Figure 1.

Reduction of [Na⁺]_o activates a large Ca_o-dependent, ouabain-resistant ²²Na⁺ efflux in squid axons with high [Na⁺]_i. Replacement of 460 mM NaCl in the artificial sea water (ASW) by LiCl greatly increased ²²Na⁺ efflux, measured as the fraction of ²²Na⁺ lost per minute. The increment was abolished by removal of the 11 mM CaCl₂ in the ASW (MgCl₂ was increased from 55 to 66 mM). This is evidence of “Na_o-Ca_o antagonism”, and indirect evidence for Na⁺/Ca²⁺ exchange. Ouabain (10⁻⁵ M) reduced the ²²Na⁺ efflux in control Na ASW (i.e., it inhibited the Na⁺ pump), but it had no effect on the large Ca_o-dependent ²²Na⁺ efflux in Li ASW; right). Reprinted from Baker et al., 1969, with permission.