The Source of Short-Circuit Current Across Locust Rectum

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Abstract
Recta of desert locusts were mounted as flat sheets in ‘Ussing-type’ chambers and various parameters of tissue viability were monitored. The trans-epithelial resistance, the electropotential difference, the short-circuit current (Isc), and unidirectional fluxes of 22Na+, 36C1− and 42K+ all remained relatively constant during the 3rd and 4th h. The direction of the Isc indicated a net transport of either anions to the haemocoel, or cations to the lumen side. This current was abolished by KCN and was sensitive to temperature (Q10 = 2.4). There was a rapid decline in Isc over the first 2 h, which could be abolished by substituting NO3− or SO42- for Cl− in the bathing medium, indicating that this fall in current is due to a decline in the rate of Cl− transport. Measurements of 36C1− fluxes under short-circuit conditions confirm this interpretation. In the steady-state (3rd and 4th h), however, the same anion substitutions had no effect on IsC. Concurrent flux measurements indicated that net Na+ and K+ transport to the haemocoel side equals or slightly exceeds that of C1− in the same direction. Consequently all of the Isc must be due to unidentified ion transport processes. Transport of H+ to the lumen, or HCO3− and organic anions to the haemocoel side, is proposed.

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