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 tubules bathed in SO42- Ringer frequently causes an immediate but transient peak in transtubular potential (Vt), before stabilisation of Vt at an increased value. 2. These transients were investigated by monitoring the associated changes in cable properties and current­voltage (I/V) relationships. Tubules were perfused and bathed in either Cl- Ringer or 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 (Rc) and transtubular resistance (Rt). As the cells returned to their initial volume, the lumen became distended, and Rc and Rt fell below their initial levels. These changes were accompanied by an increase, and a 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 Rt 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 increase in the rate of tubular secretion, reflects the operation of a specific cell-volume regulatory mechanism of transepithelial transport. 6. The cyclic-AMP-induced peak that occurs in Vt in SO42- Ringer appears to be primarily due to a transient overshoot in the fall in series resistance (i.e. an increase in basolateral Na+ conductance), accompanied by a proportionately lesser increase in shunt resistance.