Cytochrome P-450 metabolites but not NO, PGI₂, and H ₂O₂ contribute to ACh-induced hyperpolarization of pressurized canine coronary microvessels

The endothelium-dependent hyperpolarization of cells has a crucial role in regulating vascular tone, especially in microvessels. Nitric oxide (NO) and prostacyclin (PGI₂), in addition to endothelium-derived hyperpolarizing factor (EDHF), have been reported to hyperpolarize vascular smooth muscle in several organs. Studies have reported the hyperpolarizing effects of these factors are increased by a stretch in large coronary arteries. EDHF has not yet been identified and cytochrome P-450 metabolites and H ₂O₂ are candidates for EDHF. With the use of the membrane potential-sensitive fluorescent dye bis-(1,3-dibutylbarbituric acid)trimethione oxonol [DiBAC₄(3)], we examined whether NO, PGI₂, Cytochrome P-450 metabolites, and H₂O₂ contribute to ACh-induced hyperpolarization in pressurized coronary microvessels. Canine coronary arterial microvessels (60-356 μm internal diameter) were cannulated and pressurized at 60 cmH₂O in a vessel chamber perfused with physiological salt solution containing DiBAC₄(3). Fluorescence intensity and diameter were measured on a computer. There was a linear correlation between changes in the fluorescence intensity and membrane potential. ACh significantly decreased the fluorescence intensity (hyperpolarization) of the microvessels without any inhibitors. Endothelial damage caused by air perfusion abolished the ACh-induced decrease in fluorescence intensity. The inhibitors of NO synthase and cyclooxygenase did not affect the ACh-induced decreases in the fluorescence intensity. The addition of 17-octadecynoic acid, a cytochrome P-450 monooxygenase inhibitor, to those inhibitors significantly attenuated the ACh-induced decreases in fluorescence intensity, whereas catalase, an enzyme that dismutates H ₂O₂ to form water and oxygen, did not. Furthermore, catalase did not affect the vasodilation produced by ACh. These results indicate that NO and PGI₂ do not contribute to the ACh-induced hyperpolarization and that the cytochrome P-450 metabolites but not H ₂O₂ are involved in EDHF-mediated hyperpolarization in canine coronary arterial microvessels.