Midgut electrophysiology of Orchesella cincta (L.) Templeton 1835

Abstract

The objectives of this work were: To study some functional properties of the midgut. 1) First of all some essential information was needed on the electrophysiologica l properties of the midgut of 0. cincta. la) To start with, an elemental analysis of 0. cincta hemolymph was performed and an adequate physiological saline was composed, based on the hemolymph composition. lb) Furthermore we wanted to study the electrical potential differences at the basolateral and the apical membrane, i.e. to assess the importance of the conductance of the basolateral membrane for K+ and to make an estimate of the relative basolateral versus apical membrane resistance. If the basolateral resistance is lower than the apical resistance Vbi approximates Ebi (see chapter 1.2.4 ). If the basolateral resistance can be increased, for instance by the application of Ba2+, we could take advantage of this situation to learn more about the electrical events at the apical membrane. (electrogenic pumping mechanisms or conductances of other ions). lc) The measurement of pH in the hemolymph, in the cell and in the lumen, should allow to calculate the electrochemical gradient across the basolateral and the apical membranes. This is important information because pH is an important factor either regulating transporters or offering a gradient across the cell membrane, energizing secondary active transport. The items la, lb and le are covered in chapter IA. 2) to determine the specific transepithelial resistance (Rte) of the midgut epithelium. Despite its small size, the midgut of Orchesella cincta must fulfill all needs of digestion and uptake of nutrients. Inevitably, the composition of the food composition changes as a consequence of digestion and uptake of food, while it moves through the midgut. Transport processes in the epithelium might therefore be adapted to a gradual change in the luminal content at different sites along the midgut. Consequently the Rte might be unequally distributed along the midgut. Because of t he small size of the animal t he Rte had to be determined with cable analysis. One assumption of cable analysis is, that the Rte is uniformly distributed along the tissue. 2a) Because this may not necessarily be the case, we tried to evaluate, whether it is nevertheless possible to find a good estimate of the true Rte if it changes along the midgut and 2b) whether it is possible to distinguish between artifacts due to electrical leakage in the set-up and a true unequal distribution of R,, along the midgut. The items outlined in 2), 2a) and 2b) are covered in chapter IB. 3) Furthermore, a technique for direct measurement of Cd2+ fluxes was introduced at the lab to explore the possibility of a simple self-referencing ion-selective electrode set-up to detect Cd2+ fluxes at the basolateral side of the midgut, which is known to accumulate cadmium.