Differential effects of secretagogues on Na and K secretion in the Malpighian tubules ofAedes aegypti (L.)

Abstract

1. Rates of fluid secretion were measured in isolated Malpighian tubules from adult female mosquitoes and the concentrations of Na, K and Cl in the secreted fluid were measured by electron probe analysis under control conditions and in the presence of the secretagogues dibutyryl cAMP or head extract. 2. Under control conditions tubules secreted fluid at an average rate of 0.8 nl/min. The principal solutes in secreted fluid were Na at 94 mM, K at 91 mM and Cl at 161 mM. 3. Dibutyryl cAMP (1 mM) stimulated the rate of fluid secretion ∼250% (to 2.9 nl/min), increased the [Na] of secreted fluid to 178 mM, decreased the [K] to 17 mM, and did not change the [Cl]. Consequently, the net effect of dibutyryl cAMP was a highly significant stimulation of Na, Cl, and water secretion with no significant change in the secretion of K. 4. An aqueous extract of mosquito heads (0.1 head/μl) also stimulated the rate of fluid secretion ∼250% (to 2.8 nl/min), but introduced no major changes in the composition of secreted fluid with the exception of a slight, though significant decrease in the [K] from 91 to 73 mM. Consequently, the net effect of the head extract was the highly significant stimulation of Na, K, Cl, and water secretion. 5. The different effects of dibutyryl cAMP and head extract on the secretion of cations suggest that Na secretion and K secretion can be regulated independently of one another. In vivo, such a separate regulation of Na and K secretion might be important in the maintenance of hemolymph Na and K concentrations, especially following a blood meal.