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1. Isoprenaline and noradrenaline were applied iontophoretically to cardiac Purkinje fibres. Intracellular application of the drugs had no effect, while extracellular application of the same amounts of charge caused acceleration of pace-maker activity and a shift of the plateau level of the action potential. These results indicate that 
-adrenergic receptors are located at the outside of the cardiac cell membrane.
2. A systematic comparison of the effects of cyclic AMP derivatives and noradrenaline on action potentials and isometric tension of ventricular myocardial preparations showed that the nucleotides and the catecholamine increase the plateau height and the duration of the action potential and also increase tension. However, there are quantitative differences in the action of these drugs.
3. Cyclic AMP derivatives and noradrenaline increase the slow inward current, ICa, in ventricular myocardial preparations. Voltage clamp analysis of ICa showed that the kinetic parameters of this membrane current are not affected by these drugs. However, the membrane conductance to Ca ions is greatly increased by noradrenaline and to a smaller extent by dibutyryl cyclic AMP.
4. Concentration—response relations of the membrane effects of noradrenaline on plateau height of the action potential and on ICa could be fitted by the same theoretical log concentration—response curve. The Hill plot of this concentration—response curve had a slope of 2. The half maximal response occurred at 5 x 10-7 M.
5. The results are compared with other membrane effects of catecholamines and cyclic nucleotides in cardiac muscle. The effects on ICa are related to the positive inotropic effect of the drugs.
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