The kinetics of 36C1 fluxes across cAMP-stimulated, short-circuited locust rectum were studied. Raising external K+ from 0 to 100 mM increased both Kt and Vmax for net Cl transport (JnetCl) by four- to six-fold. Hill plots of JnetCl indicated non-cooperative Cl interactions. The sequence for cation stimulation of JnetCl was K > Rb > Cs > Na > NH4. Low levels of K were stimulatory only when added to the mucosal side. Cyclic AMP (cAMP) caused a small active absorption of K, although this was minor compared to the four-fold increase in transepithelial K diffusion (PK). Neither cAMP stimulation of JnetK nor of PK was sensitive to Cl removal, suggesting that K-stimulated Cl absorption and K transport are not mediated by the same co-transport mechanism. Potassium is the counter-ion for electrogenic Cl transport because JnetK was less than 10% of the JnetK during cAMP exposure under Isc conditions, but JnetK equalled JnetCl at open-circuit.

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