Numerical simulation of local calcium movements during L-type calcium channel gating in the cardiac diad

Computer simulation was used to investigate the calcium levels after sarcolemmal calcium influx through L-type calcium channels (DHPRs) into the narrow diadic space of cardiac muscle. The effect of various cytosolic and membranebound buffers, diad geometry, DHPR properties (open time and current), and surface charge were examined. The simulations showed that phospholipid binding sites on the sarcolemmal membrane are the major buffer affecting free calcium ([Ca2+]) levels in the diad. The inclusion of surface charge effects calculated from Gouy-Chapman theory resulted in a marked decrease in [Ca2+] levels at all times and a faster decay of [Ca2+] after termination of DHPR influx. For a DHPR current of 200 fA, [Ca2+] at the center of the diad reached peak levels of approximately 73 microM. In larger diads (> or = 400 nm diameter), [Ca2+] decayed more slowly than in smaller diads (100-200 nm diameter), although peak [Ca2+] levels reached during typical DHPR open times were similar. For a wide range of DHPR single-channel current magnitudes (Ica = 25-200 fA), [Ca2+] levels in the diad were approximately proportional to ICa. The decrease in calculated [Ca2+] levels due to the effects of surface charge can be interpreted as resulting from an effective "volume expansion" of the diad space. Furthermore, the layer of increased [Ca2+] close to the sarcolemmal membrane can act as a fast buffer.