Spatial Ca2+ distribution in contracting skeletal and cardiac muscle cells

The spatiotemporal distribution of intracellular Ca2+ release in contractin g skeletal and cardiac muscle cells was defined using a snapshot imaging te chnique. Calcium imaging was performed on intact skeletal and cardiac muscl e cells during contractions induced by an action potential (AP). The sarcom ere 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+ tra nsient that peaked in less than 3 ms. Distinct subsarcomeric Ca2+ gradients were evident during the first 4 ms of the skeletal Ca2+ transient. In card iac muscle, the AP-triggered Ca2+ transient was much slower and peaked in s imilar to 100 ms. In contrast to the skeletal case, there were no detectabl e subsarcomeric Ca2+ gradients during the cardiac Ca2+ transient. Theoretic al simulations suggest that the subsarcomeric Ca2+ gradients seen in skelet al muscle were detectable because of the high speed and synchrony of local Ca2+ release. Slower asynchronous recruitment of local Ca2+ release units m ay account for the absence of detectable subsarcomeric Ca2+ gradients in ca rdiac muscle. The speed and synchrony of local Ca2+ gradients are quite dif ferent in AP-activated contracting cardiac and skeletal muscle cells at nor mal resting sarcomere lengths.