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Pflügers Archiv

, Volume 334, Issue 1, pp 24–38 | Cite as

The ionic nature of slow inward current and its relation to contraction

  • W. New
  • W. Trautwein
Article

Summary

In voltage clamp experiments on cat myocardium the slow inward current as affected by the extracellular calcium and sodium concentration was studied. In some of the experiments, contraction was simultaneously recorded and related to membrane potential and slow inward current. The following results were obtained:
  1. 1.

    The amplitude of the slow inward current decreases on lowering the extracellular calcium concentration. In zero calcium slow inward current is not observed.

     
  2. 2.

    The slow inward current is not affected by lowering the sodium concentration in the bathing solution to 10% of the normal concentration.

     
  3. 3.

    The membrane conductance is increased during the slow inward current by a factor of about 2 and not fully inactivated during maintained depolarization.

     
  4. 4.

    The threshold potential for both slow inward current and contraction is at −35 mV.

     
  5. 5.

    At potentials more positive than +80 mV (at 3.6 mM extracellular calcium) and more positive than +40 mV (at 0.9 mM calcium) the amplitude of the contraction declines. Also slow inward current reverses polarity at these potentials and calcium concentrations.

     
  6. 6.

    The amplitude of the steady state contraction rises with the amplitude of clamp depolarization in an S-shaped manner from threshold to the maximum of contraction at zero millivolt. Plotted in normalized fashion (maximum amplitude of contraction = 1), the curve relating amplitude of contraction to clamp potential is shifted by 6.6 mV in negative direction when the extracellular calcium concentration is reduced by a factor of 4.

     
  7. 7.

    Reduction of extracellular sodium results in large increase of amplitude of contraction. The plot of normalized amplitude of contraction vs. clamp potential is shifted towards negative membrane potentials and both, the reversal potential of slow inward current and the maximum amplitude of contraction, occur between +10 mV and +30 mV.

     

The slow inward current is interpreted as a calcium current. The amount of calcium uptake is not less than 10−7M/l fibre volume and beat, but can be up to onehundred times larger. The calcium current is supposed to fill intracellular stores and to release calcium from such stores. For the release effect the conductance change, rather than the inflowing calcium seems to be responsible. Direct activation of myofibrils by the inflowing calcium is not likely to occur but cannot be excluded.

Key words

Slow Inward Current Calcium Current Contraction 

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References

  1. Beeler, G. W., Reuter, H.: Membrane calcium current in ventricular myocardial fibres. J. Physiol. (Lond.)207, 191–209 (1970a).Google Scholar
  2. ——: The relation between membrane potential, membrane currents and activation of contraction in ventricular myocardial fibres. J. Physiol. (Lond.)207, 211–229 (1970b).Google Scholar
  3. Blaustein, M. P., Hodgkin, A. L.: The effect of cyanide on the efflux of calcium from squid axons. J. Physiol (Lond.)200, 497–527 (1969).Google Scholar
  4. Ebashi, S., Endo, J.: Calcium ion and muscle contraction. Progr. biophys. molec. Biol.18, 123–183 (1968).Google Scholar
  5. Endo, M., Tanaka, M., Ogawa, Y.: Calcium induced release of calcium from the sarcoplasmic reticulum of skinned skeletal muscle fibres. Nature (Lond.)228, 34–36 (1970).Google Scholar
  6. Ford, L. E., Podolsky, R.: Regenerative calcium release within muscle cells. Science167, 58–59 (1969).Google Scholar
  7. Gibbons, W. R., Fozzard, H. A.: Voltage dependence and time dependence of contraction in sheep cardiac Purkinje fibres. Circulat. Res.28, 446–460 (1971).Google Scholar
  8. Hasselbach, W.: Relaxing factor and relaxation of muscle. Progr. Biophys.14, 176–222 (1964).Google Scholar
  9. Langer, G. A.: Ion fluxes in cardiac excitation and contraction and their relation to myocardial contractility. Physiol. Rev.48, 708–757 (1968).Google Scholar
  10. Mascher, D., Peper, K.: Two components of inward current in myocardial muscle fibres. Pflügers Arch.307, 190–203 (1969).Google Scholar
  11. New, W., Trautwein, W.: Inward membrane currents in mammalian myocardium. Pflügers Arch.334, 1–23 (1972).Google Scholar
  12. Niedergerke, R., Orkand, R. K.: The dual effect of calcium on the action potential of the frog heart. J. Physiol. (Lond.)184, 291–311 (1966).Google Scholar
  13. Ochi, R.: The slow inward current and the action of manganese ions on guineapig's myocardium. Pflügers Arch.316, 81–94 (1970).Google Scholar
  14. —, Trautwein, W.: The dependence of cardiac contraction on depolarization and slow inward current. Pflügers Arch.323, 187–203 (1971).Google Scholar
  15. Reuter, H., Seitz, H.: The dependence of calcium efflux from cardiac muscle on temperature and external ion composition. J. Physiol. (Lond.)195, 451–470 (1968).Google Scholar
  16. Rougier, O., Vassort, G., Garnier, G., Gargouil, Y. M., Coraboeuf, E.: Existence and role of a slow inward current during the frog atrial action potential. Pflügers Arch.308, 91–110 (1969).Google Scholar
  17. Wood, E. H., Heppner, R. L., Weidmann, S.: Inotropic effects of electric currents. Circulat. Res.24, 409–445 (1969).Google Scholar

Copyright information

© Springer-Verlag 1972

Authors and Affiliations

  • W. New
    • 1
    • 2
  • W. Trautwein
    • 1
    • 2
  1. 1.Department of PhysiologyUniversity of California at Los Angeles, Center for Health SciencesLos Angeles
  2. 2.the Los Angeles Country Cardiovascular Research LaboratoryUniversity of California at Los Angeles, Center for Health SciencesLos Angeles

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