Fig. 5.

Selected experiment with an isolated and perfused anterior midgut of larval (4th instar) Aedes aegypti showing the maximal changes of membrane voltages observed during spontaneous luminal alkalinization after luminal perfusion stop. After stabilization of V_te at –40 mV (lumen negative), an epithelial cell in the anterior portion of the preparation was impaled with a microelectrode (see lower right in photographs A and B). Transbasal potential (V_bl) stabilized at –90 mV (cell negative). From these values the transapical voltage (V_api) can be calculated to –50 mV (cell negative). Five minutes after successful impalement a photograph of the preparation was taken (A, showing yellow perfusate of neutral pH) and the perfusion pump was stopped as indicated by the black bar. Within 3 min V_te dropped to –20 mV, whereas V_bl hyperpolarized to over –120 mV (V_api calculates under these conditions to –103 mV), and the m-Cresol Purple indicated an alkaline midgut lumen (photo B). Restarting the luminal perfusion resulted in a return of the voltages to their original values. Subsequent washout of hemolymph-side serotonin reduced the lumen-negative V_te to –15 mV and the membrane voltages V_bl and V_api decreased to –57 mV and –42 mV, respectively.