Cardiovascular risk and the effect of nitric oxide synthase inhibition in female rats: The role of estrogen.

It is known that autonomic modulation is responsive to ovarian hormone levels and that estrogen increases nitric oxide (NO) bioavailability. However, little is known about the interaction of nitric oxide synthase (NOS) isoforms with autonomic modulation, oxidative stress and cardiovascular risk in females. This study aimed to investigate cardiovascular, autonomic and oxidative parameters after selective NOS inhibition. A spectral analysis of systolic arterial pressure (SAP) and heart rate variability (HRV) was performed. NO levels, total antioxidant capacity (TRAP), lipid hydroperoxides (LOOH) and paraoxonase 1 (PON1) activity were measured in the plasma of rats treated with L-NG-nitroarginine methyl ester (L-NAME), S-methylisothiourea (SMT) or saline. Wistar rats, ovariectomized (OVX) with or without estradiol treatment (1mg/kg/day) or with a false ovariectomy (SHAM), were submitted to artery and vein catheterization. Cardiovascular parameters were evaluated before and after the administration of saline or NOS inhibitors. After 2h, plasma samples were collected for biochemical measurement. At baseline, cardiovascular and autonomic parameters were not different among the groups. L-NAME, the constitutive NOS isoform (cNOS) inhibitor, promoted an increase in mean arterial pressure (MAP) and a reduction in the low frequency band (LF) of SAP of SHAM rats, but this increase was smaller in OVX animals, which also showed a reduction in PON1 activity. The decreased activity of PON1 caused by L-NAME was prevented in the OVX+E group. SMT, an inducible NOS isoform (iNOS) inhibitor, promoted an increase in MAP and in the LF of SAP, in interbeat interval (IBI) parameters at LFnu and in LF/HF ratio of HRV in all groups, but the OVX+E had lower levels of NO when compared with the OVX group. Our data suggest that while cNOS contributes to maintaining the activity of PON1 in OVX rats, iNOS activity maintains the levels of NO in OVX+E rats.