As a means of assessing ventricular performance, we analyzed the time-varying ratio of instantaneous pressure, P(t), to instantaneous volume, V(t), in the canine left ventricle. Intraventricular volume was measured by plethysmography, while the right heart was totally bypassed. The cardiac nerves were sectioned, and an epinephrine infusion was used to alter the contractile state. The instantaneous pressure-volume ratio was defined aswhere V d is an experimentally determined correction factor. We found that (1) all the E(t) curves thus defined were similar in their basic shape and attained their peak near the end of the ejection phase regardless of the mechanical load, the contractile state, or the heart rate, (2) under a constant heart rate and contractile state extensive changes in preload, afterload, or both did not alter the peak value of E ( t ) , Emax, or the time to Emax from the onset of systole, Tmax, and (3) these parameters of E(t) markedly changed with epinephrine infusion or increases in heart rate. At an epinephrine infusion rate of 2 fig/kg min" 1 , Emax increased to 12.2 ± 4.5 (SD) mm Hg/ml (N = 9) from its control value of 6.6 ± 1.2 mm Hg/ml before the infusion. Simultaneously, Tmax shortened from 191 ± 29 msec to 157 ± 26 msec. Increases in the paced heart rate proportionally shortened Tmax (45% per 100-beats/min change in heart rate) without any effect on Emax. We concluded that E ( t ) , represented by Emax and Tmax, explicitly reflects the ventricular contractility.

Background-Although abnormal L-arginine NO signaling contributes to endothelial dysfunction in the aging cardiovascular system, the biochemical mechanisms remain controversial. L-arginine, the NO synthase (NOS) precursor, is also a substrate for arginase. We tested the hypotheses that arginase reciprocally regulates NOS by modulating L-arginine bioavailability and that arginase is upregulated in aging vasculature, contributing to depressed endothelial function. Methods and Results-Inhibition of arginase with (S)-(2-boronoethyl)-L-cysteine, HCl (BEC) produced vasodilation in aortic rings from young (Y) adult rats (maximum effect, 46.4Ϯ9.4% at 10 Ϫ5 mol/L, PϽ0.01). Similar vasorelaxation was elicited with the additional arginase inhibitors N-hydroxy-nor-L-arginine (nor-NOHA) and difluoromethylornithine (DFMO). This effect required intact endothelium and was prevented by 1H-oxadiazole quinoxalin-1-one (PϽ0.05 and PϽ0.001, respectively), a soluble guanylyl cyclase inhibitor. DFMO-elicited vasodilation was greater in old (O) compared with Y rat aortic rings (60Ϯ6% versus 39Ϯ6%, PϽ0.05). In addition, BEC restored depressed L-arginine (10 Ϫ4 mol/L)-dependent vasorelaxant responses in O rings to those of Y. Arginase activity and expression were increased in O rings, whereas NOS activity and cyclic GMP levels were decreased. BEC and DFMO suppressed arginase activity and restored NOS activity and cyclic GMP levels in O vessels to those of Y.Conclusions-These findings demonstrate that arginase modulates NOS activity, likely by regulating intracellular L-arginine availability. Arginase upregulation contributes to endothelial dysfunction of aging and may therefore be a therapeutic target.

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There is increasing evidence that upregulation of arginase contributes to impaired endothelial function in aging. In this study, we demonstrate that arginase upregulation leads to endothelial nitric oxide synthase (eNOS) uncoupling and that in vivo chronic inhibition of arginase restores nitroso-redox balance, improves endothelial function, and increases vascular compliance in old rats. Arginase activity in old rats was significantly increased compared with that shown in young rats. Old rats had significantly lower nitric oxide (NO) and higher superoxide (O 2 Ϫ ) production than young. Acute inhibition of both NOS, with N G -nitro-L-arginine methyl ester, and arginase, with 2(S)-amino-6-boronohexanoic acid (ABH), significantly reduced O 2 Ϫ production in old rats but not in young. In addition, the ratio of eNOS dimer to monomer in old rats was significantly decreased compared with that shown in young rats. These results suggest that eNOS was uncoupled in old rats. Although the expression of arginase 1 and eNOS was similar in young and old rats, inducible NOS (iNOS) was significantly upregulated. Furthermore, S-nitrosylation of arginase 1 was significantly elevated in old rats. These findings support our previously published finding that iNOS nitrosylates and activates arginase 1 (Santhanam et al., Circ Res 101: 692-702, 2007). Chronic arginase inhibition in old rats preserved eNOS dimer-to-monomer ratio and significantly reduced O 2 Ϫ production and enhanced endothelial-dependent vasorelaxation to ACh. In addition, ABH significantly reduced vascular stiffness in old rats. These data indicate that iNOS-dependent S-nitrosylation of arginase 1 and the increase in arginase activity lead to eNOS uncoupling, contributing to the nitroso-redox imbalance, endothelial dysfunction, and vascular stiffness observed in vascular aging. We suggest that arginase is a viable target for therapy in age-dependent vascular stiffness. aging; nitric oxide; S-nitrosylation; NOS uncoupling VASCULAR STIFFNESS (33) and decreased nitric oxide (NO) bioavailability (3, 19) are hallmarks of the aging cardiovascular system. Reactive oxygen species (ROS) production is also enhanced in aged blood vessels (12,23 Ϫ to levels potentially detrimental to vascular cell function and viability (22). This nitroso-redox imbalance contributes to aging-related endothelial dysfunction and vascular stiffness (6).Under normal physiological conditions, nitric oxide synthase (NOS) produces the potent vasodilator NO by catalyzing L-arginine to L-citrulline. This normal function of endothelial NOS (eNOS, NOS3) requires dimerization of the enzyme, the substrate L-arginine, and the essential cofactor (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) (24). However, the enzyme arginase uses L-arginine as a substrate (4) and reciprocally regulates NOS by substrate depletion (6,15,55). There is increasing evidence that upregulation of arginase functionally inhibits NOS activity and contributes to the pathophysiology of agerelated vascular dysfunction (6,45,55). Furthermore, phar...