The pH profile, gross structure, ultrastructure and immunolabeling of the mosquito (Aedes aegypti) larval midgut are described as a first step in analyzing the role of plasma membrane H+ V-ATPase in the alkalization

mosquito larvae (Dow, 1984). The alkalinity is thought to aid in the breakdown of dietary tannins, which interfere with nutrient absorption (Berenbaum, 1980). The alkalization mechanism is partially understood in caterpillars, especially in the model lepidopteran insect Manduca sexta, in which a proton pump (H+ V-ATPase; Schweikl et al., 1989; Wieczorek et al., 1986, 1989, 1990) hyperpolarizes the apical membranes of specialized ‘goblet cells’ (Harvey et al., 1983) to 240 mV (Dow and Peacock, 1989). The voltage drives an electrophoretic exchange (antiport) of luminal H+ for cellular K+, which accounts, in part, for the alkalinity of the lumen (Wieczorek et al., 1991; Azuma et al., 1995; for a review see Wieczorek et al., 1999). The voltage also appears across the adjacent columnar cell apical membrane (brush-border membrane), where it drives electrophoretic cotransport (symport) of amino acids, coupled to K+, from the lumen into the cells (Harvey and Wieczorek, 1997; Castagna et al., 1998; D. H. Feldman, W. R. Harvey and B. R. Stevens, manuscript in preparation). Little is known about the alkalization mechanism in mosquito larvae beyond the observations of Dadd (1975, 1976) that the pH can exceed 11 in anterior midgut (e.g. in Aedes aegypti), but is lower in posterior midgut (pH 8), and that it is rapidly but reversibly lowered by agents or conditions that interfere with cellular respiration. The ultrastructure of the larval midgut of fresh water mosquito has been studied (Volkmann and Peters, 1989a,b) but not in relation to solute transport (Cioffi, 1984). Are ‘goblet cells’ present in mosquito larval midgut and do they play a role in alkalization? Do an H+ V-ATPase and H+-coupled antiporter energize the alkalization, and, if so, which plasma membranes are energized? To begin answering such questions we have 2449 The Journal of Experimental Biology 202, 2449–2460 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB2293