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Laboratory for Physiology, Institute for Cardiovascular Research, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands
Acetylcholine-induced dilation was studied in cannulated resistance arteries of rat cremaster muscle. Pressurized arteriolar segments (internal diameter: 175 ± 2 µm) developed spontaneous tone (90 ± 2 µm). Application of acetylcholine (0.1 and 0.3 µM) resulted in a transient dilation followed by a steady-state dilatory response. In the presence of NG-nitro-L-arginine (L-NNA) ~70% of the transient dilation was resistant to nitric oxide inhibition, whereas the steady-state response was abolished. Further experiments using 0.1 µM acetylcholine (no L-NNA present) were aimed to inhibit synthesis or action of the mediator of the transient component (amplitude: 39 ± 2.8 µm). A high-potassium buffer (30-50 mM) abolished this transient dilation (1.3 ± 1.3 µm), suggesting that the dilation is mediated by an endothelium-derived hyperpolarizing factor (EDHF). This putative EDHF-mediated dilation is strongly reduced by cytochrome P-450 inhibitors miconazole (11 ± 1.3 µm) and SKF-525a (4.8 ± 4.5 µm). The transient component is inhibited by tetraethylammonium but not by glibenclamide, indicating it is mediated by opening of Ca2+-activated K+ channels. Interestingly, inhibition of the transient component was followed by a subsequent decrease of the nitric oxide-mediated part of the response to acetylcholine. Thus a transient dilation, mediated by a cytochrome P-450 metabolite, precedes and possibly stimulates nitric oxide-mediated dilation in acetylcholine-induced dilation.
nitric oxide; endothelium-derived hyperpolarizing factor; cytochrome P-450; resistance vessels; potassium channels
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