Insect Malpighian tubules secrete fluid into the lumen as part of their function as excretory organs. The underlying ion transport is, when stimulated, faster than in any other known tissue. It is driven by the activity of an H+-transporting V-ATPase situated on the luminal cell membranes. This ATPase, together with cation/H+ antiporter(s), constitutes a common cation pump which can transport sodium ions, potassium ions or both. Treatments that selectively slow cation transport across the epithelium cause the secreted fluid to become alkaline, whereas those that selectively reduce the rate of anion passage lead to secretion of acid fluid.

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