Hypertension is an underlying risk factor for cardiovascular disease. Despite this, its pathogenesis remains unknown in most cases. Recently, the transient receptor potential (TRP) channel family was associated with the development of several cardiovascular diseases linked to hypertension. The melastatin TRP channels TRPM4 and TRPM5 have distinct properties within the TRP channel family: they form nonselective cation channels activated by intracellular calcium ions. Here we report the identification of TRPM4 proteins in endothelial cells, heart, kidney, and chromaffin cells from the adrenal gland, suggesting that they have a role in the cardiovascular system. Consistent with this hypothesis, Trpm4 gene deletion in mice altered long-term regulation of blood pressure toward hypertensive levels. No changes in locomotor activity, renin-angiotensin system function, electrolyte and fluid balance, vascular contractility, and cardiac contractility under basal conditions were observed. By contrast, inhibition of ganglionic transmission with either hexamethonium or prazosin abolished the difference in blood pressure between Trpm4 -/-and wild-type mice. Strikingly, plasma epinephrine concentration as well as urinary excretion of catecholamine metabolites were substantially elevated in Trpm4 -/-mice. In freshly isolated chromaffin cells, lack of TRPM4 was shown to cause markedly more acetylcholineinduced exocytotic release events, while neither cytosolic calcium concentration, size, nor density of vesicles were different. We therefore conclude that TRPM4 proteins limit catecholamine release from chromaffin cells and that this contributes to increased sympathetic tone and hypertension.

−/− ventricular myocytes. We provide evidence that this change in action potential shape leads to an increased driving force for the L-type Ca 2+ current during the action potential, which explains the altered contractility of the heart muscle. Conclusions:

Blood platelet aggregation must be tightly controlled to promote clotting at injury sites but avoid inappropriate occlusion of blood vessels. Thrombin, which cleaves and activates Gq-coupled protease-activated receptors, and collagen-related peptide, which activates the receptor glycoprotein VI, stimulate platelets to aggregate and form thrombi. Coincident activation by these two agonists synergizes, causing the exposure of phosphatidylserine on the cell surface, which is a marker of cell death in many cell types. Phosphatidylserine exposure is also essential to produce additional thrombin on platelet surfaces, which contributes to thrombosis. We found that activation of either thrombin receptors or glycoprotein VI alone produced a calcium signal that was largely dependent only on store-operated Ca(2+) entry. In contrast, experiments with platelets from knockout mice showed that the presence of both ligands activated nonselective cation channels of the transient receptor potential C (TRPC) family, TRPC3 and TRPC6. These channels principally allowed entry of Na(+), which coupled to reverse-mode Na(+)/Ca(2+) exchange to allow calcium influx and thereby contribute to Ca(2+) signaling and phosphatidylserine exposure. Thus, TRPC channels act as coincidence detectors to coordinate responses to multiple signals in cells, thereby indirectly mediating in platelets an increase in intracellular calcium concentrations and exposure of prothrombotic phosphatidylserine.

The constitutively active TRPC1/C4-dependent BGCE fine-tunes Ca(2+) cycling in beating adult cardiomyocytes. TRPC1/C4-gene inactivation protects against development of maladaptive cardiac remodelling without altering cardiac or extracardiac functions contributing to this pathogenesis.

The stress-responsive epigenetic repressor histone deacetylase 4 (HDAC4) regulates cardiac gene expression. Here we show that the levels of an N-terminal proteolytically derived fragment of HDAC4, termed HDAC4-NT, are lower in failing mouse hearts than in healthy control hearts. Virus-mediated transfer of the portion of the Hdac4 gene encoding HDAC4-NT into the mouse myocardium protected the heart from remodeling and failure; this was associated with decreased expression of Nr4a1, which encodes a nuclear orphan receptor, and decreased NR4A1-dependent activation of the hexosamine biosynthetic pathway (HBP). Conversely, exercise enhanced HDAC4-NT levels, and mice with a cardiomyocyte-specific deletion of Hdac4 show reduced exercise capacity, which was characterized by cardiac fatigue and increased expression of Nr4a1. Mechanistically, we found that NR4A1 negatively regulated contractile function in a manner that depended on the HBP and the calcium sensor STIM1. Our work describes a new regulatory axis in which epigenetic regulation of a metabolic pathway affects calcium handling. Activation of this axis during intermittent physiological stress promotes cardiac function, whereas its impairment in sustained pathological cardiac stress leads to heart failure.

