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The Journal of Experimental Biology 205, 493-501 (2002)
© 2002 The Company of Biologists Limited

Antagonistic control of fluid secretion by the Malpighian tubules of Tenebrio molitor: effects of diuretic and antidiuretic peptides and their second messengers

U. I. M. Wiehart¹, S. W. Nicolson^1,*, R. A. Eigenheer² and D. A. Schooley²

¹ Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa and
² Department of Biochemistry, University of Nevada, Reno, NV 89503, USA

*Author for correspondence and present address: Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa (e-mail: swnicolson{at}zoology.up.ac.za)

Accepted 28 November 2001

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-DH₃₇ and Tenmo-DH₄₇, were found to stimulate Tenebrio molitor tubules in vitro in a dose-dependent manner with EC₅₀ 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-DH₃₇ 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 EC₅₀ value of 85 nmol l^–1. This inhibitory effect was reversed by Tenmo-DH₃₇. 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.

Key words: diuretic peptide, antidiuretic peptide, cyclic AMP, cyclic GMP, antagonism, fluid secretion, Malpighian tubule, mealworm, Tenebrio molitor, Coleoptera.

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