Cardiomyocyte GTP cyclohydrolase 1 and tetrahydrobiopterin increase NOS1 activity and accelerate myocardial relaxation.
BACKGROUND
Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthases (NOS). Oral BH4 supplementation 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 Ca2+ handling.


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, 35°C) overexpressing GCH1 showed a faster relaxation and a PKA-mediated increase in the PLB Ser16 phosphorylated fraction and in the rate of decay of the [Ca2+]i transient. RyR2 S-nitrosylation and diastolic Ca2+ leak were larger in mGCH1-Tg and ICa density was lower; nevertheless the amplitude of the [Ca2+]i transient and contraction did not differ between genotypes, because of an increase in the SR fractional release of Ca2+ in mGCH1-Tg myocytes. Xanthine oxidoreductase inhibition abolished the difference in superoxide production but did not affect myocardial function in either group. By contrast, NOS1 inhibition abolished the differences in ICa density, Ser16 PLB phosphorylation, [Ca2+]i decay, and myocardial relaxation between genotypes.


CONCLUSIONS
Myocardial GCH1 activity and intracellular BH4 are a limiting factor for constitutive NOS1 and SERCA2A activity in the healthy myocardium. Our findings suggest that GCH1 may be a valuable target for the treatment of LV diastolic dysfunction.
Department of Cardiovascular Medicine, University of Oxford, Level 6, West Wing, John Radcliffe Hospital, OX3 9DU, United Kingdom.
Ricardo
R
Carnicer
Ashley B
AB
Hale
Silvia
S
Suffredini
Xing
X
Liu
Svetlana
S
Reilly
Mei Hua
MH
Zhang
Nicoletta C
NC
Surdo
Jennifer K
JK
Bendall
Mark J
MJ
Crabtree
Gregory B S
GB
Lim
Nicholas J
NJ
Alp
Keith M
KM
Channon
Barbara
B
Casadei
D000818:Animals; D001708:Biopterins / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D006136:GTP Cyclohydrolase / Q000235:genetics / Q000378:metabolism*; D006321:Heart / Q000187:drug effects / Q000502:physiology; D006801:Humans; D015151:Immunoblotting; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008808:Mice, Inbred CBA; D008822:Mice, Transgenic; D009206:Myocardium / Q000166:cytology / Q000201:enzymology; D032383:Myocytes, Cardiac / Q000201:enzymology; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013481:Superoxides / Q000378:metabolism
D019837:Ryanodine Receptor Calcium Release Channel; D013481:Superoxides; D001708:Biopterins; D052248:Nitric Oxide Synthase Type I; D006136:GTP Cyclohydrolase; C003402:sapropterin; D002118:Calcium