Fig. 6. The apical brush border membrane of principal cells in Malpighian tubules of Aedes aegypti. The apical membrane is characterized by a tall, mitochondrion-rich brush border (A) which brings the site of ATP synthesis in close proximity to the site of utilization (B). ATP is synthesized at the inner mitochondrial membrane by the ATP synthase, consisting of the catalytic F₁ segment and the proton-conducting F₀ segment. What drives ATP synthesis is the proton gradient generated by the electron transport chain (ETC). ATP^4- is transported across the inner membrane in exchange for ADP^3-. ATP^4- goes on to diffuse across the outer mitochondrial membrane to the apical membrane of the brush border. The hydrolysis of ATP by the V-type H⁺-ATPase powers H⁺ transport into the micro-environment of the brush border, thereby generating a large apical membrane voltage (-110.6 mV on average). The H⁺ electrochemical potential thus established across the apical membrane can drive the extrusion of Na⁺ and K⁺ via nH⁺/cation exchange transporter(s) that have yet to be isolated.

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