Transepithelial Potential and Alkalization in an in Situ Preparation of Tobacco Hornworm (manduca Sexta) Midgut

The midgut of larval lepidopteran insects rapidly raises the pH of the entering food from approximately neutral values to as high as pH 12 in some species (Waterhouse, 1949; Dow, 1984). These values are among the highest achieved by biological systems and are all the more remarkable in view of the high rate of food consumption relative to body mass in lepidopteran larvae. The isolated midgut of lepidopteran larvae secretes K+ at a high rate by an electrogenic mechanism that generates a transepithelial electric potential (TEP) which may initially exceed 100 mV in isolated preparations (Harvey et al. 1968). It has been suggested (Dow, 1984, 1992; Dow and Harvey, 1988) that alkalization is a secondary process driven by the TEP, although Dow (1992) recognized that an almost unrealistically high TEP would be required in vivo to account for measured H+ activity gradients of 3­5 orders of magnitude in freshly dissected animals. Values for the TEP of the midgut in vivo of 'around 150 mV' (Dow and Harvey, 1988) or 'between 100 and 150 mV' have been described as 'accepted' (Dow, 1992). However, to our knowledge, no experiments supporting these values have been published. Alkalization has been studied using isolated tissues, but the pH gradients and rates of alkalization achieved by those preparations have been less impressive than those measured or estimated in vivo. In the work of Dow and O'Donnell (1990), the maximum pH gradient of an isolated, double-perfused preparation was about 2 pH units, a value not inconsistent with Nernstian equilibrium with the transepithelial potentials typical of such preparations. It should be emphasized that these measurements were from small volumes of solution in the microenvironment immediately adjacent to the epithelium. During hypoxic transitions, the pH gradients and transepithelial potential were lost within minutes of imposition of N2-hypoxia and recovered on approximately the same time scale as the recovery of the TEP, suggesting that H+ equilibrated readily across the gut epithelium under hypoxic conditions.