Abstract
1. To determine whether Na$^+$-Ca$^2+$ exchange modulates
Ca$^2+$ sparks, we studied enzymatically isolated patch clamped
rat ventricular myocytes loaded with the Ca$^2+$-sensitive indicator
fluo-3, using confocal microscopy at 20-22 C. Two-dimensional images
of Ca$^2+$ sparks were recorded at 240 Hz using a laser scanning
confocal microscope, allowing observation of a large area of the
cell (820 microm2) at one time. 2. At a holding potential of -75
mV, spontaneous sparks were infrequent. Removal of extracellular
Na$^+$ for 520 ms, which in the absence of pipette Na$^+$
should block Na$^+$-Ca$^2+$ exchange bidirectionally, was
associated with a fourfold increase in spark frequency, without a
significant change in cytoplasmic Ca$^2+$, sarcoplasmic reticulum
(SR) Ca$^2+$ content, or spark intensity, size or time course.
3. These findings are consistent with a model of excitation-contraction
coupling in which Na$^+$-Ca$^2+$ exchange locally regulates
the resting Ca$^2+$ concentration in the diadic cleft (T-tubule-SR
junction), thereby modulating the threshold for triggering Ca$^2+$
sparks.
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