ELECTROPHYSIOLOGY OF K+ TRANSPORT BY MIDGUT EPITHELIUM OF LEPIDOPTERAN INSECT LARVAE .4. A MULTICOMPARTMENT MODEL ACCOUNTS FOR TETRAMETHYLAMMONIUM ENTRY INTO GOBLET CAVITIES

A quantitative model was developed to explain the kinetics of tetramet hylammonium (TMA(+)) movement into and out of the goblet cavities of p osterior midgut cells of Manduca sexta based on the data of the accomp anying paper, which indicated that TMA(+) does not enter the goblet ca vity directly from the lumen. The model has two cellular compartments between the lumen and goblet cavity; these have been tentatively ident ified as the columnar cell and goblet cell cytoplasm. Five transmembra ne pathways are included: from lumen to columnar cell, from columnar c ell to goblet cell, from goblet cell cytoplasm to goblet cell cavity, and across the basal membrane of each cell type. These pathways need n ot be channels; they could use endocytotic or exocytotic mechanisms or , in the case of the cell-to-cell passage, septate junctions. However, in all cases, transfer is proportional to the electrochemical gradien t. The model was tested against the results obtained after exposure to TMA(+) in short-circuited and open-circuited tissues as well as resul ts from an open-circuited tissue that did not develop a large transepi thelial potential. Although driving forces for TMA(+) across the membr ane barriers were quite different in the different experimental condit ions, the transfer coefficients from lumen to columnar cell, from colu mnar to goblet cell and from both cells across the basal membrane were the same. The only transfer coefficient that changed between short-ci rcuit and open-circuit conditions was that from goblet cell cytoplasm to goblet cavity. This value was high under short-circuit conditions ( when K+ transport activity is high), but low under open-circuit condit ions (when K+ transport activity is low). The model suggests a hypothe sis in which TMA(+) enters the goblet cavity by an indirect route acro ss the cell membrane of columnar cells, and thence passes to the goble t cell cytoplasm through intercellular junctions. Results from experim ents with cytochalasin E suggest that oo the actin-based cytoskeleton is involved in limiting cell-cell coupling. In this model, TMA(+) pass es from the goblet cell cytoplasm to the goblet cavity via the K+/nH() antiport believed to mediate active transepithelial K+ transport. Ho wever, although actively transported K+ is believed to pass from goble t cavity to lumen, TMA(+) cannot.