2792 INTRODUCTION Lutzomyia longipalpis (Lutz and Neiva 1912) is the main phlebotomine vector of Leishmania infantum (Nicolle 1908) [syn. L. chagasi (Cunha and Chagas 1937)], the etiologic agent of the visceral leishmaniasis in the Americas (Soares and Turco, 2003). As for any other haemathophagous insects, nutrient digestion and absorption after blood feeding is one of the most important events for L. longipalpis, which uses these nutrients to produce eggs. Since the digestive processes are essentially enzymatic and enzyme activities are influenced by the hydrogen ion concentration (pH) in the intestinal environment, studies of pH and the mechanisms involved in pH control become extremely relevant. This importance increases if it is considered that Leishmania parasites ingested through an infective blood meal develop exclusively in the phlebotomine gut, from where they are transmitted to a vertebrate host by biting (Bates and Rogers, 2004). In fact, pH could be one of the most important factors in Leishmania development within the vector, as well as for the normal functioning of the gut. It has been shown that Leishmania promastigotes cultivated at pH 5.5 differentiate into metacyclic forms in much larger numbers than those cultivated at pH 7.6 (Bates and Tetley, 1993; Zakai et al., 1998). Acidification of the medium seems to be one of the main stimuli that determines the differentiation of Leishmania in vitro and probably plays a similar role during their development in the sand fly gut (Gontijo et al., 1998). Although unknown in mammals, a high gut pH (above 9.0) is common in some insects, especially in larvae from the orders Lepidoptera and Diptera (suborder Nematocera only) (Terra et al., 1996). Some models have been proposed in order to explain the high physiological pH observed in these insects. All of them include the participation of the enzyme carbonic anhydrase and the hydration of CO 2 molecules (Boudko et al., 2001a; Boudko et al., 2001b). In adult females of phlebotomine sand flies and mosquitoes (Nematocera), the physiology of the midgut is different. These insects go from a diet composed basically of carbohydrates to one of blood. This implies great modifications in the midgut physiology especially in the production of digestive enzymes and the promotion of a slightly alkaline pH necessary for their action. Studying the pH of the blood bolus inside the midgut of some blood-fed mosquitoes, Billker and colleagues proposed that the alkalization observed [pH 7.4 to 7.52 and 7.58 in Aedes aegypti (Linnaeus 1762) and Anopheles stephensi (Liston 1901), respectively] might be attributed to the phenomenon of CO 2 volatilization (Billker et al., 2000). The loss of CO 2 from blood equilibrating with air leads to a reduction in H + concentration in accordance with the equation: CO 2 +H 2 O}H 2 CO 3 }HCO 3 – +H + . The authors showed that the pH of 2–3 μl of blood reached a value of 8.0 when exposed to the air for 10 min. On the other hand, del Pilar Corena and colleagues observed a higher alkalization, to pH 8 or more, in the midgut of seven species of adult mosquito after feeding (del Pilar Corena et al., 2005). Taking into account the fact that these mosquitoes were fed not with blood but with a substitute solution containing indicator dyes, the CO 2 volatilization mechanism cannot be considered the only mechanism responsible for the alkalization observed in blood-fed insects. Probably, both the CO 2 volatilization and a second mechanism The Journal of Experimental Biology 211, 2792-2798 Published by The Company of Biologists 2008 doi:10.1242/jeb.019836 The physiology of the midgut of Lutzomyia longipalpis (Lutz and Neiva 1912): pH in different physiological conditions and mechanisms involved in its control Vânia C. Santos, Ricardo N. Araujo, Luciane A. D. Machado, Marcos H. Pereira and Nelder F. Gontijo* Department of Parasitology, Federal University of Minas Gerais–UFMG, Avenue Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil *Author for correspondence (e-mail: nelder@icb.ufmg.br) Accepted 25 June 2008 SUMMARY Nutrient digestion and absorption after blood feeding are important events for Lutzomyia longipalpis, which uses these nutrients to produce eggs. In this context, the pH inside the digestive tract is an important physiological feature as it can markedly influence the digestive process as well as interfere with Leishmania development in infected phlebotomines. It was described previously that unfed females have an acidic midgut (pH 6). In this study, the pH inside the midgut of blood-fed females was measured. The abdominal midgut (AM) pH varied from 8.15±0.31 in the first 10 h post-blood meal to 7.7±0.17 after 24 h. While the AM was alkaline during blood digestion, the pH in the thoracic midgut (TM) remained acidic (5.5–6.0). In agreement with these findings, the enzyme α-glucosidase, which has an optimum pH of 5.8, is mainly encountered in the acidic TM. The capacity of unfed females to maintain the acidic intestinal pH was also evaluated. Our results showed the presence of an efficient mechanism that maintains the pH almost constant at about 6 in the midgut, but not in the crop. This mechanism is promptly interrupted in the AM by blood ingestion. RT-PCR results indicated the presence of carbonic anhydrase in the midgut cells, which apparently is required to maintain the pH at 6 in the midgut of unfed females. Investigations on the phenomenon of alkalization observed after blood ingestion indicated that two mechanisms are involved: in addition to the alkalization promoted by CO 2 volatilization there is a minor contribution from a second mechanism not yet characterized. Some inferences concerning Leishmania development and pH in the digestive tube are presented. Key words: Lutzomyia longipalpis, midgut pH, pH control mechanisms, acidification, alkalization, Leishmania development. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY