Canavanine potentiates the response of insect malpighian tubules to diuretic hormone and cyclic AMP

Abstract

Insects regulate the amount of water they excrete via their nalpighian tubules, the degree of stringency depending on the hydration of their diet and on the frequency at which they feed. The excretion of water is controlled by a diuretic hormone, which is released as required into the haemolymph by the corpora cardiaca^1,2. Previous reports^3–5 have shown that non-protein amino acids, known to be neurotoxic in vertebrates⁶ as well as in insects⁷, lead to a disruption of water balance in nsects. However, the exact mode of action of these amino acids has not been elucidated. We report here on preliminary experiments using isolated malpighian tubules of locusts Locusta migratoria migratorioides (R & F)) which 9 show that he non-protein amino acid L-canavanine inhibits fluid secretion in vitro by isolated tubules. Furthermore, when tubules, previously inhibited by canavanine, are subsequently stimulated by exogenous diuretic hormone (methanol extracts of storage lobes of corpora cardiaca^1,2) or cyclic AMP (which mimics the action of diuretic hormone¹), the stimulation obtained is much greater than that normally expected from diuretic hormone or cyclic AMP.