10

Role of Nitric Oxide

Production and Matrix

Protease Activity in Cruciate

Ligament Degeneration

David E. Sprengand Simone Forterre

Nitric oxide: An overview

Nitric oxide (NO) is an important mediator in
physiologic and pathophysiologic pathways in
the body. As a nonpolar, highly reactive gas,
NO diffuses freely through cell membranes and
reacts with transition metals, binds to reac-
tive cysteine thiol, or forms reactive oxygen
molecules. Initially, it has been identified as
the small-molecular-weight mediator released
from endothelial cells that promotes the relax-
ation of adjacent vascular smooth muscle.
This explains its earlier acronym ‘endothelium-
derived relaxing factor.’ The generation of NO
occurs when nitric oxide synthase (NOS) cat-
alyzes the conversion of L-arginine to NO and
L-citrulline in a two-step process (Figure 10.1;
Marletta 1993). Under aerobic conditions, NO
is not stable and is promptly oxidized to higher
nitrogen oxides called reactive nitrogen or nitro-
gen oxide intermediates such as peroxynitrite
(ONOO–).
Three isoforms of NOS with characteristic
tissue distribution have been described: the
constitutive endothelial (eNOS) and neuronal
(nNOS) isoforms, and the inducible isoform
(iNOS). The activities of isoforms differ in that
the constitutive isoforms are Ca^2 +-dependent
and produce relatively low levels of NO.

Physiological low amounts of NO released by

eNOS in vascular endothelial cells regulate

blood vessel tone, platelet aggregation, or

cell diapedesis. In the neuronal system, NO

derived from nNOS has been identified as a

neurotransmitter of certain non-adrenergic,

non-cholinergic nerve fibers in various organs

such as cerebral arteries, myocardium, and

pancreas in the dog. In contrast, the iNOS

forms a tight complex with calmodulin inde-

pendent from intracellular calcium levels, and

once activated produces large amounts of NO

over extended periods of time (Weinberget al.

2007). Pro-inflammatory cytokines known to

induce iNOS include interleukin-1 beta (IL-1β),

tumor necrosis factor alpha (TNFα) and inter-

feron gamma (IFNγ), whereas transforming

growth factor beta (TGFβ), IL-4, IL-10 or even

glucocorticoids can inhibit the expression of

iNOS (Kronckeet al. 1995).

iNOS is expressed in nearly all eukaryotic

cells, including monocytes/macrophages,

hepatocytes, synovial cells, and chondrocytes

(Kronckeet al. 1995). Depending on the amount

of NO released and the NOS involved, effects of

NO vary at sites of inflammation. NO in higher

concentrations regulates the T-cell balance by

Th 1 downregulation and Th 2 upregulation,

inhibits viral replication in macrophages,

Advances in the Canine Cranial Cruciate Ligament, Second Edition. Edited by Peter Muir. © 2018 ACVS Foundation.
This Work is a co-publication between the American College of Veterinary Surgeons Foundation and Wiley-Blackwell.

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