Progressive impairment of atrial myocyte function during left ventricular hypertrophy and heart failure.

Hypertensive heart disease (HHD) can cause left ventricular (LV) hypertrophy and heart failure (HF). It is unclear, though, which factors may contribute to the transition from compensated LV hypertrophy to HF in HHD. We hypothesized that maladaptive atrial remodeling with impaired atrial myocyte function would occur in advanced HHD and may be associated with the emergence of HF. Experiments were performed on atrial myocytes and tissue from old (15-25months) normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) with advanced HHD. Based on the absence or presence of elevated lung weight, a sign of lung congestion and heart failure, SHR were divided into a non-failing (SHR-NF) and failing (SHR-HF) group. Compared with WKY, SHR exhibited elevated blood pressure, LV hypertrophy and left atrial (LA) hypertrophy with increased LA expression of markers of hypertrophy and fibrosis. SHR-HF were distinguished from SHR-NF by aggravated hypertrophy and fibrosis. SHR-HF atrial myocytes exhibited reduced contractility and impaired SR Ca2+ handling. Moreover, in SHR the expression and phosphorylation of SR Ca2+-regulating proteins (SERCA2a, calsequestrin, RyR2 and phospholamban) showed negative correlation with increasing lung weight. Increasing stimulation frequency (1-2-4Hz) of atrial myocytes caused a progressive increase in arrhythmogenic Ca2+ release (including alternans), which was observed most frequently in SHR-HF. Thus, in old SHR with advanced HHD there is profound structural and functional atrial remodeling. The occurrence of HF in SHR is associated with LA and RA hypertrophy, increased atrial fibrosis, impaired atrial myocyte contractility and SR Ca2+ handling and increased propensity for arrhythmogenic Ca2+ release. Therefore, functional remodeling intrinsic to atrial myocytes may contribute to the transition from compensated LV hypertrophy to HF in advanced HHD and an increased propensity of atrial arrhythmias in HF.

Enhanced nucleoplasmic Ca2+ signaling in ventricular myocytes from young hypertensive rats.

Arterial hypertension causes left ventricular (LV) myocyte hypertrophy. Alterations in nuclear Ca2+ may be involved in regulation of histone acetylation, transcription and hypertrophy. Regulation of nuclear Ca2+ in hypertension, however, is unknown. Therefore, we elucidated cellular mechanisms underlying nuclear Ca2+ regulation in LV myocytes from hypertensive versus normotensive rats and evaluated possible consequences for Ca2+-dependent regulation of histone acetylation. LV myocytes and myocyte nuclei were isolated from young spontaneously hypertensive rats (SHR) shortly after development of hypertension. Normotensive Wistar-Kyoto rats (WKY) served as controls. Cytoplasmic and nucleoplasmic Ca2+ transients (CaTs) were imaged simultaneously using linescan confocal microscopy and Fluo-4. LV myocytes and nuclei from SHR exhibited hypertrophy. Cytoplasmic and nucleoplasmic CaTs were increased in SHR. The increase in nucleoplasmic Ca2+, however, exceeded the increase in cytoplasmic Ca2+, indicating enhanced nuclear Ca2+ signaling in SHR. Ca2+ load of sarcoplasmic reticulum and perinuclear Ca2+ stores was also increased in SHR, while fractional release from both stores remained unchanged. Intranuclear Ca2+ propagation was accelerated in SHR, associated with preserved density of nuclear envelope invaginations and elevated nuclear expression of nucleoporins and SR-Ca2+-ATPase, SERCA2a. Nuclear Ca2+/calmodulin-dependent protein kinase II delta (CaMKIIδ) expression was elevated and histone deacetylases exhibited redistribution from nucleus to cytosol associated with increased histone acetylation in SHR. Thus, in early hypertension, there is remodeling of nuclear Ca2+ handling resulting in enhanced nuclear Ca2+ signaling. Enhanced nuclear Ca2+ signaling, in turn, increases nuclear localization and activity of CaMKIIδ driving nuclear export of histone deacetylases and increased histone acetylation.

Early subcellular Ca2+ remodelling and increased propensity for Ca2+ alternans in left atrial myocytes from hypertensive rats.