Cardiac atrial natriuretic peptide (ANP) locally counteracts cardiac hypertrophy via the guanylyl cyclase-A (GC-A) receptor and cGMP production, but the downstream signalling pathways are unknown. Here, we examined the influence of ANP on β-adrenergic versus Angiotensin II (Ang II)-dependent (Gs vs. Gαq mediated) modulation of Ca2+i-handling in cardiomyocytes and of hypertrophy in intact hearts. L-type Ca2+ currents and Ca2+i transients in adult isolated murine ventricular myocytes were studied by voltage-clamp recordings and fluorescence microscopy. ANP suppressed Ang II-stimulated Ca2+ currents and transients, but had no effect on isoproterenol stimulation. Ang II suppression by ANP was abolished in cardiomyocytes of mice deficient in GC-A, in cyclic GMP-dependent protein kinase I (PKG I) or in the regulator of G protein signalling (RGS) 2, a target of PKG I. Cardiac hypertrophy in response to exogenous Ang II was significantly exacerbated in mice with conditional, cardiomyocyte-restricted GC-A deletion (CM GC-A KO). This was concomitant to increased activation of the Ca2+/calmodulin-dependent prohypertrophic signal transducer CaMKII. In contrast, β-adrenoreceptor-induced hypertrophy was not enhanced in CM GC-A KO mice. Lastly, while the stimulatory effects of Ang II on Ca2+-handling were absent in myocytes of mice deficient in TRPC3/TRPC6, the effects of isoproterenol were unchanged. Our data demonstrate a direct myocardial role for ANP/GC-A/cGMP to antagonize the Ca2+i-dependent hypertrophic growth response to Ang II, but not to β-adrenergic stimulation. The selectivity of this interaction is determined by PKG I and RGS2-dependent modulation of Ang II/AT1 signalling. Furthermore, they strengthen published observations in neonatal cardiomyocytes showing that TRPC3/TRPC6 channels are essential for Ang II, but not for β-adrenergic Ca2+i-stimulation in adult myocytes.

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The major L-type voltage-gated calcium channels in heart consist of an ␣1C (Ca V 1.2) subunit usually associated with an auxiliary ␤ subunit (Ca V ␤2). In embryonic cardiomyocytes, both the complete and the cardiac myocyte-specific null mutant of Ca V ␤2 resulted in reduction of L-type calcium currents by up to 75%, compromising heart function and causing defective remodeling of intra-and extra-embryonic blood vessels followed by embryonic death. Here we conditionally excised the Ca V ␤2 gene (cacnb2) specifically in cardiac myocytes of adult mice (KO). Upon gene deletion, Ca V ␤2 protein expression declined by >96% in isolated cardiac myocytes and by >74% in protein fractions from heart. These latter protein fractions include Ca V ␤2 proteins expressed in cardiac fibroblasts. Surprisingly, mice did not show any obvious impairment, although cacnb2 excision was not compensated by expression of other Ca V ␤ proteins or changes of Ca V 1.2 protein levels. Calcium currents were still dihydropyridine-sensitive, but current density at 0 mV was reduced by <29%. The voltage for half-maximal activation was slightly shifted to more depolarized potentials in KO cardiomyocytes when compared with control cells, but the difference was not significant. In summary, Ca V ␤2 appears to be a much stronger modulator of L-type calcium currents in embryonic than in adult cardiomyocytes. Although essential for embryonic survival, Ca V ␤2 down-regulation in cardiomyocytes is well tolerated by the adult mice.The L-type calcium channels in heart are high voltage-activated, and their minimal composition includes the pore-forming Ca V 1.2␣ 1 subunit and the auxiliary Ca V ␤2 subunit (1). Ca V ␤ subunits are believed to enhance the trafficking of the channels to the plasma membrane and to produce shifts in the voltage dependence of channel activation. From the four mammalian genes coding for Ca V ␤ subunits, the cacnb2 gene is predominantly expressed in the heart (2). Overexpressing the rat neuronal Ca V ␤2a transgene in cardiomyocytes showed increased Ca 2ϩ entry and progressive cell necrosis that led to pump dysfunction and premature death (3). Generation of a ubiquitous and a cardiomyocyte-specific null mutant of cacnb2 (4) causes early embryonic death because of a morphologically and functionally compromised heart. In contrast, targeted deletion of the cacnb1 or cacnb3 genes and the functional inactivation of cacnb4 caused by the spontaneous lethargic (lh) mutation appear not to effect cardiac function (5). In Ca V ␤2 null cardiomyocytes at the embryonic days preceding death, the calcium current was reduced to approximately one-fourth to onethird of the current in wild type cells (4). The remaining calcium current was ineffective to support cardiomyocyte contraction and cardiac pump function; as a consequence, embryonic heart failure associated with pericardial effusion and, finally, embryonic death after embryonic day 10.5 occurred (4). Although it was still sensitive to dihydropyridines (4), the remaining current appears not to depend...