Additional file 5: of Alternating bipolar field stimulation identifies muscle fibers with defective excitability but maintained local Ca2+ signals and contraction

Reduction of extracellular [Na+] “converts” UNI myofibers to ALT myofibers. Description: left, uniform electrically induced Ca2+ transients measured in physiological extracellular Na + concentration, [Na+]e. Confocal time series of a skeletal muscle myofiber loaded with the Ca2+ indicator rhod-2 illustrating uniform Ca2+ transients in response to bipolar electrical field stimulation of alternate polarity and before the addition of Na-free recording solution. Center, alternate electrically induced Ca2+ transients measured in Na+-free extracellular conditions. Confocal time series of the same myofiber displayed in left panel, illustrating asymmetric Ca2+ transients in response to bipolar electrical field stimulation of alternate polarity 5 min after the addition of the addition of Na+-free recording solution. Right, uniform electrically induced Ca2+ transients measured when myofiber is returned to recording solution with physiological extracellular [Na+]. Confocal time series of same myofiber displayed in center panel, illustrating uniform Ca2+ transients in response to bipolar electrical field stimulation of alternate polarity 5 min after the return to control solution. Ca2+ transients were elicited by a pair of pulses (1 ms; 15V/cm) interspaced by a 400-ms interval. High-speed time-lapse imaging was conducted for 2 s at 16.7 ms/frame using a ×10 objective magnification. (AVI 4415 kb)