Pharmacological actions of a new class of neuropeptides, the leucokinins I-IV, on the visceral muscles of Leucophaea maderae.

1. Leucokinins I-IV did not activate visceral muscles uniformly as a class 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 5-10% 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 100-1000 fold less sensitive than the hindgut, and each of the former organs required more than 10(-8) M to elicit a detectable excitation. The heart, by comparison, failed to give consistent responses with any of the 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. 4. This result shows that nonsynaptic receptors for the peptides exist in visceral muscle. The leucokinins showed no evidence of facilitating the reentry of calcium into calcium depleted hindgut preparations.