Mechanisms and regulation of chloride transport in the Malpighian tubules of the larval cabbage looper Trichoplusia ni

Malpighian tubules (MTs) and the hindgut together constitute the excretory system of insects. Larvae of lepidopterans (butterflies and moths) demonstrate the so-called cryptonephric arrangement, where the distal blind end of each MT is embedded into the rectal complex. The rest of the free tubule is modified into several distinct regions that differ greatly in the transport of cations and water. However, relatively little is known about the transport of counter-anions (e.g., Cl− and HCO3−) by the MTs of lepidopteran larvae. In the current study we used ion-selective microelectrodes to characterize Cl− transport in the distinct regions of the free MT of the larval Trichoplusia ni. Firstly, we note that Cl− transport in the MTs is sensitive to the Cl− concentration of the bathing saline, and several regions of the MTs are capable of either secreting or reabsorbing Cl−. In the distal ileac plexus (DIP), a region previously characterized by cellular heterogeneity and its ability to switch between cation secretion and reabsorption, principal cells (PCs) toggled between Cl− reabsorption (in high-Cl- saline) and Cl− secretion (in low-Cl- saline). In contrast, secondary cells (SCs) in the DIP secreted Cl− regardless of saline Cl− concentration. Mechanistically, we have detected a number of ‘leak’ and ligand-gated Cl− channels (ClC) and demonstrated that Cl− channels are involved in Cl− secretion. Additionally, we demonstrated that the lumen-positive transepithelial potential increased in response to glycine. Using the scanning ion-selective electrode technique we demonstrated that glycine stimulated Cl− secretion by SCs, but not by PCs. In contrast, when MTs were deprived of glycine, a decrease in Cl− secretion, coupled with a decrease in the TEP, was observed. In contrast to the effects of glycine, an active dose of helicokinin reduced Cl− secretion by PCs, but not by SCs. Lastly, we detected expression of chloride-bicarbonate exchangers (CBE) in all regions of the free tubule. Scans of H+ transport across the tubule indicated that base equivalents are likely reabsorbed across the ileac plexus. Blocking ClC or CBE led to secretion of a more basic fluid, indicating lack of base reabsorption. We suggest that the transport of Cl− in the MTs of larval lepidopterans (i) may be correlated with the reabsorption of base, (ii) may be sensitive to Cl− concentration in the haemolymph, and (iii) could be regulated by helicokinin and glycine.