Spatial Ca2+ distribution in contracting skeletal and cardiac muscle cells

The spatiotemporal distribution of intracellular Ca2+ release in contracting skeletal and cardiac muscle cells was defined using a snapshot imaging technique. Calcium imaging was performed on intact skeletal and cardiac muscle cells during contractions induced by an action potential (AP). The sarcomere length of the skeletal and cardiac cells was similar to 2 mu m. Imaging Rhod-2 fluorescence only during a very brief (7 ns) snapshot of excitation light minimized potential image-blurring artifacts due to movement and/or diffusion. In skeletal muscle cells, the AP triggered a large fast Ca2+ transient that peaked in less than 3 ms. Distinct subsarcomeric Ca2+ gradients were evident during the first 4 ms of the skeletal Ca2+ transient. In cardiac muscle, the AP-triggered Ca2+ transient was much slower and peaked in similar to 100 ms. In contrast to the skeletal case, there were no detectable subsarcomeric Ca2+ gradients during the cardiac Ca2+ transient. Theoretical simulations suggest that the subsarcomeric Ca2+ gradients seen in skeletal muscle were detectable because of the high speed and synchrony of local Ca2+ release. Slower asynchronous recruitment of local Ca2+ release units may account for the absence of detectable subsarcomeric Ca2+ gradients in cardiac muscle. The speed and synchrony of local Ca2+ gradients are quite different in AP-activated contracting cardiac and skeletal muscle cells at normal resting sarcomere lengths.