Signaling by intracellular Ca2+ and H+ in larval mosquito (Aedes aegypti) midgut epithelium in response to serosal serotonin and lumen pH

The midgut of larval mosquitoes (Aedes aegypti) mediates a cycle of alkali secretion in the anterior segment (AMG) followed by partial reacidification in the posterior segment (PMG); both processes are serotonin-dependent. Here we report that intracellular Ca2+(Cai2+) as indicated by Fura-2 fluorescence, is elevated in both tissues in response to serotonin, but the time courses differ characteristically in the two gut segments, and Ca2+-free solution abolishes the serotonin response in AMG, but not in PMG, whereas Thapsigargin, an inhibitor of endoplasmic Ca2+ transport, abolished responsiveness to 5-HT in PMG. These results suggest the origins for the Ca2+ signal differ between the two tissues. Quantitative real-time RT-PCR revealed expression of 5 putative 5-HT receptor types in AMG, including 5-HT2-like receptors which would be expected to initiate a Ca2+ signal. None of these receptors were highly expressed in PMG. Cyclic AMP (cAMP) is a secretagogue for both tissues, but H89, an inhibitor of Protein Kinase A (PKA), is also a secretagogue, suggesting that the stimulatory effect of cAMP involves a non-PKA pathway. Cytochalasins B and D block the effect of 5-HT in AMG, suggesting a vesicle-fusion mechanism of activation of the basal V-ATPase in this tissue. Finally, in PMG, elevation of luminal pH increases (Cai2+) and decreases intracellular pH as measured by BCECF fluorescence. These responses suggest that the rate of acid secretion by PMG might be responsive to local demand for luminal reacidification as well as to serosal serotonin.

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► Ca2+ 2nd messages are required for serotonin-stimulated acid–base secretion in mosquito midgut.
► Cyclic AMP is also a secretagogue but cannot substitute for the Ca2+ signals.
► Putative 5-HT receptors were found in the anterior midgut but not the posterior midgut.
► The posterior midgut may regulate luminal pH through changes in cellular Ca2+ and H+.