Midgut electrophysiology of a small insect Orchesella cincta (L.): regional differences and influence of molting

Abstract

Properties of the isolated midgut of the springtail are described for the first time. Effects of molting stage were assessed and/or of the location of the cells along the short length of the epithelium (about 1 to 1.5 mm). Hemolymph, cleared from hematocytes, was pooled (ca5µl from 200 individuals; 27±1.7SEMnl per individual, n=22). The samples were analyzed with a vapor pressure osmometer, ICP-OES*, and micropotentiometry (Cl–.). pH was measured in situ in live animals with ion selective electrodes. The ionic composition in mM±SEM (Na+ 154±2; K+7.0±1.2; Ca++ 9.8±3.5; Mg++ 3.5±1.1 (n=3), Cl– 114±1 (n=11)), osmolality 296±1 mOsm (n=5), pH 7.3±0.1 (n=9) resembled that of other ancestral groups. The physiological solution mimicked this composition. Luminal perfusion and cable analysis of the isolated midgut was done. Under control conditions the basolateral membrane potential (Vbl) was –64±1mV (n = 51), the transepithelial potential (Vte) - 11±1mV (n = 59) lumen negative. The apical potential (Vap) was calculated to be –53mV, cell negative. The transepithelial resistance (Rte) was 389±35cm2 (n = 69). Correlations between the electrical parameters, molting stage and location of the cells along the gut were tested with linear multiple regression. Near molting Vbl became less negative (p=0.0006,n=22), Vte and Rte were not affected. Vbl was less negative at the aboral side of the midgut (p=0.009,n=22). Rte increased towards the aboral end (p<0.00001,n=57). The results show changes with molting and with progressive processing of food along the midgut.