Local regulation of the threshold for calcium sparks in rat ventricular myocytes: role of sodium-calcium exchange

1. To deter mine whether Na+-Ca2+ exchange modulates Ca2+ sparks, we studie d enzymatically isolated patch clamped rat ventricular myocytes loaded with the Ca2+-sensitive indicator fluo-3, using confocal microscopy at 20-22 de grees C. Two-dimensional images of Ca2+ sparks were recorded at 240 Hz usin g a laser scanning confocal microscope, allowing observation of a large are a of the cell (820 mu m(2)) at one time. 2. At a holding potential of -75 mV, spontaneous sparks were infrequent. Re moval of extracellular Na+ for 520 ms, which in the absence of pipette Nashould block Na+-C2+ exchange bidirectionally, was associated with a fourfo ld increase in spark frequency, without a significant change in cytoplasmic [Ca2+], sarcoplasmic reticulum (SR) Ca2+ content, or spark intensity, size or time course. 3. These findings are consistent with a model of excitation-contraction cou pling in which Na+-Ca2+ exchange locally regulates the resting Ca2+ concent ration in the diadic cleft (T-tubule-SR junction), thereby modulating the t hreshold fur triggering Ca2+ sparks.