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 conce ntration 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. Replac ement of NaHCO3 with NaCl in Hepes buffered Ringer solution reduced th e rate of urine production by 23%. Fluid secretions were reduced in th e presence of 10(-4)-10(-2) M acetazolamide, a carbonic anhydrase inhi bitor. The combined effect of acetazolamide in the absence of HCO3- ap pears to be additive. A 1 mM concentration of thiocyanate, an ionic in hibitor, reduced fluid secretion by 35%. Amiloride interferes with the electrogenic entry of Na+ into the cell and a 1 mM solution reduced f luid secretion by 94% with secretion completely inhibited in 80% of th e tubules tested.