Fluid secretion in Rhodnius upper malpighian tubules (UMT): water osmotic permeabilities and morphometric studies

We have measured the osmotic permeability of the basolateral cell membrane (Poscb) and compared it with the transepithelial permeability (Poste) to calculate the paracellular (Posp) permeability of the upper malpighian tubules (UMT) of the 5th instar of Rhodnius prolixus under several experimental conditions, namely, at rest and after stimulation to secrete with 5-HT, each under control conditions (no treatment), after treatment with pCMBS, and after addition of pCMBS and DTT. Secretion rate is negligible at rest. During stimulation mean secretion rate is 43.5 nl/cm2 sec. Secretion is severely curtailed by pCMBS and fully restored by DTT. Poscb = 9.4 (resting, control); 5.8 (control + pCMBS); 10.7 (control + pCMBS + DTT); 20.6 (stimulated, control); 14.7 (stimulated + pCMBS); 49.1 (stimulated + pCMBS + DTT) (x10?4 cm3/cm2 sec Osm). Calculated Posp are higher than the transcellular permeability, Posc, at rest and after stimulation. Electron micrograph morphometry of UMT sections show that cells significantly decrease their volume after stimulation. Lateral intercellular space (LIS) and basolateral extracellular labyrinth (BEL) are barely discernible at rest. LIS and BEL are widely dilated in stimulated UMT. Thus, ions have restricted access to the deep and narrow basolateral cell membrane indentations at rest, but they have ready access to cell membrane indentations after stimulation, because of the opening of LIS and BEL. These findings are discussed in relation to isosmotic secretion. The rate-limiting step for paracellular movement is located at the smooth septate junctions.