Tissue specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomous riparius exposed 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.