Cardiac Na+-Ca2+ exchange (NCX) activity is regulated by [Ca2+]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca2+ activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca2+. We probed Ca2+ activation without voltage clamp by applying Na+-free (0 Na+) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca2+]i transients due to Ca2+ influx via NCX. In rested rabbit myocytes, Ca2+ influx was undetectable even after 0 Na+ applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca2+ via L-type Ca2+ channels, 0 Na+ bouts activated Ca2+ influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na+]i, either in rabbit cardiomyocytes as a result of inhibition of Na+-K+ pumping, or in mouse cardiomyocytes where normal [Na+]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na+ bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca2+ that would otherwise cause activation. We modelled Ca2+ activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca2+ activation on a multibeat time scale, NCX might better maintain a stable long-term Ca2+ balance while contributing to the ability of myocytes to produce Ca2+ transients over a wide range of intensity.
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