Barium modifies the concentration dependence of active potassium transport by insect midgut

The rate of active K+ transport by the isolated lepidopteran midgut shows a rectangular hyperbolic relation to [K+] over the range 20 to 70 mM K+ in the absence of any divalent cation. Addition of Ba++ to the hemolymph (K+ uptake) side introduces a linear component to the concentration dependence, such that active K+ transport is decreased at [K+] of 55 mM or less, but increased transiently at higher [K+]. As [Ba++] is increased over the range 2 to 8 mM the linear component increases and the saturating component decreases; in 8 mM Ba++ the concentration dependence is dominated by the linear component. The effect of Ba++ cannot easily be accounted for by simple competition with K+ for basal membrane uptake sites. Similar effects might be exercised by other alkali earth cations, since the concentration dependence of active K+ transport possesses a substantial linear component in solutions containing 5 mM Ca++ and 5 mM Mg++ (the alkali earth metal concentrations of standard lepidopteran saline).