Physiology of the Malpighian tubule

The Malpighian tubule secretes a nearly isosmotic fluid with Na⁺, K⁺, and Cl^- serving as the principal osmolytes. Considerable progress has been made in characterizing the molecular identities and biochemical properties of the apical membrane transport pathways for Na⁺ and K⁺. Transport of these cations from the cell into the tubule lumen is achieved by a vacuolar H⁺ ATPase that operates in parallel with a cation/H⁺ exchanger. A variety of diuretic peptides influence fluid secretion rates by Malpighian tubules in vitro. Many of these peptides influence electrogenic ion transport across the apical or basolateral membranes of the epithelial cells. Detailed electrophysiological studies on tubules of several species have provided insights into the contributions of constitutive and regulated electrolyte transport pathways to the process of fluid and electrolyte secretion. This review focuses on some essential features of several models that describe membrane and transepithelial electrolyte transport in the Malpighian tubule.