Rate-dependent changes in cell shortening, intracellular Ca2+ levels and membrane potential in single, isolated rainbow trout (Oncorhynchus mykiss) ventricular myocytes

Claire L. Harwood, F. Chris Howarth, John D. Altringham, Ed White

Research output: Contribution to journalArticlepeer-review

43 Citations (Scopus)

Abstract

The effects of increasing stimulation frequency (from 0.2 to 1.4 Hz) on the contractility, intracellular Ca2+ concentration ([Ca2+](i)) and membrane potential of single ventricular myocytes isolated from the heart of rainbow trout (Oncorhynchus mykiss) were measured. Cell shortening, expressed as a percentage of resting cell length, was our index of contractility. The fluorescent Ca2+ indicator Fura-2 was used to monitor changes in [Ca2+](i). Action potentials and L-type Ca2+ currents (I(Ca)) were recorded using the whole-cell patch-clamp technique. Experiments were performed at 15 °C. Increasing the stimulation frequency caused a significant increase in diastolic [Ca2+](i) and a significant decrease in diastolic cell length and membrane potential. During systole, there was a significant fall in the amplitude of the [Ca2+](i) transient, cell shortening and action potential with a decrease in the duration of the action potential at both 20% and 90% repolarisation. Caffeine was used to assess the Ca2+ content of the sarcoplasmic reticulum. We observed that sarcoplasmic reticulum Ca2+ load was greater at 1.0 Hz than at 0.6 Hz, despite a smaller electrically evoked [Ca2+](i) transient. The amplitude of I(Ca) was found to decrease with increased stimulation frequency. At 0.6 Hz, electrically evoked [Ca2+](i) transients in the presence of 10 mmol l-1 caffeine or 10 μmol l-1 ryanodine and 2 μmol l-1 thapsigargin were reduced by approximately 15%. We have described the changes in contractility, [Ca2+](i) and action potential configuration in a fish cardiac muscle system. Under the conditions tested (0.6 Hz, 15°C), we conclude that the sarcoplasmic reticulum contributes at least 15% of the Ca2+ associated with the [Ca2+](i) transient. The rate-dependent decrease in contraction amplitude appears to be associated with the fall in the amplitude of the [Ca2+](i) transient. This, in turn, may be influenced by changes in the action potential configuration via mechanisms such as altered Ca2+ efflux and Ca2+ influx. In support of our conclusions, we present evidence that there is a rate-dependent decrease in Ca2+ influx via I(Ca) but that the Ca2+ load of the sarcoplasmic reticulum is not reduced at increased contraction frequencies.

Original languageEnglish
Pages (from-to)493-504
Number of pages12
JournalJournal of Experimental Biology
Volume203
Issue number3
Publication statusPublished - Feb 1 2000

Keywords

  • Action potential
  • Ca transient
  • Excitation-contraction coupling
  • Fura-2
  • Heart
  • Myocyte
  • Oncorhynchus mykiss
  • Rainbow trout
  • Sarcoplasmic reticulum
  • Stimulation frequency

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Physiology
  • Aquatic Science
  • Animal Science and Zoology
  • Molecular Biology
  • Insect Science

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