Determination of mosquito bloodmeal pH in situ by ion-selective microelectrode measurement: implications for the regulation of malarial gametogenesis

Malarial gametocytes circulate in the peripheral blood of the vertebrate ho st as developmentally arrested intra-erythrocytic cells, which only resume development into gametes when ingested into the bloodmeal of the female mos quito vector. The ensuing development encompasses sexual reproduction and m ediates parasite transmission to the insect. In vitro the induction of game togenesis requires a drop in temperature and either a pH increase from phys iological blood pH (ca pH 7.4) to about pH 8.0, or the presence of a gameto cyte-activating factor recently identified as xanthurenic acid (XA). Howeve r, it is unclear whether either the pH increase or XA act as natural trigge rs in the mosquito bloodmeal. We here use pH-sensitive microelectrodes to d etermine bloodmeal pH in intact mosquitoes. Measurements taken in the first 30 min after ingestion, when malarial gametogenesis is induced in vivo, re vealed small pH increases from 7.40 (mouse blood) to 7.52 in Aedes aegypti and to 7.58 in Anopheles stephensi. However, bloodmeal pH was clearly subop timal if compared to values required to induce gametogenesis in vitro. Xant hurenic acid is shown to extend the pH-range of exflagellation in vitro in a dose-dependent manner to values that we have observed in the bloodmeal, s uggesting that in vivo malarial gametogenesis could be further regulated by both these factors.