EFFECTS OF LEUCOMYOSUPPRESSIN ON THE EXCITATION-CONCENTRATION COUPLING OF INSECT LEUCOPHAEA-MADERAE VISCERAL MUSCLE

1. Leucomyosuppressin (LMS) inhibited neurally evoked contractions of the hindgut of the cockroach Leucophaea maderae. The threshold for this inhibition of LMS was in the range of 1 x 10(-10) M. 2. LMS caused a sharp reduction in both L-glutamate and proctolin induced contractions. Dose-response profiles of the effect of LMS (held constant at 10(-8) M) on variable amounts of proctolin showed an inhibitory effect at 10(-9) M proctolin and below, but at 5 x 10(-9) M proctolin and above, LMS caused no inhibition. 3. Potassium (158 mM) depolarized hindguts treated with LMS (10(-8) M) showed a marked reduction (76% +/- 2.1) in the proctolin (10(-8) M) response. 4. When calcium depleted preparations were returned to normal calcium levels (2 mM) in the presence of proctolin (10(-8) M) a contraction occurred that was 45% +/- 4 of the maximum in normal saline solution. However, LMS (10(-8) M) reduced this response to only 28% +/- 2 of the maximum. 5. Proctolin (10(-8) M) induced contractions in the presence of the manganous ions (2 mM) fell to 63% +/- 4 of the maximum but on the addition of LMS (10(-8) M), such responses fell to only 16% +/- 5 of the maximum. 6. These results offer evidence for a non-synaptic site of action for LMS and a perturbation of key calcium dependent events in the excitation-contraction coupling sequence of visceral muscle by this peptide.