Revisiting the cellular mechanisms of strong luminal alkalinization in the anterior midgut of larval mosquitoes

Here we critically review two recent hypotheses about the mechanism of strong alkalinization by the anterior midgut of mosquito larvae and our tests of these hypotheses. We present experimental evidence against the major components of transport models proposed in these hypotheses. Measurements of the transapical and transbasal proton electrochemical gradients provide an indication of driving forces faced by and generated by the transport mechanisms of the tissue. These measurements confirmed that basal V-ATPase energizes alkalinization. Serotonin stimulates the V-ATPase, as indicated by the ensuing increase in proton-motive force across the basal membrane. Moreover, the neurohormone resulted in a surprisingly large increase in the intracellular pH. The results of inhibitor studies indicate that, contrary to previous proposals, carbonic anhydrase is apparently not involved in supplying acid-base equivalents to the respective transporters. Furthermore, any apical processes proposed to be involved in alkali secretion or acid absorption must be Cl- independent and insensitive to DIDS, amiloride, Zn2+ and ouabain. These results argue against the involvement of putative apical Cl-/HCO3- exchangers, apical H+ channels, apical cation/proton exchangers and the importance of the apical Na+/K+ pump. The studies analyzed here thus provide both a limitation and direction for further studies of the mechanism of strong alkalinization in this system.