CONCEALED TRANSEPITHELIAL POTENTIALS AND CURRENT RECTIFICATION IN TSETSE-FLY MALPIGHIAN TUBULES

1. Electrophysiological techniques have been applied to tsetse fly Mal pighian tubules for the first time. 2. In either Cl- or SO42- Ringer, both non-perfused and perfused tubules displayed transtubular potentia ls (V-t) at or close to O mV. Exposure to cyclic AMP elicited a marked secretory response and, in SO42- Ringer, a sharp (lumen-positive) inc rease in V-t. In Cl- Ringer, despite more than double the secretory re sponse, there was little or no change in V-t. 3. Replacing Cl- with SO 42- Ringer, in the presence of cyclic AMP, promptly increased V-t. In perfused tubules, this occurred irrespective of the Cl- or SO42- compo sition of the perfusate. 4. In Cl- Ringer, the transepithelial resista nce (R(trans)) was less than half that previously reported in Malpighi an tubules of other species. Cyclic AMP reduced R(trans) still further , whether tubules were bathed in Cl- or SO42- Ringer. 5. Current-volta ge (I/V) plots often displayed current rectification, both before and more frequently after exposure to cyclic AMP, thus permitting estimati on of both the electromotive force of the Na+ transport mechanism (E(N a)) and of the shunt resistance (R(shunt)) Both E(Na) and R(shunt) wer e markedly lower in tubules bathed in Cl- than in SO42- Ringer. Cyclic AMP was without effect on E(Na) and R(shunt), in either Cl- or SO42- Ringer. 6. In terms of the equivalent electrical circuit, the secretor y response to cyclic AMP was due solely to a fall in resistance of the active transport pathway (R(series)) The absence of an appreciable V- t, in Cl- Ringer, is consistent with an apical Cl- shunt.