Effects of anions, acetazolamide, thiocyanate and amiloride on fluid secretion by the Malpighian tubules of Locusta migratoria L.

The effect of Cl^?, HCO₃^?, Br^?, acetazolamide, thiocyanate and amiloride on urine formation in Locusta migratoria Malpighian tubules have been determined. The rate of fluid secretion depends markedly on the concentration of Cl^? in the bathing solution with concentrations less than 90 mM resulting in reduced fluid secretion. Substitution of Br^? for Cl^? had no significant effect on the rate of the fluid secretion. Replacement of NaHCO₃ with NaCl in Hepes buffered Ringer solution reduced the rate of urine production by 23%. Fluid secretions were reduced in the presence of 10^?4–10^?2 M acetazolamide, a carbonic anhydrase inhibitor. The combined effect of acetazolamide in the absence of HCO₃^? appears to be additive. A 1 mM concentration of thiocyanate, an ionic inhibitor, reduced fluid secretion by 35%. Amiloride interferes with the electrogenic entry of Na⁺ into the cell and a 1 mM solution reduced fluid secretion by 94% with secretion completely inhibited in 80% of the tubules tested.