Short Communication Lumtnal Alkalinization by the Isolated Mtdgut of the Tobacco Hornworm (manduca Sexta)

The midgut contents of lepidopteran larvae are often several pH units more basic than the haemolymph (Berenbaum, 1980). One species, Manduca sexta, maintains a luminal pH of 11.3 (Dow, 1984) while the haemolymph has a pH of 6.7-6.8 (Dow, 1984; Dow et al. 1984). The midgut of M. sexta is divided into three morphologically distinct regions (Cioffi, 1979), and Dow (1984) has shown that the luminal pH is greatest in the anterior and middle sections of the midgut. The posterior midgut lumen, while still more basic than the haemolymph, has a pH lower than the middle and anterior sections. Whereas these results suggest that alkaline secretion may differ along the length of the midgut, no direct measurements of alkaline secretion have been made. In this study the rate of luminal alkalinization in the three midgut sections was measured in vitro. Tobacco hornworms were grown from eggs or larvae purchased from Carolina Biological Supply Company (Burlington, North Carolina). The larvae were maintained at 28°C on a 16h:8h light: dark cycle and fed an artificial diet (Carolina Biological Supply Co.). Fifth-stage larvae weighing 3-5.3 g were used in all experiments. Larvae were decapitated and a dorsal incision made to expose the midgut. The anterior, middle or posterior midgut was then cleaned of adhering tracheae and Malpighian tubules, opened, removed from the animal and mounted as a flat sheet in an Ussing type chamber. Each half of the chamber had a volume of 2.5 ml and was a modification of a design described previously (Hanrahan et al. 1984). The midgut was secured to a notched collar with cotton thread and the opening (0.196 cm) was backed with fine nylon screen to support the tissue. The tissue was bathed bilaterally with saline which was constantly bubbled with 100 % oxygen to maintain high oxygen tension and to circulate the saline. After reaching a steady transepithelial potential difference (PD) or short-circuit current (7^; usually 60-90 min), the midgut was bilaterally perfused with saline at a rate of 3 ml min" using a Gilson Minipuls peristaltic pump to ensure that any residual alkali was