Strophanthidin inotropy: role of intracellular sodium ion activity and sodium-calcium exchange

The relation among the ratio of extra- and intra-cellular sodium ion activities (aoNa/aiNa), contractile force and action of strophanthidin was studied in cardiac Purkinje fibers when transmembrane Na+ and Ca2+ gradients were changed. The aiNa, contractile force and action potential were simultaneously measured. Simultaneous reduction of [Na+]o and [Ca2+]o to 80.8 and 1.08 mM respectively, decreased aiNa from 8.0 +/- 1.1 mM (mean +/- S.D., n = 17) to 6.0 +/- 0.9 mM (n = 17) whereas contractile force transiently increased and then recovered toward the level similar to that in Tyrode solution. Reduction of [Ca2+]o alone increased aiNa by 1.7 +/- 0.4 mM (n = 5) and decreased contractile force by 87 +/- 5% (n = 5). Raising osmolarity of Tyrode solution with sucrose increased both aiNa and contractile force. Substitution of sucrose with Na+ (high [Na+] solution) increased aiNa by 1.2 +/- 0.3 mM (n = 5) and decreased contractile force by 31 +/- 9% (n = 5). Strophanthidin (2 X 10(-7) M) increased aiNa by 0.4 +/- 0.1 mM (n = 6) and contractile force by 24 +/- 8 (n = 6) in a low [Na+] - [Ca2+] solution. These changes were smaller than those in Tyrode solution (1.1 +/- 0.3 mM); 96 +/- 32%, n = 6). On the other hand, strophanthidin increased aiNa and contractile force more in a low [Ca2+] (2.7 +/- 0.5 mM; 220 +/- 24%, n = 5) or a high [Na+] (2.3 +/- 0.9 mM; 164 +/- 37%, n = 5) solution than in Tyrode solution. In the solutions containing the altered [Na+]o and/or [Ca2+]o, the increases in aiNa and force by strophanthidin were parallel. Therefore, the parallel increase in aiNa and contractile force due to strophanthidin depends on the initial level of aiNa, suggesting the dependence of digitalis inotropy on the rate of Na+ extrusion by the Na+ -K+ pump. The results also indicate that the ratio of aoNa/aiNa is an important and powerful factor in the control of contractile force. Presumably this is mediated through the Na+ -Ca2+ exchange.