Tetrahydrobiopterin Rescues β3-/- Mice from Cardiac Hypertrophy and Impaired Function Induced by Pressure Overload

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β3-Adrenergic receptor signaling in the mammalian cardiovascular system has a depressant effect on ventricular contractility, antagonizing the stimulatory role traditionally ascribed to β1/β2-AR's. Via NOS-dependent increases in NO availability, β3-AR activation may induce an adaptive braking response to excessive catecholamine stimulation present in failing hearts. We have recently presented data revealing impaired cardiac functional compensation and increases in NOS-derived superoxide in β3-/- mice after pressure overload with transverse aortic constriction (TAC). The reducing agent and essential NOS cofactor, tetrahydrobiopterin (BH4), maintains functional coupling and NO-generating catalytic activity. We therefore hypothesized that treatment with BH4 would preserve cardiac function and reduce hypertrophy in β3-/- mice after TAC. Compared to WT mice, 3 weeks of TAC in β3-/- mice produced more marked cardiac hypertrophy as well as impaired function as assessed by echocardiography. These changes were accompanied by an increase in NOS-dependent superoxide production of 39.0% in hearts of TAC'd β3-/- mice over baseline levels, while NOS-dependent superoxide production was undetectable in WT mice. Total cardiac superoxide production in the TAC'd β3-/- hearts trended towards an increase (29.0%) as well, though this was not statistically significantly different from WT hearts or from baseline values. In addition, nNOS and iNOS protein levels were increased in response to TAC in β3-/- mice, though overall cardiac NOS-activity was unchanged, as assessed by arginine-to-citrulline conversion. BH4 treatment completely inhibited the increase in NOS-dependent superoxide production and rescued β3-/- mice from cardiac hypertrophy and impaired fractional shortening when compared to WT. Measurement of BH4 levels revealed no absolute differences between strains, though the BH4/(BH2+B) ratio was slightly lower in β3-/- mice and was unchanged with TAC. These results implicate NOS-derived superoxide in the diminished cardiac compensation after TAC in mice lacking the β3-AR, and that reduction of this superoxide reduces hypertrophy and preserves myocardial systolic function.