The nitrergic transmitter of the anococcygeus--NO or not?
Nonadrenergic noncholinergic (NANC) relaxations of the anococcygeus muscle are reduced by inhibitors of nitric oxide synthase (NOS). Since NOS can be detected within 6-hydroxydodpamine-resistant nerve tracts running through the muscle, it seems clear that these NANC relaxations result from activation of the L-arginine/NO pathway within the prejunctional nerve terminal, an example of so-called "nitrergic" transmission. However, a number of substances (hydroquinone, superoxide anions, hydroxocobalamin) profoundly reduce relaxations to exogenous NO but do not affect those to nitrergic field stimulation; such observations have raised questions over the nature of the substance actually released from the nitrergic nerves. Several possible explanations are discussed: (1) NO is released attached to a carrier molecule, perhaps in the form of a nitrosothiol; (2) NO is released in a modified redox form; (3) NO is released as a free radical, but is protected within the neuroeffector junction by other substances which preferentially interact with scavenger molecules; and (4) NO is released as a free radical and, because of a rapid and unhindered rate of diffusion over short distances (100-200 microM), it is less susceptible than exogenous NO to scavenger molecules. As yet, there is insufficient experimental evidence to decide which, if any, of these explanations is correct.
Gibson
A
A
Biomedical Sciences Division, King's College London, U.K.
Brave
S R
SR
McFadzean
I
I
Tucker
J F
JF
Wayman
C
C
D000594:Amino Acid Oxidoreductases / Q000378:metabolism; D000818:Animals; D001120:Arginine / Q000494:pharmacology*; D009130:Muscle, Smooth / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism*; D019001:Nitric Oxide Synthase
D018377:Neurotransmitter Agents; D009569:Nitric Oxide; D001120:Arginine; D019001:Nitric Oxide Synthase; D000594:Amino Acid Oxidoreductases