Interactions between angiotensin II and nitric oxide during exercise in normal and heart failure rats

Abstract

We hypothesized that nitric oxide (NO) opposes ANG II-induced increases in arterial pressure and reductions in renal, splanchnic, and skeletal muscle vascular conductance during dynamic exercise in normal and heart failure rats. Regional blood flow and vascular conductance were measured during treadmill running before (unblocked exercise) and after 1) ANG II AT1-receptor blockade (losartan, 20 mg/kg ia), 2) NO synthase (NOS) inhibition [ N G-nitro-l-arginine methyl ester (l-NAME); 10 mg/kg ia], or 3) ANG II AT1-receptor blockade + NOS inhibition (combined blockade). Renal conductance during unblocked exercise (4.79 ± 0.31 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1) was increased after ANG II AT1-receptor blockade (6.53 ± 0.51 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1) and decreased by NOS inhibition (2.12 ± 0.20 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1) and combined inhibition (3.96 ± 0.57 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1;all P < 0.05 vs. unblocked). In heart failure rats, renal conductance during unblocked exercise (5.50 ± 0.66 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1) was increased by ANG II AT1-receptor blockade (8.48 ± 0.83 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1) and decreased by NOS inhibition (2.68 ± 0.22 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1; both P < 0.05 vs. unblocked), but it was unaltered during combined inhibition (4.65 ± 0.51 ml ⋅ 100 g−1 ⋅ min−1 ⋅ mmHg−1). Because our findings during combined blockade could be predicted from the independent actions of NO and ANG II, no interaction was apparent between these two substances in control or heart failure animals. In skeletal muscle, l-NAME-induced reductions in conductance, compared with unblocked exercise ( P < 0.05), were abolished during combined inhibition in heart failure but not in control rats. These observations suggest that ANG II causes vasoconstriction in skeletal muscle that is masked by NO-evoked dilation in animals with heart failure. Because reductions in vascular conductance between unblocked exercise and combined inhibition were less than would be predicted from the independent actions of NO and ANG II, an interaction exists between these two substances in heart failure rats.l-NAME-induced increases in arterial pressure during treadmill running were attenuated ( P < 0.05) similarly in both groups by combined inhibition. These findings indicate that NO opposes ANG II-induced increases in arterial pressure and in renal and skeletal muscle resistance during dynamic exercise.