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Cardiology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892
The identity of
endothelium-dependent hyperpolarizing factor (EDHF) in the human
circulation remains controversial. We investigated whether EDHF
contributes to endothelium-dependent vasomotion in the forearm
microvasculature by studying the effect of K+ and
miconazole, an inhibitor of cytochrome P-450, on the
response to bradykinin in healthy human subjects. Study drugs were
infused intra-arterially, and forearm blood flow was measured using
strain-gauge plethysmography. Infusion of KCl (0.33 mmol/min) into the
brachial artery caused baseline vasodilation and inhibited the
vasodilator response to bradykinin, but not to sodium nitroprusside.
Thus the incremental vasodilation induced by bradykinin was reduced from 14.3 ± 2 to 7.1 ± 2 ml · min
1 · 100 g
1
(P < 0.001) after KCl infusion. A similar inhibition
of the bradykinin (P = 0.014), but not the sodium
nitroprusside (not significant), response was observed with KCl after
the study was repeated during preconstriction with phenylephrine to
restore resting blood flow to basal values after KCl. Miconazole (0.125 mg/min) did not inhibit endothelium-dependent or -independent responses
to ACh and sodium nitroprusside, respectively. However, after
inhibition of cyclooxygenase and nitric oxide synthase with aspirin and
NG-monomethyl-L-arginine, the
forearm blood flow response to bradykinin (P = 0.003),
but not to sodium nitroprusside (not significant), was significantly
suppressed by miconazole. Thus nitric oxide- and
prostaglandin-independent, bradykinin-mediated forearm vasodilation is
suppressed by high intravascular K+ concentrations,
indicating a contribution of EDHF. In the human forearm
microvasculature, EDHF appears to be a cytochrome P-450 derivative, possibly an epoxyeicosatrienoic acid.
vascular tone; endothelium; cytochrome P-450; miconazole; potassium; bradykinin; sodium nitroprusside
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