Malpighian tubules of Rhodnius prolixus and Drosophila melanogaster. Secretion rates of Rhodnius tubules in amino-acid-free saline increase after addition of serotonin

contains a total concentration of amino acids 50–100 times that which is normal for mammalian plasma (Wigglesworth, 1972). Total haemolymph amino acid concentrations vary from 16 mmol l–1 in aquatic midges (Chironomus tentans; Firling, 1977) to 33 mmol l–1 in black flies (Simulium venustum; Gordon and Bailey, 1974), 54 mmol l–1 in blow flies (Calliphora vicina; Evans and Crossley, 1974), 90 mmol l–1 in locusts (Locusta migratoria; Zanotto et al., 1997) and 150 mmol l–1 in tsetse flies (Glossina austeni; Tobe, 1978). Concentrations of proline, the predominant amino acid in the haemolymph of Rhodnius and tsetse flies are 14–21 mmol l–1 (Barrett, 1974) and 45–90 mmol l–1 (Tobe, 1978), respectively. In Calliphora, the most abundant amino acids are glutamine (8 mmol l–1), proline (7 mmol l–1), alanine (6 mmol l–1) and glycine (6 mmol l–1; Evans and Crossley, 1974). Amino acids may perform a number of functions and so may be particularly important for tissues such as the Malpighian tubules (MTs) that are bathed in the haemolymph. The nonprotein amino acid canavanine has been shown to inhibit fluid secretion in isolated Locusta MTs but to potentiate the subsequent response of the tubules to stimulation with cAMP or diuretic hormone (Rafaeli and Applebaum, 1980). In most cells, amino acids act as intracellular compatible osmolytes (Yancey et al., 1982). Glutamine, for example, is a major compatible osmolyte engaged in the role of cell volume control as a response to cell shrinkage (Fumarola et al., 2001). Taurine is a non-protein amino acid and compatible osmolyte that modulates ion transport by many tissues (Guizouarn et al., 2000; Law, 1994). Proline is an important compatible osmolyte in both intracellular and extracellular fluids of mosquito larvae (Patrick and Bradley, 2000). Some amino acids play pivotal 79 The Journal of Experimental Biology 206, 79-91 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00058