NG-monomethyl-L-arginine (L-NMMA) blocks vasodilatation by endothelium-derived nitric oxide but not nitrergic nerves in the bovine ciliary artery

Jill Overend & William Martin, Division of Neuroscience & Biomedical Systems, Institute of Biomedical & Life Sciences, West Medical Building, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK.

The nitric oxide synthase inhibitor, L-NMMA, selectively blocks vasodilatation produced by endothelium-derived nitric oxide but not nitrergic nerves in the bovine penile artery (Liu et al., 1991). The present study investigated whether L-NMMA had a similar selective action in the bovine ciliary artery.

Rings of bovine ciliary artery ( ~400μM internal diameter) were suspended in a wire myograph for tension recording as previously reported (Overend et al., 2005). Sub-maximal tone was induced with the thromboxane mimetic, U46619 (0.1 – 1μM). Neurogenic vasodilatation was elicited by electrical field stimulation (EFS) in the presence of the adrenergic neurone blocker, guanethidine (30μM). E ndothelium-dependent, nitric oxide-mediated dilatation was evoked using bradykinin (1μM), following blockade of EDHF with apamin and charybdotoxin (both 10μM) and cyclooxygenase with indomethacin (10μM). Data are expressed as mean ± s.e. mean, n ≥6, and differences determined using one-way ANOVA with Bonferroni’s post-test.

EFS (10 – 15 V, 0.3 ms pulse width, 10 s train length) of bovine ciliary artery rings evoked frequency (0.5 – 32 Hz)-dependent dilatation, optimal at 32 Hz. As reported previously (Overend et al., 2005), this dilatation was biphasic, comprising an initial rapid (nitrergic) component peaking at 10 s, followed by a slower component peaking at 50 s. The first component of neurogenic dilatation was abolished at all frequencies by the NOS inhibitor, L-NAME (100 m M). Furthermore, when stimulated at a single frequency (16 Hz, 10 s), L-NAME (0.1 – 100μM) produced concentration-dependent inhibition, with a pIC50 of 5.74 ± 0.16. In contrast, L-NMMA (10μM – 1 mM) failed to inhibit nitrergic dilatation at any frequency. Pretreatment with L-arginine or L-NMMA (both 1 mM, 1 h) protected against subsequent inhibition of nitrergic dilatation (16 Hz, 10 s) by L-NAME, shifting its apparent pIC50 to 4.07 ± 0.11 and 3.50 ± 0.26, respectively (P < 0.001 for both). The potencies of L-arginine and L-NMMA in protecting against inhibition of nitrergic dilatation by L-NAME were not significantly different. Bradykinin (1μM) induced endothelium-dependent, nitric oxide-mediated dilatation (58 ± 4 %), and this was inhibited both by 100μM L-NAME and L-NMMA (reduced to 12.9 ± 3.8 % and 12.5 ± 1.1 %, respectively, P < 0.001 for both).

In conclusion, in the bovine ciliary artery, L-NMMA acts as a selective blocker of endothelium-dependent, eNOS-mediated vasodilatation that has no inhibitory effect on dilatation produced by the nNOS of the nitrergic nerves. In fact, L-NMMA behaves similarly to the endogenous substrate, L-arginine, in protecting nitrergic vasodilatation against blockade by L-NAME. Our present findings with L-NMMA parallel those obtained in the bovine penile artery (Liu et al., 1991) and suggest that in this species this agent may have general functional selectivity for eNOS over nNOS.

Overend, J et al., (2005). Br. J. Pharmacol.,145, 1001-1008.
Liu X. et al., (1991). Br. J. Pharmacol.,104, 53-58.