Diuretic action of the peptide locustatachykinin I: cellular localisation and effects on fluid secretion in Malpighian tubules of locusts.

In insects primary urine is produced by the Malpighian tubules under hormonal control. Here we have analysed the effects of the peptide locustatachykinin I (Lom-TK-I) on secretion in isolated Malphigian tubules. We also mapped the distribution of Lom-TK immunoreactivity in the gut in comparison with Locusta diuretic hormone (Lom-DH) and serotonin, two other factors that are active on locust tubules. Lom-TK-I produces an immediate, potent and long-lasting stimulation of fluid secretion. Furthermore, we show that Lom-TK-I acts synergistically with Lom-DH on fluid secretion and demonstrate that Lom-TKs are co-localised with Lom-DH in endocrine cells of the midgut ampullae. Thus, the two peptides might be released together to act synergistically on fluid secretion. Also serotonin and Lom-DH act synergistically and we can demonstrate a plexus of serotonin-containing axon processes over the midgut.