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Diastolic heart failure is a major cause of mortality in the elderly population. It is often preceded by diastolic dysfunction, which is characterized by impaired active relaxation and increased stiffness. We tested the hypothesis that senescence-prone (SAMP8) mice would develop diastolic dysfunction compared with senescence-resistant controls (SAMR1). Pulsed-wave Doppler imaging of the ratio of blood flow velocity through the mitral valve during early (E) vs. late (A) diastole was reduced from 1.3 ± 0.03 in SAMR1 mice to 1.2 ± 0.03 in SAMP8 mice (P < 0.05). Tissue Doppler imaging of the early (E') and late (A') diastolic mitral annulus velocities found E' reduced from 25.7 ± 0.9 mm/s in SAMR1 to 21.1 ± 0.8 mm/s in SAMP8 mice and E'/A' similarly reduced from 1.1 ± 0.02 to 0.8 ± 0.03 in SAMR1 vs. SAMP8 mice, respectively (P < 0.05). Invasive hemodynamics revealed an increased slope of the end-diastolic pressure-volume relationship (0.5 ± 0.05 vs. 0.8 ± 0.14; P < 0.05), indicating increased left ventricular chamber stiffness. There were no differences in systolic function or mean arterial pressure; however, diastolic dysfunction was accompanied by increased fibrosis in the hearts of SAMP8 mice. In SAMR1 vs. SAMP8 mice, interstitial collagen area increased from 0.3 ± 0.04 to 0.8 ± 0.09% and perivascular collagen area increased from 1.0 ± 0.11 to 1.6 ± 0.14%. Transforming growth factor-β and connective tissue growth factor gene expression were increased in the hearts of SAMP8 mice (P < 0.05 for all data). In summary, SAMP8 mice show increased fibrosis and diastolic dysfunction similar to those seen in humans with aging and may represent a suitable model for future mechanistic studies. [ABSTRACT FROM AUTHOR]

Angiotensin II (ANG II) increases oxidative stress and is associated with increased risk of sudden cardiac death. The cardiac Na+ channel promoter contains elements that confer redox sensitivity. We tested the hypothesis that ANG II- mediated oxidative stress may modulate Na+ channel current through altering channel transcription. In H9c2 myocytes treated for 48 h with ANG II (100 nmol/l) or H2O2 (10 μmol/l) showed delayed macroscopic inactivation, increased late current, and 59.6% and 53.8% reductions in Na+ current, respectively (P ⩽ 0.01). By quantitative real-time RT-PCR, the cardiac Na+ channel (scn5a) mRNA abundance declined by 47.3% (P < 0.01) in H9c2 myocytes treated for 48 h with 100 nmol/l ANG II. A similar change occurred with 20 μmol/l H2O2 (46.9%, P < 0.01) after 48 h. Comparable effects were seen in acutely isolated ventricular myocytes. The effects of ANG II could be inhibited by prior treatment of H9c2 cells with scavengers of reactive oxygen species or an inhibitor of the NADPH oxidase. Mutation of the scn5a promoter NF-κB binding site prevented decreased activity in response to ANG II and H2O2. Gel shift and chromosomal immunoprecipitation assays confirmed that nuclear factor (NF)-κB bound to the scn5a promoter in response to ANG II and H2O2. Overexpression of the p50 subunit of NF-κB in H9c2 cells reduced scn5a mRNA (77.3%, P < 0.01). In conclusion, ANG II can decrease scn5a transcription and current. This effect appears to be through production of H2O2 resulting in NF-κB binding to the Na+ channel promoter. [ABSTRACT FROM AUTHOR]

Renin-angiotensin (RAS) system activation is associated with an increased risk of sudden death. Previously, we used cardiac-restricted angiotensin-converting enzyme (ACE) overexpression to construct a mouse model of RAS activation. These ACE 8/8 mice die prematurely and abruptly. Here, we have investigated cardiac electrophysiological abnormalities that may contribute to early mortality in this model. In ACE 8/8 mice, surface ECG voltages are reduced. Intracardiac electrograms showed atrial and ventricular potential amplitudes of 11% and 24% compared with matched wild-type (WT) controls. The atrioventricular (AV), atrio-Hisian (AH), and Hisian-ventricular (HV) intervals were prolonged 2.8-, 2.6-, and 3.9-fold, respectively, in ACE 8/8 vs. WT mice. Various degrees of AV nodal block were present only in ACE 8/8 mice. Intracardiac electrophysiology studies demonstrated that WT and heterozygote (HZ) mice were noninducible, whereas 83% of ACE 8/8 mice demonstrated ventricular tachycardia with burst pacing. Atrial connexin 40 (Cx40) and connexin 43 (Cx43) protein levels, ventricular Cx43 protein level, atrial and ventricular Cx40 mRNA abundances, ventricular Cx43 mRNA abundance, and atrial and ventricular cardiac Na+ channel (Scn5a) mRNA abundances were reduced in ACE 8/8 compared with WT mice. ACE 8/8 mice demonstrated ventricular Cx43 dephosphorylation. Atrial and ventricular L-type Ca2+ channel, Kv4.2 K+ channel α-subunit, and Cx45 mRNA abundances and the peak ventricular Na+ current did not differ between the groups. In isolated heart preparations, a connexin blocker, 1-heptanol (0.5 mM), produced an electrophysiological phenotype similar to that seen in ACE 8/8 mice. Therefore, cardiac-specific ACE overexpression resulted in changes in connexins consistent with the phenotype of low-voltage electrical activity, conduction defects, and induced ventricular arrhythmia. These results may help explain the increased risk of arrhythmia in states of RAS activation such as heart failure. [ABSTRACT FROM AUTHOR]

T-type Ca[sup 2+] channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents (I[sub Ca,T]) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). I[sub Ca,T] was studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. I[sub Ca,T] showed characteristics of Ca[sub v]3.1, such as resistance to Ni[sup 2+] block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Ca[sub v]3.1 mRNA abundance correlated (r² = 0.81) with I[sub Ca,T]. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that I[sub Ca,T] is developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that I[sub Ca,T] might play a role in the spontaneous contractions during CM development. [ABSTRACT FROM AUTHOR]

Determines unitary conductances of native Na+ channel isoforms (alphaNa) under a variety of conditions. Basis of a linear least-squares fit; Comparison of nerve and cardiac Na+ channels; Mean open time; Unitary current-voltage relationships for neuroblastoma.