Components of acetylcholine-induced dilation in isolated rat arterioles.

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 N G-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.