Flow-induced enhancement of vasoconstriction and blockade of endothelium-derived hyperpolarizing factor (EDHF) by ascorbate in the rat mesentery.
OBJECTIVE
We previously reported that ascorbate inhibits flow- and agonist-induced, EDHF-mediated vasodilatation in the bovine ciliary circulation. This study examined whether ascorbate had similar actions in the rat mesenteric vasculature.


METHODS
The effects of ascorbate were examined both in rat second order mesenteric arterial rings suspended in a static wire myograph and the rat mesentery perfused at different rates of flow.


RESULTS
Ascorbate (50 microM) had no effect on U46619-induced tone or acetylcholine-induced, EDHF-mediated vasodilatation in either rings of mesenteric artery or the perfused mesentery at rates of flow below 10 ml min(-1). At higher rates of flow, ascorbate produced two distinct effects in the rat mesentery: a rapid and maintained enhancement of vasoconstrictor tone and a slow (max at 3 h) inhibition of acetylcholine-induced, EDHF-mediated vasodilatation. The enhancement of vasoconstrictor tone appeared to be due to inhibition of flow-induced EDHF-like activity, since it was endothelium-dependent, but could be elicited during blockade of nitric oxide synthase and cyclooxygenase. Despite this, the classical inhibitors of EDHF, apamin and charybdotoxin, failed to affect the ascorbate-induced enhancement of tone, although they inhibited acetylcholine-induced vasodilatation.


CONCLUSIONS
Ascorbate inhibits both flow- and agonist-induced EDHF in the rat mesentery. The strikingly different timecourses of these two effects, together with their differential sensitivity to apamin and charybdotoxin, suggest that the flow- and agonist-induced EDHFs in the rat mesenteric vasculature may either be different entities or operate by different mechanisms.
Stirrat
A
A
Division of Neuroscience and Biomedical Systems, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, Scotland, UK.
Nelli
S
S
Dowell
F J
FJ
Martin
W
W
D000109:Acetylcholine; D000818:Animals; D000975:Antioxidants / Q000008:administration & dosage / Q000494:pharmacology*; D001205:Ascorbic Acid / Q000008:administration & dosage / Q000494:pharmacology*; D001685:Biological Factors / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D008638:Mesenteric Arteries / Q000187:drug effects / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000187:drug effects / Q000378:metabolism; D010477:Perfusion; D011451:Prostaglandin-Endoperoxide Synthases / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D014661:Vasoconstriction / Q000187:drug effects; D014664:Vasodilation / Q000187:drug effects*
D000975:Antioxidants; D001685:Biological Factors; C067410:endothelium-dependent hyperpolarization factor; D019001:Nitric Oxide Synthase; D011451:Prostaglandin-Endoperoxide Synthases; D000109:Acetylcholine; D001205:Ascorbic Acid