Secretion of Water and Ions by Malpighian Tubules of Larval Mosquitoes: Effects of Diuretic Factors, Second Messengers, and Salinity

The effects of changes in the salinity of the rearing medium on Malpighian tubule fluid secretion and ion transport were examined in larvae of the freshwater mosquitoAedes aegyptiand the saltwater speciesOchlerotatus taeniorhynchus. For unstimulated tubules of both species, the K+concentration of secreted fluid was significantly lower when larvae were reared in 30% or 100% seawater (O. taeniorhynchusonly), relative to tubules from freshwater‐reared larvae. The Na+concentration of secreted fluid from unstimulated tubules ofO. taeniorhynchusreared in 30% or 100% seawater was higher relative to tubules from freshwater‐reared larvae. The results suggest that changes in salinity of the larval rearing medium lead to sustained changes in ion transport mechanisms in unstimulated tubules. Furthermore, alterations of K+transport may be utilized to either conserve Na+under freshwater (Na+‐deprived) conditions or eliminate more Na+in saline (Na+‐rich) conditions. The secretagogues cyclic AMP [cAMP], cyclic GMP [cGMP], leucokinin‐VIII, and thapsigargin stimulated fluid secretion by tubules of both species. Cyclic AMP increased K+concentration and decreased Na+concentration in the fluid secreted by tubules isolated fromO. taeniorhynchuslarvae reared in 100% seawater. Interactions between rearing salinity and cGMP actions were similar to those for cAMP. Leucokinin‐VIII and thapsigargin had no effect on secreted fluid Na+or K+concentrations. Results indicate that changes in rearing medium salinity affect the nature and extent of stimulation of fluid and ion secretion by secretagogues.