Fast sodium influx provides an initial step to trigger contractions in cat ventricle

Ana Maria Vites*, John A Wasserstrom

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


We examined the possibility that Na+ current (I(Na)) may play a role in excitation-contraction coupling in cat ventricular myocytes. A voltage step from -70 to -40 mV produced a fast I(Na), followed by a small transient inward current, a brief loss in voltage control to more positive potentials, and a transient contraction (reduction in cell length, ΔL). We established that 10 μM nifedipine completely blocked Ca2+ current but did not prevent ΔL; nifedipine reduced it by ~15%. This nifedipine-insensitive ΔL was abolished by 1-10 μM ryanodine, 1-10 μM saxitoxin (STX), and 0.1-1.0 mM Cd2+. The size of ΔL increased with more negative holding potential (V(H); ΔL-V(H) relation). Maximal ΔL was achieved at a V(H) of approximately -70 mV. Anthopleura toxin A (APA, 3-10 nM), which selectively slows inactivation of I(Na), increased the size of the nifedipine-insensitive ΔL at all V(H), thus producing a +7-mV shift in the ΔL-V(H) relation that was not affected by the state of the sarcoplasmic reticulum (SR). APA also produced an increase in maximal ΔL, which was no longer observed once the SR was significantly loaded. These effects of APA were prevented by preexposure to STX. The state of the SR Ca2+ stores did not affect the presence of a nifedipine-insensitive ΔL but determined its magnitude, suggesting that ΔL was not associated with Ca2+ overload. In summary, cat and guinea pig ventricular myocytes are alike in that they both exhibit distinct I(Na)- dependent contractions. Whether these contractions are due to a sudden increase in subsarcolemmal Na+ as a result of fast I(Na) or the depolarization and thus reversal of the Na+/Ca2+ exchange remains undetermined.

Original languageEnglish (US)
Pages (from-to)H674-H686
JournalAmerican Journal of Physiology - Heart and Circulatory Physiology
Issue number2 40-2
StatePublished - Aug 1 1996


  • Anthopleura toxin A
  • calcium
  • excitation-contraction coupling
  • heart
  • sarcoplasmic reticulum

ASJC Scopus subject areas

  • Physiology
  • Cardiology and Cardiovascular Medicine
  • Physiology (medical)


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