The study is believed to be the first to suggest future therapies for heart failure using chemical cofactors that control the enzymes’ action.
Nitric oxide (NO)’s extensive portfolio of natural effects includes the ability to expand coronary arteries, which improves blood flow, and to help regulate the strength of the heart’s contraction, but there is a biological cost to this activity in some situations when the enzyme changes form, according to the reseachers.
In several experiments, the researchers simulated hypertrophy (enlarged heart) for up to nine weeks in groups of 10 to 40 male mice, some bred with and some bred without the gene for the most prominent of the NO-making enzymes, nitric-oxide synthase-3 (NOS3).
NOS3 stops functioning normally when levels of its cofactor, called tetrahydrobiopterin (BH4), decrease. Results not only showed that BH4 levels drop in hypertrophied hearts, but also that NOS3 uncouples, or splits apart, in the absence of its cofactor. Less NO is produced, and instead, the enzyme produces factors that contribute to oxidative stress in the heart. When the researchers restored levels of BH4, it reversed these harmful effects.
In the first experiment, mice without NOS3 better compensated for the damaging stress of hypertrophy, showing less muscle growth, and fibrosis (scar tissue) and better heart function than mice with the enzyme.
Normal mice with the gene for NOS3 could not adapt to the stress, leading the researchers to conclude that the enzymes had lost their protective value to the heart during hypertrophy.
Biochemical analysis revealed that mice with NOS3 had a mix of two chemical forms of the enzyme. The form of NOS3 that works best with the BH4 cofactor dominated in the non-enlarged hearts but uncoupled when levels of its cofactor declined. The scientists believe this enzymatic uncoupling is key to explaining what happens to cause heart enlargement and pumping failure.
In a second experiment to see if the effects of hypertrophy could be reversed, the researchers attempted to preserve normal NOS3 enzymatic function and fed supplements of cofactor BH4 to the group of mice with the enzyme. After three weeks of therapy, results showed that hypertrophy was markedly reduced and heart function improved.
“This study shows that nitric-oxide-making enzymes can have both beneficial and detrimental effects on the heart,” says Dr David Kass, a professor at The Johns Hopkins University School of Medicine and its Heart Institute. “However, the harmful effects can, at least in mice, be treated with its naturally occurring cofactor, BH4, suggesting a possible therapy in the future.”
The researchers plan further experiments to evaluate the therapeutic effects of BH4 in hypertrophy and how it, together with NOS3, compensates for the damage that leads to heart failure.