Summary

The permeability of the midgut of the fleshfly, Sarcophaga falculata, to the orally toxic polypeptide (M_r7000), a cobra venom-cardiotoxin, was investigated by LM and EM autoradiography, using the radio-iodinated toxin. The histology of the normal and toxin affected midgut was also investigated.

1. 1. The midgut could be classified into segments that were permeable, partially-permeable and non-permeable to the toxin.

2. 2. Histological comparison between the epithelial cells of the permeable and non-permeable segments revealed strong differences in the form of the cells, and in the distribution, form and organization of their microvilli, organelles and basal foldings.

3. 3. The movement of the toxin in the permeable region of the gut is progressive and includes the crossing of the peritrophic membrane, penetration into the apical region of the cell through the microvilli (5 min), movement in the cell's cytoplasm and finally the passage of the midgut as shown by its appearance in extraintestinal tissues (30 min). The data strongly suggest that the crossing of the midgut by cardiotoxin did not follow the common pattern of pinocytotic uptake and vesicular transport.

4. 4. When applied orally in superlethal doses (10 LD₅₀ units per fly), cardiotoxin strongly affected the integrity of the epithelial cells in the toxin permeable segment, as expressed in their collapse, rupture and final disintegration. The toxin had no histopathological effect on any other region of the midgut.

These data are interpreted in terms of a postulated specific composition and arrangement of phospholipids in the outer plasma membranes of the epithelial cells in the cardiotoxin-permeable segment of the midgut.