Comparative pharmacological actions of leucokinins V-VIII on the visceral muscles of Leucophaea maderae.

1. Leucokinins V-VIII (Lem-K-V to VIII) did not activate visceral muscles of the cockroach Leucophaea maderae uniformly as a group but rather showed a selective action on the muscles of the hindgut. This organ showed a contractile response to all of the leucokinins at 3 x 10(-10) M that was 2-20% above the mean level of spontaneous activity. The maximum response for each peptide was recorded at 2.1 x 10(-7) M. 2. Both the foregut and the oviduct were 100- to 1000-fold less sensitive than the hindgut, and each of the former required more than 10(-8) M to elicit a detectable excitation. The heart, by comparison, did not respond to any of these peptides. 3. The leucokinins caused a protracted excitation of contractile events in the hindgut that lasted for more than 60 min. Moreover, all four peptides evoked contractions from hindguts after membrane depolarization with 158 mM potassium. These results suggest that nonsynaptic receptors for the peptides exist in visceral muscle.