Involvement of Ca2+ buffering and Na+/Ca2+ exchange in the positive staircase of contraction in guinea-pig ventricular myocytes
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The mean sarcomere length (SL) of guinea-pig cardiac myocytes was recorded simultaneously with the whole-cell current under voltage-clamp conditions. After blocking both sarcoplasmic reticulum (SR) and L-type Ca2+ channels with ryanodine, cyclopiazonic acid and nicardipine, strong depolarizing pulses induced only the tonic component of SL shortening through the reverse mode of Na+/Ca2+ exchange (NCX). A positive staircase of SL shortening was observed on applying a train pulses to +60~+100 mV at 2 Hz and trans-membrane Ca2+ flux was calculated from the time integral of the Na+/Ca2+ exchange current (I NCX). Changes in cytosolic [Ca2+] ([Ca2+]i) were determined indirectly using the experimental [Ca2+]i/SL relationship. Cellular Ca2+ buffering was characterized by a lumped single-component system with a maximum binding capacity of 200 µM and a dissociation constant of 613 nM. Despite the decrease in driving force, the amplitude of the outwards I NCX at +60 mV gradually increased along with the positive staircase. The model simulation suggested that this increase of outwards I NCX is caused by a dramatic increase in Ca2+-mediated activation of NCX.
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