Effects of the mycotoxin destruxin A on Locusta migratoria visceral muscles

Destruxins, a family of cyclic peptides, are produced by various species of entomopathogenic fungi. These peptides have been shown to influence calcium-dependent processes in insect cell lines and tissues, such as skeletal muscles. To better understand the mechanism of action of these peptide toxins on insect muscular tissues, we have evaluated the effects of destruxin A on the contractions of oviducts and hindgut of Locusta migratoria. In oviducts, destruxin A increased the frequency of spontaneous contractions and induced a dose-dependent tonic contraction; the EC(50) for lower lateral and upper lateral oviducts was 0.7 microM and 8.7 microM, respectively. In hindgut, destruxin A also caused an increase in the frequency of spontaneous contractions; the EC(50) was 3.2 microM. The action of destruxin A was abolished in Ca(2+)-free saline or when the Ca(2+) channel blocker CoCl(2) was added to the incubation saline. Likewise, the presence of 50 microM nifedipine or 100 microM verapamil in the medium reduced the magnitude of destruxin A's effect, particularly in hindgut. The depolarization of muscle membranes by 100 mM K(+) saline prevented the action of destruxin A. Preincubation of lower lateral oviducts in the intracellular Ca(2+) antagonist TMB-8 did not have any effect on destruxin A action; however, preincubation in the calmodulin inhibitor trifluoperazine greatly reduced the effect of destruxin A. Taken together, these results show that destruxin A has an excitatory effect on contractions of insect visceral muscles of L. migratoria. Destruxin A-induced contractions appear to be dependent on extracellular, but not on intracellularly-released Ca(2+), which suggest that this peptide toxin might be acting on insect visceral muscle by facilitating an influx of extracellular Ca(2+).