Fig. 1. Inotropic response to β-adrenoceptor stimulation (isoproterenol) in young adult (A ) and senescent rats (B ), under high load. AF = isometric active force normalized per cross-sectional area; l-NAME = N G-nitro-l-arginine methylester, nonspecific nitric oxide synthase (NOS) inhibitor; l-VNIO = vinyl-l - N -5(1-imino-3-butenyl)-l-ornithine, specific NOS1 inhibitor; 1400W = specific NOS2 inhibitor; NS = not significant. Data are mean percentage of baseline value ± SD (n = 8 in each group). The P values refer to the comparison of the maximum effect on active force per cross-sectional area reported in table 2. *P < 0.05 versus control group.