Voltages and resistances of the anterior Malpighian tubule of Drosophila melanogaster

The small size of Malpighian tubules in the fruit fly Drosophila melanogaster has discouraged measurements of the transepithelial electrical resistance. The present study introduces two methods for measuring the transepithelial resistance in isolated D. melanogaster Malpighian tubules using conventional microelectrodes and PClamp hardware and software. The first method uses three microelectrodes to measure the specific transepithelial resistance normalized to tubule length or luminal surface area for comparison with resistances of other epithelia. The second method uses only two microelectrodes to measure the relative resistance for comparing before and after effects in a single Malpighian tubule. Knowledge of the specific transepithelial resistance allows the first electrical model of electrolyte secretion by the main segment of the anterior Malpighian tubule of D. melanogaster. The electrical model is remarkably similar to that of the distal Malpighian tubule of Aedes aegypti when tubules of Drosophila and Aedes are studied in vitro under the same experimental conditions. Thus, despite 189 millions of years of evolution separating these two genera, the electrophysiological properties of their Malpighian tubules remains remarkably conserved.