Cytochalasin D as the depressant of contraction for the optical monitoring of action potentials in isolated rat atrium

The mechanical uncoupler blebbistatin is associated with significant electrophysiological effects in the isolated rabbit heart

Blebbistatin (BS) is a recently discovered inhibitor of the myosin II isoform and has been adopted as the mechanical uncoupler of choice for optical mapping, because previous studies suggest that BS has no significant cardiac electrophysiological effects in a number of species. The aim of this study was to determine whether BS affects cardiac electrophysiology in isolated New Zealand White rabbit hearts. Langendorff-perfused hearts (n= 39) in constant-flow mode had left ventricular monophasic action potential duration (MAPD) measured at apical and basal regions during constant pacing (300 ms cycle length). Standard action potential duration restitution was obtained using the single extrastimulus method with measurement of the maximal restitution slope. Ventricular fibrillation threshold was measured as the minimal current inducing sustained ventricular fibrillation with burst pacing (30 stimuli, at 30 ms intervals). Optical action potentials were recorded using the voltage-sensitive dye di-4-ANEPPS. Measurements were taken at baseline and after 60 min perfusion with BS (5 μm). Blebbistatin significantly prolonged left ventricular apical (mean ± SEM; from 129.9 ± 2.9 to 170.7 ± 4.1 ms, P < 0.001, n= 8) and basal MAPD (from 135.0 ± 2.3 to 163.3 ± 5.6 ms, P < 0.001) and effective refractory period (from 141.3 ± 4.8 to 175.6 ± 3.7 ms, P < 0.001) whilst increasing the maximal slope of restitution (apex, from 0.79 ± 0.09 to 1.57 ± 0.16, P < 0.001; and base, from 0.71 ± 0.06 to 1.44 ± 0.24, P < 0.001) and ventricular fibrillation threshold (from 5.3 ± 1.1 to 17.0 ± 2.9 mA, P < 0.001). In other hearts, blebbistatin significantly prolonged optically recorded action potentials (from 136.5 ± 6.3 to 173.0 ± 7.9 ms, P < 0.05, n= 4). In control experiments, the increase of MAPD with blebbistatin was present whether the hearts were perfused in constant-pressure mode (n= 5) or in unloaded conditions (n= 5). These data show that blebbistatin significantly affects cardiac electrophysiology. Its use in optical mapping studies should be treated with caution.

Optical mapping study of blebbistatin-induced chaotic electrical activities in isolated rat atrium preparations

We have studied the spatiotemporal pattern of blebbistatin-induced anomalous electrical activities in isolated rat atrial preparations using the optical mapping of excitation spread. Atrial preparations including the right or left auricle were dissected from adult rat hearts. Each preparation was then stained with a fast merocyanine-rhodanine voltage-sensitive dye (NK2761). Using a multi-element (16 x 16) photodiode array, we assessed the spread of excitation optically by timing the initiation of the action potential-related extrinsic absorption changes. The contraction-related optical signals were suppressed by adding (S)-(-)-blebbistatin (10-100 miocroM) to the bathing solution. Blebbistatin had an effective delay time of about 1.5 h following its application, at which time anomalous electrical activities occurred. These took the form of triggered activities and rhythmical spontaneous excitations. We optically mapped the spatiotemporal patterns of the excitation spread during these anomalous electrical activities. When the triggered activities occurred, the site of ectopic focus, where the triggered action potential first appeared, and the area of excitation spread varied in every event. When the rhythmical spontaneous excitations occurred, the excitation spread from the anomalous pacemaker and, occasionally, their spatial shift was observed. In addition, the combination pattern of the spontaneous excitations and triggered activities was also observed. We suggest that these phenomena are due to the disturbed intracellular calcium dynamics induced by the application of blebbistatin.