The Effect of Ouabain on the Efflux of Sodium Ions in the Nerve Cords of Two Insect Species (Periplaneta Americana and Carausius Morosus)

The intracellular efflux of 22Na from the nerve cord is reduced by the presence of ouabain in the bathing medium. With Periplaneta 10−6M ouabain caused a reduction in the rate constant for sodium efflux from 3·94 × 10−3 sec.−1 to 1·80 × 10−3 sec.−1 and in Carausius from 1·14 × 10−3 sec.−1 to 6·18 × 10−1 sec.−1. Uncoupling of the energy supply to the sodium pump, by the addition of cyanide, did not further reduce the sodium efflux from ouabain-treated axons, suggesting that the greater part of the active extrusion of this cation is effected by a single ouabain-sensitive carrier mechanism. The relative insensitivity to ouabain of the insect axons, as compared with squid giant axons, is shown to result from the presence of a rather leaky axon membrane in which the carrier-mediated component forms a much smaller part of the total flux. The concentration gradient of sodium across the insect axon membrane is related to the combined effects of the activity of the sodium pump and the passive permeability of the membrane.