title: Activation of reverse Na+-Ca2+ exchange by the Na+ current augments the cardiac Ca2+ transient: evidence from NCX knockout mice.
abstract: The hypothesis that Na(+) influx during the action potential (AP) activates reverse Na(+)-Ca(2+) exchange (NCX) and subsequent entry of trigger Ca(2+) is controversial. We tested this hypothesis by monitoring intracellular Ca(2+) before and after selective inactivation of I(Na) prior to a simulated action potential in patch-clamped ventricular myocytes isolated from adult wild-type (WT) and NCX knockout (KO) mice. First, we inactivated I(Na) using a ramp prepulse to 45 mV. In WT cells, inactivation of I(Na) decreased the Ca(2+) transient amplitude by 51.1 +/- 4.6% (P < 0.001, n = 14) and reduced its maximum release flux by 53.0 +/- 4.6% (P < 0.001, n = 14). There was no effect on diastolic Ca(2+). In striking contrast, Ca(2+) transients in NCX KO cardiomyocytes were unaffected by the presence or absence of I(Na) (n = 8). We obtained similar results when measuring trigger Ca(2+) influx in myocytes with depleted sarcoplasmic reticulum. In WT cells, inactivation of I(Na) decreased trigger Ca(2+) influx by 37.8 +/- 6% and maximum rate of flux by 30.6 +/- 7.7% at 2.5 mm external Ca(2+) (P < 0.001 and P < 0.05, n = 9). This effect was again absent in the KO cells (n = 8). Second, exposure to 10 mum tetrodotoxin to block I(Na) also reduced the Ca(2+) transients in WT myocytes but not in NCX KO myocytes. We conclude that I(Na) and reverse NCX modulate Ca(2+) release in murine WT cardiomyocytes by augmenting the pool of Ca(2+) that triggers ryanodine receptors. This is an important mechanism for regulation of Ca(2+) release and contractility in murine heart.
authors.affiliation: Cardiovascular Research Laboratories, MRL 3-645, David Geffen School of Medicine at UCLA, Los Angeles, CA 90095-1760, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Larbig
authors.forename: Natalia
authors.initials: N
authors.lastname: Torres
authors.forename: John H B
authors.initials: JH
authors.lastname: Bridge
authors.forename: Joshua I
authors.initials: JI
authors.lastname: Goldhaber
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Philipson
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D018398:Homeodomain Proteins / Q000235:genetics*; D051379:Mice; D018345:Mice, Knockout; D032342:Muscle Cells / Q000166:cytology / Q000502:physiology; D032383:Myocytes, Cardiac / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism / Q000502:physiology; D015222:Sodium Channels / Q000378:metabolism / Q000502:physiology*; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D015854:Up-Regulation / Q000502:physiology
chemical_list: D018398:Homeodomain Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D015222:Sodium Channels; D019831:Sodium-Calcium Exchanger; C107836:Tlx2 protein, mouse; C074530:potassium-dependent sodium-calcium exchanger; D002118:Calcium