Artifactual contractions triggered by field stimulation of cardiomyocytes

Although cell shortening in patch-clamped cells (current-clamp mode) is triggered by an ordinary action potential, the trigger mechanism in field-stimulated cells is not so obvious. The contraction characteristics of the two methods differ, and we, therefore, examined the triggering sequence in field-stimulated cells. Isolated rat cardiomyocytes were plated on laminin-coated coverslips that were mounted on an inverted light microscope and superfused with HEPES-Tyrode buffer (pH 7.4; 37 degrees C). The cells were stimulated to contract either by a 0.5-ms current injection (CC cells) through high-resistance electrodes or a 5-ms biphasic field-stimulation pulse (FS cells), and drugs were added to block sarcolemmal proteins involved in excitation-contraction coupling. Time to peak contraction (TTP) was significantly longer in FS cells and was not affected by the polarity or the length of the stimulus pulse. Tetrodotoxin (TTX; 20 microM) blocked cell shortening in CC cells but not in FS cells. Ni(2+) (5 mM) blocked cell shortening in FS cells, whereas KB-R7943 (KB; 5 microM) had no effect either on cell shortening or TTP. In FS cells, nifedipine (Nif; 100 microM) and Cd(2+) (300 microM) reduced fractional shortening by 34 and 63%, respectively, but only Cd(2+) affected TTP (reduced by 48%). A combination of Nif and KB reduced cell shortening by 50%, whereas a combination of Cd(2+) and KB almost abolished cell shortening. We conclude that field stimulation per se prolongs TTP and that cell shortening in FS cells is not dependent on Na(+) current but is triggered by a combination of L-type Ca(2+) current and reverse mode Na(+)/Ca(2+) exchange.