In a neuromuscular junction, the vesicular acetylcholine transporter (VAChT) moves ACh from the cytoplasm into synaptic vesicles. The energy for this transport comes from:

• A The electrochemical H+ gradient across the vesicle membrane (acidic interior), driven by V-type H+-ATPase — proton antiport mechanism ✓
• B Direct ATP hydrolysis by VAChT, which is a primary active transporter
• C Sodium gradient maintained by Na+/K+-ATPase on the vesicle membrane providing the driving force
• D Passive diffusion down the concentration gradient as ACh is synthesised by ChAT in the cytoplasm

Explanation

Loading of acetylcholine into synaptic vesicles is performed by the vesicular ACh transporter (VAChT), which is a secondary active transporter (proton antiporter). The vesicle interior is maintained acidic (pH ~5.5) by a V-type H+-ATPase that pumps H+ into the vesicle using ATP. VAChT then exchanges 2 H+ out of the vesicle for 1 ACh molecule into the vesicle, harnessing the inward proton concentration gradient and the vesicle membrane potential. This is the same general mechanism used by all vesicular monoamine and amine transporters (VMATs). Option B is incorrect as VAChT itself does not hydrolyse ATP. Option C is incorrect; the vesicle membrane does not have Na+/K+-ATPase. Option D is incorrect as active concentration against gradient requires energy.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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