Serotonin-induced high intracellular pH aids in alkali secretion in the anterior midgut of larval yellow fever mosquito Aedes aegypti L.

The anterior midgut of the larval yellow fever mosquito Aedes aegypti generates a luminal pH in excess of 10 in vivo and similar values are attained by isolated and perfused anterior midgut segments after stimulation with submicromolar serotonin. In the present study we investigated the mechanisms of strong luminal alkalinization using the intracellular fluorescent indicator BCECF-AM. Following stimulation with serotonin, we observed that intracellular pH (pH(i)) of the anterior midgut increased from a mean of 6.89 to a mean of 7.62, whereas pH(i) of the posterior midgut did not change in response to serotonin. Moreover, a further increase of pH(i) to 8.58 occurred when the pH of the luminal perfusate was raised to an in vivo-like value of 10.0. Luminal Zn(2+) (10 micromol l(-1)), an inhibitor of conductive proton pathways, did not inhibit the increase in pH(i), the transepithelial voltage, or the capacity of the isolated tissue to alkalinize the lumen. Finally, the transapical voltage did not significantly respond to luminal pH changes induced either by perfusion with pH 10 or by stopping the luminal perfusion with unbuffered solution which results in spontaneous luminal alkalinization. Together, these results seem to rule out the involvement of conductive pathways for proton absorption across the apical membrane and suggest that a serotonin-induced alkaline pH(i) plays an important role in the generation of an alkaline lumen.