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

Abstract

Summary A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na+-K+-ATPase (NKA) and V-type H+-ATPase (VA) in the alimentary canal and (2) rectal K+ transport with scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut (FAMG), the posterior midgut (PMG), the Malpighian tubules (MT) and the hindgut (HG). Both enzymes exhibited 10 - 20 times greater activity in the HG relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in HG enzyme activity was observed. However, in brackish water (BW) reared animals, NKA and VA activity in the HG significantly decreased. Immunolocalization of NKA and VA in the HG revealed that the bulk of protein was located in the rectum. Therefore K+ transport across the rectum was examined using SIET. Measurement of K+ flux along the rectum revealed a net K+ reabsorption which was reduced four-fold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K+ channels with charybdotoxin, diminished rectal K+ reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

Allatostatin A-like immunoreactivity in the nervous system and gut of the larval midge, Chironomus riparius (Meigen): Modulation of hindgut motility, rectal K+ transport and implications for exposure to salinity

“…The rectum of larval C. riparius is a site of K + absorption (see Jonusaite et al, 2013). The mean baseline K + flux (absorption, lumen to bath) measured at the recta in this study was 104.3±13.7 pmol cm −2 s −1 (n=38).…”

“…Therefore, salinity and AST7 have similar effects on rectal NKA and VA activities. The actions of AST7 occur rapidly and hence cannot be mediated by changes in NKA and VA expression; however, it was suggested that larvae reared in brackish water may have lower expression of rectal NKA and VA than those reared in freshwater (Jonusaite et al, 2013). In an effort to determine whether the mechanisms of AST7-and salinity-induced changes in NKA and VA rectal activities are similar or different, NKA and VA protein expression studies on freshwaterand brackish-water-reared larvae were carried out.…”

“…6). NKA and VA protein expression in recta of freshwater-and brackish-water-reared larvae A previous study demonstrated that recta of brackish-water-reared larvae have lower activities of NKA and VA compared with recta of freshwater-reared larvae (Jonusaite et al, 2013). Therefore, salinity and AST7 have similar effects on rectal NKA and VA activities.…”

“…4). Application of 1 μmol l −1 Dippu-AST7 did not abolish contractions but inhibited the contractions by ~95%.Effects of Dippu-AST7 on K + absorption by the rectum, and NKA and VA activities in the rectumThe rectum of larval C. riparius is a site of K + absorption (see Jonusaite et al, 2013). The mean baseline K + flux (absorption, lumen to bath) measured at the recta in this study was 104.3±13.7 pmol cm −2 s −1 (n=38).…”

“…Abrupt transfer to brackish water is accompanied by a sharp reduction in whole-body VA activity and a less pronounced decrease in whole-body NKA activity (Jonusaite et al, 2011). The greatest amount of NKA and VA activity can be attributed to the rectum, and the recta of larvae reared in brackish water have greatly reduced NKA and VA activities, which coincide with reduced ion reabsorption (Jonusaite et al, 2013).Although the larval response to abrupt and long-term (rearing) exposure to brackish water is the same, the mechanisms with which this is achieved are likely to be different. Abrupt exposure to elevated salinity would require a rapid response that could be…”