EFFECTS OF EXTERNAL IONS ON THE EXCITATIONCONTRACTION COUPLING OF CARDIAC MUSCLE OF THE RABBIT

Abstract

The effects of concentration of constituent ions of Tyrode solution on the membrane potentials and mechanical events were studied on the ventricular muscle of rabbits. When NaCl was less or KC1 was more than normal, the plateau level lowered and the duration became shorter as expected. When NaCl was more or KC1 was less than normal, the reverse effects were observed. These changes appeared more rapidly and conspicuously in Ca++-deficient condition. Substitution of I-, NO3- or Br- for Cl-had little effects both on the membrane potentials and tensions. Accordingly, most of the effects of NaCl or KC1 concentration were considered attributable to each cations. Even in K+ deficient condition and in all other cases examined, the plateau could not appear without Na+. Therefore, Na+ is indispensable for producing the plateau. Na+ or K+ removal within the limit of 3/4 of normal improved the twitch tension conspicuously. Depletion of Cl" did not produce any appreciable poten-tiation of contraction as observed in skeletal muscle. Ca++ was only cation in Tyrode solution, the removal of which caused a straight and complete disappearance of the contraction. Mg++ did not produce any eminent effects on the membrane potentials except the shortening of action potential, but caused an increase of resting tension and a decrease of contraction in both cases of excess and lack of the ions. Differences between the effects of various ions on the cardiac and skeletal muscles were discussed, and difficulties to relate the potentials to the mechanical events were pointed out.