AIMS: Hypertension is a major risk factor for atrial fibrillation. We hypothesized that arterial hypertension would alter atrial myocyte calcium (Ca2+) handling and that these alterations would serve to trigger atrial tachyarrhythmias. METHODS AND RESULTS: Left atria or left atrial (LA) myocytes were isolated from spontaneously hypertensive rats (SHR) or normotensive Wistar-Kyoto (WKY) controls. Early after the onset of hypertension, at 3 months of age, there were no differences in Ca2+ transients (CaTs) or expression and phosphorylation of Ca2+ handling proteins between SHR and WKY. At 7 months of age, when left ventricular (LV) hypertrophy had progressed and markers of fibrosis were increased in left atrium, CaTs (at 1 Hz stimulation) were still unchanged. Subcellular alterations in Ca2+ handling were observed, however, in SHR atrial myocytes including (i) reduced expression of the α1C subunit of and reduced Ca2+ influx through L-type Ca2+ channels, (ii) reduced expression of ryanodine receptors with increased phosphorylation at Ser2808, (iii) decreased activity of the Na+ / Ca2+ exchanger (at unaltered intracellular Na+ concentration), and (iv) increased SR Ca2+ load with reduced fractional release. These changes were associated with an increased propensity of SHR atrial myocytes to develop frequency-dependent, arrhythmogenic Ca2+ alternans. CONCLUSIONS: In SHR, hypertension induces early subcellular LA myocyte Ca2+ remodelling during compensated LV hypertrophy. In basal conditions, atrial myocyte CaTs are not changed. At increased stimulation frequency, however, SHR atrial myocytes become more prone to arrhythmogenic Ca2+ alternans, suggesting a link between hypertension, atrial Ca2+ homeostasis, and development of atrial tachyarrhythmias.

Urocortin 2 stimulates nitric oxide production in ventricular myocytes via Akt- and PKA-mediated phosphorylation of eNOS at serine 1177.

Urocortin 2 (Ucn2) is a cardioactive peptide exhibiting beneficial effects in normal and failing heart. In cardiomyocytes, it elicits cAMP- and Ca(2+)-dependent positive inotropic and lusitropic effects. We tested the hypothesis that, in addition, Ucn2 activates cardiac nitric oxide (NO) signaling and elucidated the underlying signaling pathways and mechanisms. In isolated rabbit ventricular myocytes, Ucn2 caused concentration- and time-dependent increases in phosphorylation of Akt (Ser473, Thr308), endothelial NO synthase (eNOS) (Ser1177), and ERK1/2 (Thr202/Tyr204). ERK1/2 phosphorylation, but not Akt and eNOS phosphorylation, was suppressed by inhibition of MEK1/2. Increased Akt phosphorylation resulted in increased Akt kinase activity and was mediated by corticotropin-releasing factor 2 (CRF2) receptors (astressin-2B sensitive). Inhibition of phosphatidylinositol 3-kinase (PI3K) diminished both Akt as well as eNOS phosphorylation mediated by Ucn2. Inhibition of protein kinase A (PKA) reduced Ucn2-induced phosphorylation of eNOS but did not affect the increase in phosphorylation of Akt. Conversely, direct receptor-independent elevation of cAMP via forskolin increased phosphorylation of eNOS but not of Akt. Ucn2 increased intracellular NO concentration ([NO]i), [cGMP], [cAMP], and cell shortening. Inhibition of eNOS suppressed the increases in [NO]i and cell shortening. When both PI3K-Akt and cAMP-PKA signaling were inhibited, the Ucn2-induced increases in [NO]i and cell shortening were attenuated. Thus, in rabbit ventricular myocytes, Ucn2 causes activation of cAMP-PKA, PI3K-Akt, and MEK1/2-ERK1/2 signaling. The MEK1/2-ERK1/2 pathway is not required for stimulation of NO signaling in these cells. The other two pathways, cAMP-PKA and PI3K-Akt, converge on eNOS phosphorylation at Ser1177 and result in pronounced and sustained cellular NO production with subsequent stimulation of cGMP signaling.