The Source of Short-Circuit Current Across Locust Rectum

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 (I_sc), and unidirectional fluxes of ²²Na⁺, ³⁶C1⁻ and ⁴²K⁺ all remained relatively constant during the 3rd and 4th h. The direction of the I_sc 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 I_sc over the first 2 h, which could be abolished by substituting NO₃⁻ or SO₄^2- 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 I_sC. 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 I_sc 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.