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

The effects of increasing stimulation frequency (from 0.2 to 1.4 Hz) on the contractility, intracellular Ca2+ concentration ([Ca2+](i)) and membrane p otential 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, u as our index of contractility, The flu orescent Ca2+ indicator Fura-2 was used to monitor changes in [Ca2+](i), Ac tion potentials and L-type Ca2+ currents (I-Ca) were recorded using the who le-cell patch-clamp technique. Experiments were performed at 15 degrees C. Increasing the stimulation frequency caused a significant increase in diast olic [Ca2+](i) and a significant decrease in diastolic cell length and memb rane potential, During systole, there was a significant fall in the amplitu de 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 % repo larisation, Caffeine was used to assess the Ca2+ content of the sarcoplasmi c 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) t ransient. The amplitude of I-Ca was found to decrease with increased stimul ation frequency. At 0.6 Hz, electrically evoked [Ca2+](i) transients in the presence of 10 mmol l(-1) caffeine or 10 mu mol l(-1) ryanodine and 2 mu m ol l(-1) thapsigargin were reduced by approximately 15 %, We have described the changes in contractility, [Ca2+](i) and action potent ial configuration in a fish cardiac muscle system. Under the conditions tes ted (0.6 Hz, 15 degrees C), we conclude that the sarcoplasmic reticulum con tributes at least 15 % of the Ca2+ associated with the [Ca2+](i) transient. The rate-dependent decrease in contraction amplitude appears to be associa ted with the fall in the amplitude of the [Ca2+](i) transient, This, in tur n, may be influenced by changes in the action potential configuration via m echanisms such as altered Ca2+ efflux and Ca2+ influx, In support of our co nclusions, we present evidence that there is a rate-dependent decrease in C a2+ influx via I-Ca but that the Ca2+ load of the sarcoplasmic reticulum is not reduced at increased contraction frequencies.