Rationale: Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthases (NOS). Oral BH4supplementation preserves cardiac function in animal models of cardiac disease; however, the mechanisms underlying these findings are not completely understood.Objective: To study the effect of myocardial transgenic overexpression of the rate-limiting enzyme in BH4 biosynthesis, GTP cyclohydrolase 1 (GCH1), on NOS activity, myocardial function, and Ca 2؉ handling. Methods and Results: GCH1overexpression significantly increased the biopterins level in left ventricular (LV)myocytes but not in the nonmyocyte component of the LV myocardium or in plasma. The ratio between BH4 and its oxidized products was lower in mGCH1-Tg, indicating that a large proportion of the myocardial biopterin pool was oxidized; nevertheless, myocardial NOS1 activity was increased in mGCH1-Tg, and superoxide release was significantly reduced. Isolated hearts and field-stimulated LV myocytes (3 Hz Key Words: tetrahydrobiopterin Ⅲ neuronal NOS Ⅲ nitric oxide Ⅲ relaxation Ⅲ phospholamban T etrahydrobiopterin (BH4) is an essential cofactor of all nitric oxide synthases (NOS). Although its role remains incompletely defined, BH4 is thought to facilitate electron transfer from the NOS' reductase domain, maintain the heme prosthetic group in its redox active form, stimulate NO synthesis, and promote the formation of active NOS homodimers. 1 Decreased BH4 bioavailability can lead to NOS uncoupling; a phenomenon whereby reduction of oxygen by NOS is uncoupled from L-arginine oxidation, resulting in the formation of superoxide rather than nitric oxide (NO). 2 A reduction in BH4 bioavailability has been demonstrated in a number of vascular disease states [3][4][5] and, more recently, also in the myocardium in the presence of left ventricular (LV) pressure overload, 6,7 severe ischemia, 8 hyperglycemia, 9 and atrial fibrillation. 10 Under these conditions, supplementation of BH4 has been shown to prevent adverse remodeling and improve cardiac function 6 -8 ; however, whether this is due to a specific increase in myocardial BH4 and NO bioavailability or to the anti-inflammatory and antioxidant effects of BH4 11,12 is still a matter of debate. Indeed, to what extent BH4 can be transported across the plasma membrane remains uncertain. 13 To date, investigations have largely focused on the role of BH4 on endothelial NOS3 activity, [3][4][5][6] whereas the regulation of myocardial constitutive NOS1 activity by BH4 has received comparatively little attention. Reduced availability of Original received June 3, 2012; revision received July 3, 2012; accepted July 10, 2012. In June 2012, the average time from submission to first decision for all original research papers submitted to Circulation Research was 13.35 days.,From the Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, UK. 7,16 -18 and exacerbate adverse LV remodeling after myocardial infarction. 16,19 Whether myocardial BH4 availability is a limiting factor f...

Nitric oxide (NO) is a highly reactive, free radical signalling molecule that is constitutively released in cardiomyocytes by both the endothelial and neuronal isoforms of nitric oxide synthase (eNOS and nNOS, respectively). There are increasing data indicating that NO modulates various proteins involved in excitation-contraction coupling (ECC), and here we discuss the evidence that NO may modulate the function of the ryanodine receptor Ca(2+) release channel (RyR2) on the cardiac sarcoplasmic reticulum (SR). Both constitutive isoforms of NOS have been shown to co-immunoprecipitate with RyR2, suggesting that the channel may be a target protein for NO. eNOS gene deletion has been shown to abolish the increase in spontaneous Ca(2+) spark frequency in cardiomyocytes exposed to sustained stretch, whereas the effect of nNOS-derived NO on RyR2 function remains to be investigated. Single channel studies have been performed with RyR2 reconstituted in planar lipid bilayers and exposed to various NO donors and, under these conditions, NO appears to have a dose-dependent, stimulatory effect on channel open probability (P(open)). We discuss whether NO has a direct effect on RyR2 via covalent S-nitrosylation of reactive thiol residues within the protein, or whether there are downstream effects via cyclic nucleotides, phosphodiesterases, and protein kinases. Finally, we consider whether the proposed migration of nNOS from the SR to the sarcolemma in the failing heart may have consequences for the nitrosative vs. oxidative balance at the level of the RyR2, and whether this may contribute to an increased diastolic Ca(2+) leak, depleted SR Ca(2+) store, and reduced contractility in heart failure.

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