title: Antagonistic control of fluid secretion by the Malpighian tubules of Tenebrio molitor: effects of diuretic and antidiuretic peptides and their second messengers.
abstract: Fluid secretion by insect Malpighian tubules is controlled by haemolymph-borne factors. The mealworm Tenebrio molitor provides the first known example of antagonistic interactions between endogenous neuropeptides acting on Malpighian tubules. The two corticotropin-releasing-factor (CRF)-related diuretic peptides previously isolated from Tenebrio molitor, Tenmo-DH(37) and Tenmo-DH(47), were found to stimulate Tenebrio molitor tubules in vitro in a dose-dependent manner with EC(50) values of 0.12 nmol l(-1) and 26 nmol l(-1) respectively. However, no synergistic or additive effect was observed when these two peptides were tested simultaneously. We then investigated antagonism between second messengers: dose-response curves were constructed for stimulation of Tenebrio molitor tubules by cyclic AMP and their inhibition by cyclic GMP. When both cyclic nucleotides were included in the bathing Ringer, the stimulatory effect of cyclic AMP was neutralised by cyclic GMP. Similarly, the stimulatory effect of Tenmo-DH(37) was reversed on addition of an antidiuretic peptide (Tenmo-ADF), which was recently isolated from Tenebrio molitor and acts via cyclic GMP. The cardioacceleratory peptide CAP(2b), originally isolated from Manduca sexta, also increases intracellular cyclic GMP levels and inhibited fluid secretion by Tenebrio molitor tubules, with an EC(50) value of 85 nmol l(-1). This inhibitory effect was reversed by Tenmo-DH(37). Endogenous diuretic and antidiuretic peptides, effective at low concentrations and acting via antagonistic second messengers, have the potential for fine control of secretion rates in the Malpighian tubules of Tenebrio molitor.
authors.affiliation: Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa.
authors.forename: U I M
authors.initials: UI
authors.lastname: Wiehart
authors.forename: S W
authors.initials: SW
authors.lastname: Nicolson
authors.forename: R A
authors.initials: RA
authors.lastname: Eigenheer
authors.forename: D A
authors.initials: DA
authors.lastname: Schooley
mesh_terms: D000367:Age Factors; D000818:Animals; D001826:Body Fluids / Q000187:drug effects / Q000378:metabolism*; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology / Q000502:physiology; D000242:Cyclic AMP / Q000494:pharmacology; D006152:Cyclic GMP / Q000494:pharmacology; D004232:Diuretics / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D007301:Insect Hormones / Q000494:pharmacology / Q000502:physiology; D019476:Insect Proteins / Q000302:isolation & purification / Q000494:pharmacology*; D008317:Malpighian Tubules / Q000187:drug effects / Q000502:physiology*; D009479:Neuropeptides / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology; D011761:Pyrrolidonecarboxylic Acid / Q000031:analogs & derivatives; D015290:Second Messenger Systems / Q000502:physiology*; D013712:Tenebrio / Q000502:physiology*
chemical_list: D004232:Diuretics; D007301:Insect Hormones; D019476:Insect Proteins; D009479:Neuropeptides; D009842:Oligopeptides; C095817:cardioacceleratory peptide 2b; D003346:Corticotropin-Releasing Hormone; D000242:Cyclic AMP; D006152:Cyclic GMP; D011761:Pyrrolidonecarboxylic Acid