Stimulation of Malpighian tubules from larval Aedes aegypti by secretagogues

Abstract

Dibutyryl-cAMP (db-cAMP) and 5-hydroxytryptamine (5-HT) stimulate secretion rates of fluid, sodium, and potassium by Malpighian tubules from larval female Aedes aegypti, without affecting the sodium and potassium concentrations of the secreted fluid. High-potassium saline extracts of larval and adult tissues also cause increased fluid secretion rates by these tubules. The maximal rate of fluid secretion is agonist-dependent, and tubules secrete at the greatest rates in response to stimulation by 5-HT. Bumetanide has no effect on the fluid secretion rates of larval female tubules stimulated with 5-HT or db-cAMP. The head is the most concentrated source of diuretic neurosecretory material in larvae. The amount of material released into high-potassium saline from a single larval head causes a maximal response by a Malpighian tubule in vitro. The diuretic material in high-potassium saline extracts of larval heads is a peptide or peptides, apparently of small molecular weight. Enzymatic digestion studies reveal that it probably has a capped N-terminus, and a sialyl group is not required for activity. Size exclusion chromatography followed by reversed-phase HPLC yields a single peak of diuretic activity.