KC1 transport across an insect epithelium: characterization of K-stimulated Cl absorption and active K transport.

The kinetics of (36)C1 fluxes across cAMP-stimulated, short-circuited locust rectum were studied. Raising external K(+) from 0 to 100 mM increased both K(t) and V(max) for net Cl transport (J(CI)net) by four- to six-fold. Hill plots of J(CI)net indicated non-cooperative Cl interactions. The sequence for cation stimulation of J(CI)net 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 J(K)net 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 J(K)net was less than 10% of the J(CI)net during cAMP exposure under Isc conditions, but J(K)net equalled J(CI)net at open-circuit.