Upregulation of the Nitric Oxide–cGMP Pathway in Aged Myocardium: Physiological Response to l-Arginine

Abstract —Cardiovascular aging is associated with decreased endothelial vasoreactivity and prolonged diastolic relaxation. As diminished NO signaling contributes to age-associated endothelial dysfunction, we tested the hypothesis that impaired NO signaling or bioactivity also contributes to slowed ventricular relaxation with age. Accordingly, we measured myocardial NO synthase (NOS) enzyme activity, protein abundance, and cGMP production in old (22 to 25 months) and young adult (4 to 7 months) male Wistar rats. Both NOS3 protein abundance and calcium-dependent NOS activity were elevated in old compared with young adult hearts (7.2±1.1 versus 4.2±0.6 pmol/mg protein, respectively, P =0.03). However, NOS activity and protein abundance were similar in isolated myocytes, indicating that endothelial NOS likely explains the age difference. Cardiac effluent cGMP (enzyme immunoassay) was 4.8-fold higher (1794±373 fmol/min per mg heart tissue) in older versus younger hearts ( P =0.003). To assess NO pathway responsiveness, we administered the NOS substrate l-arginine (100 μm) to isolated perfused rat hearts. Baseline isovolumic relaxation (τ) was prolonged in old (42.9±2.5 ms, n=16) versus young hearts (36.0±1.9 ms, n=11, P =0.03). l-Arginine decreased τ ( P P H -[1,2,4]oxadiazolo-[4,3,- a ]quinoxalin-1-one (n=7, P