1. The potential difference across the hindgut of Sarcophaga is 30 mV, in vivo, the lumen being negative with respect to the haemolymph. A potential difference of the same polarity exists in the isolated hindgut.

2. The potential difference is not a simple diffusion potential, since it is maintained in the absence of any ionic concentration difference across the gut, and is dependent on energy supplies.

3. The potential across the gut is the algebraic sum of two separate electrogenic pumping mechanisms; a cation system which moves K+ or NH4+ and an anion system which moves Cl- into the gut lumen.

4. Since a potential exists across the gut, the rate or amount of cation and anion movement into the gut cannot be equal; alternatively, various shunt pathways may exist for one or more of the ions involved.

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