Abstract 1351: Ca2+ Dissociated from Myofilaments Enhances Delayed Afterdepolarizations by Triggering Ca2+ Waves in Rat Cardiac Muscle
Background: Non-uniform excitation-contraction coupling (ECC) causes arrhythmias by triggering Ca2+ waves from the border zone (BZ) between contracting and stretched regions. To elucidate the triggering mechanism of Ca2+ waves under the condition of non-uniform ECC, we examined the role of Ca2+ dissociated from myofilaments in delayed afterdepolarizations (DADs) and Ca2+ waves.
Methods: Force was measured in 25 rat trabeculae with a strain gauge, sarcomere length with a laser diffraction technique, membrane potential with a ultracompliant microelectrode, and [Ca2+]i with microinjected fura-2 and a CCD camera. To produce non-uniform ECC, a restricted region (~300 μm length) of muscle was exposed to a jet of Hepes solution containing 20 mM 2,3-butanedione monoxime (BDM), causing sarcomere stretch inside the region during twitches. DADs were induced by 7.5 s-2 Hz stimulus trains in the presence or absence of the BDM jet. Streptomycin (100 μM) was then added to eliminate the effect of stretch-activated channels. To modulate myofilament-Ca2+ affinity inside the BDM jet region, muscle length was varied transiently from 2.0 to 2.1 or 2.2 μm during a twitch by the last stimulus of the trains (24 °C, [Ca2+]o=2.0 mM).
Results: The regional exposure of muscle to the BDM jet reduced the twitch force by 41 %, but enhanced DADs (n=6, P<0.05) and aftercontractions (n=6, P<0.05) and occasionally induced action potentials. Streptomycin did not suppress the significant enhancement of DADs and aftercontractions caused by the BDM jet (n=6). Stretch of sarcomere length for 210 ms increased the velocity of Ca2+ waves and the amplitude of aftercontractions in proportion to the increase in force during the stretch (n=12, r=0.90, P<0.01). In the presence of 1 mM caffeine, quick return of muscle length after the stretch for 400 ms induced a small and quick increase in [Ca2+]i at BZ, whose amplitude was correlated with the change in force during the stretch (n=15, r=0.76, P<0.01).
Conclusions: These results suggest that Ca2+ dissociated from myofilaments due to quick release of active sarcomere enhances DADs by triggering Ca2+-induced Ca2+ release in the BZ and thereby induces triggered arrhythmias. Thus, non-uniform ECC can be arrhythmogenic by the dissociated Ca2+ in the BZ.