Cellular mechanism of HCO 3 − and Cl− transport in insect salt gland

Active HCO ₃ ^t- secretion in the anterior rectal salt gland of the mosquito larva,Aedes dorsalis, is mediated by a 1∶1 Cl⁻/HCO ₃ ⁻ exchanger. The cellular mechanisms of HCO ₃ ⁻ and Cl⁻ transport are examined using ion- and voltage-sensitive microelectrodes in conjunction with a microperfused preparation which allowed rapid saline changes. Addition of DIDS or acetazolamide to, or removal of CO₂ and HCO ₃ ⁻ from, the serosal bath caused large (20 to 50 mV) hyperpolarizations of apical membrane potential (V _a) and had little effect on basolateral potential (V _bl). Changes in luminal Cl⁻ concentration alteredV _a in a repid, linear manner with a slope of 42.2 mV/decaloga _Cl ^l −. Intracellular Cl⁻ activity was 23.5mm and was approximately 10mm lower than that predicted for a passive distribution across the apical membrane. Changes in serosal Cl⁻ concentration had no effect onV _bl, indicating an electrically silent basolateral Cl⁻ exit step. Intracellular pH in anterior rectal cells was 7.67 and the calculated $a_{HCO_3 }^c $ was 14.4mm. These results show that under control conditions HCO₃ enters the anterior rectal cell by an active mechanism against an electrochemical gradient of 77.1 mV and exits the cell at the apical membrane down a favorable electrochemical gradient of 27.6 mV. A tentative cellular model is proposed in which Cl enters the apical membrane of the anterior rectal cells by passive, electrodiffusive movement through a Cl⁻-selective channel, and HCO ₃ ⁻ exits the cell by an active or passive electrogenic transport mechanism. The electrically silent nature of basolateral Cl⁻ exit and HCO₃ entry, and the effects of serosal addition of the Cl⁻/HCO₃ exchange inhibitor, DIDS, on $J_{net}^{CO_2 } $ and transepithelial potential (V _ic) suggest strongly that the basolateral membrane is the site of a direct coupling between Cl⁻ and HCO ₃ ⁻ movements.