| Cardiac Na+/Ca2+ exchanger (NCX) is central to [Ca2+] homeostasis by regulating Ca2+ efflux. Since NCX expression and activity are often altered in disease, recent studies suggest NCX inhibition as a possible therapeutic approach. However, the role of NCX Ca2+ influx in excitation-contraction (EC) coupling during an action potential (AP) remains uncertain.;Initial AP repolarization rate is a key regulator of EC coupling. To test the hypothesis that NCX-ICa,L interaction is responsible for AP modulation of EC coupling, Ca2+ spikes were triggered by APs with varying repolarization rates. In myocytes containing 20 mmol/L [Na +]i, DeltaF/F0 displayed a biphasic relationship with AP duration at 50% repolarization (APD50), increasing initially then decreasing as APD50 increased from 4 to 52 ms. Maximal DeltaF/F 0 was achieved at 16 ms APD50 (DeltaF/F0=0.77+/-0.06). In the presence of XIP, APD enhancement of DeltaF/F0 was blunted (DeltaF/F0=0.60+/-0.06 when APD50=16ms). With only 5 mmol/L [Na+]i, APD enhancement of DeltaF/F 0 was also blunted and was unaffected by XIP. In myocytes with 20 mmol/L [Na+]i, and XIP, DeltaF/F0 was similar to that in myocytes containing 5 mmol/L [Na+] i with or without XIP.;This thesis resolves the controversy regarding NCX in EC coupling, providing unequivocal evidence that NCX Ca2+ influx enhances Ca 2+ release during AP stimulation. NCX enhancement of release is achieved by supra-additive interaction between NCX and ICa,L and is sensitive to early AP repolarization rate.;Contraction of cardiac myocytes is initiated primarily by L-type Ca 2+ current (ICa,L) which triggers intracellular Ca 2+ release from the sarcoplasmic reticulum (SR). SR release flux was measured by Ca2+ spikes in patch-clamped rat myocytes to isolate the contribution of NCX to Ca2+ release. In response to AP stimulation, normalized Ca2+ spike amplitudes (DeltaF/F 0) increased from 0.40+/-0.08 to 0.80+/-0.06 as [Na +], increased from 0 to 20 mmol/L (EC50=9.08+/-0.97 mmol/L). Enhancement of Ca2+ release was independent of I Na or SR Ca2+ load. However, NCX inhibition with KB-R7943 or inhibitory peptide XIP reduced DeltaF/F0 in myocytes containing 20 (but not 5) mmol/L [Na+]i. ICa,L inhibition with Cd2+ abolished spikes even when [Na+] i was elevated, demonstrating that Ca2+ influxes through NCX and ICa,L interact supra-additively to enhance triggered release. |
