ELECTRICAL TRANSIENTS IN THE CELL-VOLUME RESPONSE TO CYCLIC-AMP OF THE TSETSE-FLY MALPIGHIAN TUBULE

1. Using cyclic AMP to stimulate perfused tsetse fly Malpighian tubule s bathed in SO42- Ringer frequently causes an immediate but transient peak in transtubular potential (V-t), before stabilisation of V-t at a n increased value. 2. These transients were investigated by monitoring the associated changes in cable properties and current-voltage (I/V) relationships, Tubules were perfused and bathed in either Cl- Ringer o r SO42- Ringer (containing 8 mmol l(-1) Cl-). 3. Tubules bathed in Cl- Ringer showed a transient swelling of the cells on exposure to cyclic AMP, Cable analysis confirmed the visually observed narrowing of the tubular lumen and revealed transient increases in core resistance (R(c )) and transtubular resistance (Rt). As the cells returned to their in itial volume, the lumen became distended, and R(c) and R(t) fell below their initial levels, These changes were accompanied by an increase, and subsequent decrease, in the slope of the I/V plot. 4. None of the above changes was apparent in SO42- Ringer, other than a fall in R(t) and in the slope of the I/V plot. 5. The results suggest that, in Cl- Ringer, cyclic AMP induces swelling of the tubular cells by promoting increased basolateral solute (and water) entry and that the subsequent rapid return to normal cell volume, with a concomitant progressive in crease in the rate of tubular secretion, reflects the operation of a s pecific cell-volume regulatory mechanism of transepithelial transport. 6. The cyclic-AMP-induced peak that occurs in V-t in SO42- Ringer app ears to be primarily due to a transient overshoot in the fall in serie s resistance (i.e. an increase in basolateral Na+ conductance), accomp anied by proportionately lesser increase in shunt resistance.