Figure 2.

(A) Effect of the external Ca²⁺ concentration, [Ca²⁺]_o, on contraction of rabbit aorta. Contractile tension (gf) is increased by reduction of [Na⁺]_o. Either 80% (filled circles) or 100% (open circles) of the NaCl in the standard medium (137 mM) was replaced by equiosmotic sucrose. The greater effectiveness of low [Ca²⁺]_o in inducing contraction when [Na⁺]_o was lowered is evidence of “Na_o-Ca_o antagonism”. (B) Reduction of [Na⁺]_o decreases ⁴⁵Ca²⁺ efflux (fraction of ⁴⁵Ca²⁺ lost per min) in a rabbit aortic strip (left), but not in the isolated adventitia (right). The NaCl in the standard medium (137 mM) was replaced by equimolar LiCl or choline-Cl during the periods indicated by the bars at the top. To avoid influx of Ca²⁺ and dilution of intracellular tracer ⁴⁵Ca²⁺, the efflux (extracellular) solutions were Ca-free and contained 0.5 mM EGTA, except during the period indicated by the “Ca” bar at the end of the experiment on the left. The Na_o-dependent ⁴⁵Ca²⁺ efflux in intact intact aortic strip is evidence of Na⁺/Ca²⁺ exchange. Reprinted from Reuter et al., 1973, with permission.