Phenotypic plasticity in response to dietary salt stress: Na+ and K+ transport by the gut ofDrosophila melanogasterlarvae

Abstract

SUMMARYDrosophila provides a useful model system for studies of the mechanisms involved in regulation of internal ion levels in response to variations in dietary salt load. This study assessed whether alterations in Na + and K + transport by the gut of larval D. melanogaster reared on salt-rich diets contribute to haemolymph ionoregulation. Na + and K + fluxes across the isolated guts of third instar larvae reared on control or salt-rich diets were measured using the scanning ion-selective electrode technique (SIET). K + absorption across the anterior portion of the posterior midgut of larvae reared on diet in which the concentration of KCl was increased 0.4moll -1 above that in the control diet was reduced eightfold relative to the same gut segment of larvae reared on the control diet. There was also an increase in the magnitude and extent of K + secretion across the posterior half of the posterior midgut. Na + was absorbed across the ileum of larvae reared on the control diet, but was secreted across the ileum of larvae reared on diet in which the concentration of NaCl was increased 0.4moll -1 above that in the control diet. There was also a small reduction in the extent of Na + absorption across the middle midgut of larvae reared on the NaCl-rich diet. The results indicate considerable phenotypic plasticity with respect to K + and Na + transport by the gut epithelia of larval D. melanogaster. SIET measurements of K + and Na + fluxes along the length of the gut show that ion transport mechanisms of the gut are reconfigured during salt stress so that there are reductions in K + and Na + absorption and increases in K + and Na + secretion. Together with previously described changes in salt secretion by the Malpighian tubules, these changes contribute to haemolymph ionoregulation.

Tissue specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larvalChironomous ripariusexposed to varying salinity

“…For example, ion transport mechanisms of the gut epithelia of larval Drosophila are reconfigured during dietary salt stress so that there are reductions in K + and Na + absorption and increased K + and Na + secretion (Naikkhwah and O'Donnell, 2012). In the present study, we showed that NKA and VA activity in the FAMG of C. riparius larvae acclimated to varying salinity remain largely unaltered (Fig.2B, Fig.3B).…”

The rectal complex and Malpighian tubules of the cabbage looper (Trichoplusia ni): regional variations in Na+ and K+ transport and cation reabsorption by secondary cells

“…SIET makes use of an ion-selective microelectrode that is moved between two positions within the unstirred layer; the measured difference in ion concentration between the two positions is converted into a corresponding ion flux across the tissue using the Fick equation. Hardware and software used for SIET have been described previously (Naikkhwah and O'Donnell, 2012). K + -selective microelectrodes were backfilled with 150 mmol l…”

“…Positioning was followed by a 4.0 s wait period, during which no measurements were made, to allow the re-establishment of ion gradients at the tissue surface following the localized disturbance of the microelectrode movement. A wait time of 3 s is sufficient to allow the gradients to fully reestablish (Naikkhwah and O'Donnell, 2012). The voltage at the microelectrode tip was then recorded during a sample period of 0.5 s following the wait period.…”

Parallel ionoregulatory adjustments underlie phenotypic plasticity and evolution ofDrosophilacold tolerance

“…Although fluid secretion at the Malpighian tubules is primarily energized by the proton-motive V-ATPase coupled to a H + /K + exchanger, Na + /K + -ATPase in the basolateral membrane of Malpighian tubule principal cells is important in modulating rates of Na + and water secretion (Linton and O'Donnell, 1999;Beyenbach et al, 2010 (Linton and O'Donnell, 1999). High Na + /K + -ATPase activity in the basal membrane of the hindgut is responsible for transporting Na + back into the haemolymph of the mosquito Aedes aegypti (Diptera: Culicidae) (Patrick et al, 2006), and net Na + is transported into the haemolymph in the larval hindgut of D. melanogaster (Naikkhwah and O'Donnell, 2012 + gradients that drive Na + leak and water balance disruption at low temperatures, and thereby improve cold tolerance. This hypothesis is supported by the repeated observation that cold acclimation of insects results in less-polarized muscle Na + equilibrium potentials (Koštál et al, 2004;Koštál et al, 2006;Coello Alvarado, 2012).…”