Identification of cardiotonic sodium channel activators by potassium depolarization in isolated guinea-pig atria

The inotropic actions of various drugs known to increase force of contraction in isolated mammalian cardiac muscle were investigated in electrically driven (1 Hz) guinea-pig left atria under both normal [K⁺]_o (4.7 mM) and high [K⁺]_o (22 mM). Under normal [K⁺]_o a concentration-dependent increase in force of contraction could be confirmed with the β-adrenoceptor agonist, isoprenaline, the cyclase activator, forskolin, the inhibitors of the cyclic AMP-phosphodiesterase (PDE), amrinone, IBMX, and OPC 8212, the Na⁺ channel activators, DPI 201−106, SDZ 210−921, veratridine, and ATX II, the Na⁺-ionophore monensin, the inhibitor of $\text{Na}{}^{\mathrm{+}}\text{K}{}^{\mathrm{+}}$-ATPase, ouabain, and the Ca²⁺ channel activators, Bay K 8644, CGP 28 H 392, and SDZ 202−791. Partial depolarization of the muscle preparations by increasing [K⁺]_o in the organ bath to 22 mM completely abolished the positive inotropic action of the Na⁺ channel-activating drugs. In contrast, the effects of the other compounds were still present, although changes in the maximal force development were observed. The efficacy of the PDE inhibitors amrinone and IBMX were slightly increased; the maximal effects of isoprenaline, monensin, forskolin, and OPC 8212 were unchanged; the effect of ouabain decreased to about half maximal values; while the efficacy of the Ca²⁺ channel activators were either unchanged (CGP 28 392) or decreased (Bay K 8644 and SDZ 202−791). The results suggest that inactivation of cardiac fast Na⁺ channels by partially depolarizing isolated, electrically driven atria is a suitable model to distinguish between cardiotonic agents acting through activation of Na⁺ channels and those with other mechanisms of action.