Clark, T.M.: Integrative Aspects of Epithelial Transport in Larval Aedes aegypti

In contrast to the extensively studied hormonal regulation of Malpighian tubule function, regulation of midgut transport has been largely ignored. 5-hydroxytryptamine (5-HT; serotonin) is perhaps best known among insect transport physiologists as one of a suite of hormones regulating Malpighian tubule function. In larval Aedes aegypti, Malpighian tubule secretion rates are stimulated by increasing hemolymph 5-HT concentrations as ambient salinity increases. First principles dictate that larvae must coordinate changes in Malpighian tubule secretion rates with selective recovery of water and ions in other regions of the excretory system to remain in homeostasis as ambient salinity changes. Recent evidence suggests that the midgut is involved in this coordinated regulation of hemolymph composition. 5-HT at concentrations known to exist in hemolymph and to stimulate Malpighian tubule transport hyperpolarizes the transepithelial potential of both the anterior and the posterior stomach regions. 5-HT regulates different mechanisms in the two regions. The actions of 5-HT in anterior stomach are confined to the basal membrane of a subset of cells. The physiological significance of stimulation of midgut and Malpighian tubule function by a single hormone is not yet understood. However, stimulation by 5-HT of both midgut and Malpighian tubule transport provides support for a model in which the actions of multiple systems are coordinated by asingle hormone to regulate hemolymph composition. Furthermore, stimulation by 5-HT of midgut and Malpighian tubule transport in response to saline loads in exopterygotes (Rhodnius), and endopterygotes (Aedes), suggests that its primary function as a regulator of hemolymph composition through coordinated actions on organ systems traditionally considered separately may be very highly conserved among insects.