Our study suggests that specific micro- and macrostructural features of the ventricle determine the incidence and spatiotemporal characteristics of conduction block, independent of spatial heterogeneities in ion channel expression.

Simultaneous mapping of transmembrane voltage (Vm) and intracellular Ca2+ concentration (Cai) has been used for studies of normal and abnormal impulse propagation in cardiac tissues. Existing dual mapping systems typically utilize one excitation and two emission bandwidths, requiring two photodetectors with precise pixel registration. In this study we describe a novel, single-detector mapping system that utilizes two excitation and one emission bandwidth for the simultaneous recording of action potentials and calcium transients in monolayers of neonatal rat cardiomyocytes. Cells stained with the Ca2+-sensitive dye X-Rhod-1 and the voltage-sensitive dye Di-4-ANEPPS were illuminated by a programmable, multicolor LED matrix. Blue and green LED pulses were flashed in opposite phase at a rate of 488.3Hz using a custom-built dual bandpass excitation filter that transmitted blue (482±6nm) and green (577±31nm) light. A long-pass emission filter (>605 nm) and a 504-channel photodiode array were used to record combined signals from cardiomyocytes. Green excitation yielded Cai transients without significant crosstalk from Vm. Crosstalk present in Vm signals obtained with blue excitation was removed by subtracting an appropriately scaled version of the Cai transient. This method was applied to study delay between onsets of action potentials and Cai transients in anisotropic cardiac monolayers.