX was completely reversed by washing and incubation in normal medium. DTX (1 mug ml-1) effectively completely prevented (93% inhibition) the large stimulatory effects on ecdysteroid secretion of exposure to dibutyryl cAMP in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). DTX also significantly inhibited ecdysteroid secretion stimulated by the adenylyl cyclase activator forskolin, and by the calcium ionophore A23187. These results suggest that in Manduca PG, DTX may act at a level beyond these second messengers.