Abstract-Depressed contractility is a central feature of the failing human heart and has been attributed to altered [Ca 2ϩ

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Key Words: CaMKII Ⅲ calcineurin Ⅲ NFAT Ⅲ myocytes Ⅲ heart disease I ntermittent changes in the amplitude and duration of the systolic Ca 2ϩ transient are the principle mechanism for regulating the strength of contraction (contractility) of the heart in health. Cardiovascular diseases that cause persistent increases in systolic wall stress require sustained increases in Ca 2ϩ influx and sarcoplasmic reticulum uptake, storage and release to produce the necessary increases in [Ca 2ϩ ] required to maintain the pump function of the heart under these conditions. 1 The persistent increases in [Ca 2ϩ ] that are required to maintain cardiac pump function in pathological cardiovascular stress also activate complex signaling pathways that lead to cardiac hypertrophy, structural and functional remodeling, and cell death. 2 The signaling cascades that link changes in myocyte Ca 2ϩ to activation of hypertrophic and survival signaling are the topic of this study.Increases in myocyte [Ca 2ϩ ] activate both the type 2B Ca 2ϩ /calmodulin (CaM)-dependent phosphatase, calcineurin (Cn), and the Ca 2ϩ /CaM-dependent protein kinase II (CaMKII). Activation of these signaling pathways is linked to electric and contractile disturbances in pathological cardiac hypertrophy. 3,4 Activated Cn induces pathological hypertrophy by dephosphorylation and subsequent nuclear translocation of transcription factors associated with the NFAT family (nuclear factor of activated T cells) that activates specific hypertrophic target genes. 3,[5][6][7][8] In mice, transgenic cardiac overexpression of a constitutively active form of Cn has been shown to cause hypertrophy, mechanical dysfunction, arrhythmias, and premature death. 3,9 In larger animal models 10 and in patients, 11 increased Cn activity has been linked to structural heart disease and the development of heart failure.

Ca2+ enters both normal and failing cardiac myocytes during the late portion of the AP plateau via reverse mode NCX. In (normal) myocytes with good SR function, this Ca(2+) influx helps maintain and regulate SR Ca2+ load. In (failing) human myocytes with poor SR function this Ca2+ influx directly contributes to contraction. These studies suggest that the Ca2+ transient of the failing human ventricular myocytes has a higher than normal reliance on Ca2+ influx via the reverse mode of the NCX during the terminal phases of the AP.

Objective. The transverse-axial tubule system (TATS) of cardiomyocytes allows a spatially coordinated conversion of electrical excitation into an intracellular Ca2+ signal and consequently contraction. Previous reports have indicated alterations of structure and/or volume of the TATS in cardiac hypertrophy and failure, suggesting a contribution to the impairment of excitation contraction coupling. To test whether structural alterations are present in human heart failure, the TATS was visualized in myocytes from failing and non-failing human hearts. Methods and Results. In freshly isolated myocytes, the plasmalemmal membranes were labeled with Di-8-ANEPPS and imaged using two-photon excitation at 780 nm. Optical sections were taken every 300 nm through the cells. After deconvolution, the TATS was determined within the 3D data sets, revealing no significant difference in normalized surface area or volume. To rule out possible inhomogeneity in the arrangement of the TATS, Euclidian distance maps were plotted for every section, allowing to measure the closest distance between any cytosolic and any membrane point. There was a trend towards greater spacing in cells from failing hearts, without statistical significance. Conclusion. Only small changes, but no significant changes in the geometrical dimensions of the TATS were observed in cardiomyocytes from failing compared to non-failing human myocardium.

OSA is associated with a high prevalence of PAD. This implies substantial diseasés under-recognition and a presumable atherogenic role of OSA in the pathogenesis of PAD. However, vasoprotective impact of OSA treatment remains to be determined.

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