Regional differences in the histochemical localization of carbonic anhydrase in the midgut of tobacco hornworm (Manduca sexta)

Carbonic anhydrase (CA) activity was assayed by a histochemical method in mounted cryostat sections of larval midgut of Manduca sexta (Insecta: Sphingidae). In anterior and middle midgut most CA activity was associated with apical microvilli of goblet cells; with occasional patchy staining of the columnar cell brush border in anterior midgut. In contrast, in the posterior midgut, the apical microvilli of goblet cells showed no CA activity, but intense staining was associated with the brush border of columnar cells. Staining of mitochondria and nuclei was seen in all midgut regions. In addition, midguts from late fifth instar larvae showed intense staining of spherites (mineral concretions). Acetazolamide (10 −5 M) completely inhibited staining at all sites except nuclei and spherites. Thiocyanate (100 mM) completely inhibited staining at all sites except anterior and middle goblet cell microvilli, which remained partially reactive. These results suggest that the anteriormiddle and posterior regions play different roles in generating and managing the alkalinization of midgut contents characteristic of lepidopteran midgut. The different sensitivities of CA activity in anterior‐middle and posterior midgut to thiocyanate raises the possibility that more than one form of the enzyme may be involved.