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 mu m) developed spontaneous tone (90 +/- 2 mu m). Application of acetylcholine (0.1 and 0.3 mu M) resulted in a transient dilation followed by a steady-state dilatory response. In the presence of N-G-nitro-L-arginine (L-NNA) similar to 70% of the transient dilation was resistant to nitric oxide inhibition, whereas the steady-state response was abolished. Further experiments using 0.1 mu M acetylcholine (no L-NNA present) were aimed to inhibit synthesis or action of the mediator of the transient component (amplitude: 39 +/- 2.8 mu m). A high-potassium buffer (30-50 mM) abolished this transient dilation (1.3 +/- 1.3 mu m), suggesting that the dilation is mediated by an endothelium-derived hyperpolarizing factor (EDHF). This putative EDHF-mediated dilation is strongly reduced by cytochrome P-460 inhibitors miconazole (11 +/- 1.3 mu m) and SKF-525a (4.8 +/- 4.5 mu m). 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.