Human Coronary Arteriolar Dilation to Arachidonic Acid Depends on Cytochrome P-450 Monooxygenase and Ca ²⁺ -Activated K ⁺ Channels

Abstract

Endothelium-dependent hyperpolarization of vascular smooth muscle cells (VSMCs) plays a crucial role in regulating vascular tone, especially in resistance vessels. It has been proposed that metabolites of arachidonic acid (AA), formed by cytochrome P-450 monooxygenase (P450), are endothelium-derived hyperpolarizing factors (EDHFs). These metabolites have been reported to mediate dilation to endogenous vasoactive compounds, such as bradykinin and acetylcholine. However, it is not known whether these metabolites of AA contribute to dilation of human resistance vessels. This is important since it has been proposed that EDHF serves as a compensatory mechanism to maintain dilation in disease states. Therefore, we studied the effect of AA on vessel diameter and VSMC membrane potential in isolated human coronary microvessels. Arterioles (81+/-5 microm, n=70) were dissected from right atrial appendages at the time of cardiac surgery and cannulated at a distending pressure of 60 mm Hg and zero flow. Changes in internal diameter were recorded with videomicroscopy. Some vessels were impaled with glass microelectrodes to measure membrane potential of VSMCs while internal diameters were simultaneously recorded. After constriction (47+/-2%) with endothelin-1, AA (10(-10)to 10(-5)mol/L) induced substantial dilation of human coronary microvessels, which was abolished by removal of the endothelium. Treatment with 17-octadecynoic acid (17-ODYA, 10(-5) mol/L; a P450 inhibitor) attenuated maximal dilation to AA (49+/-9% versus 91+/-4% [control]; P<0.05 versus control), whereas indomethacin (INDO, 10(-5) mol/L; a cyclooxygenase inhibitor) and N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) mol/L; a NO synthase inhibitor) were without effect. Both 17-ODYA and miconazole (10(-5) mol/L, a chemically distinct P450 inhibitor) further reduced the dilation to AA in the presence of INDO. The presence of 40 mmol/L KCl or charybdotoxin (10(-8) mol/L, a blocker of large-conductance Ca2+-activated K+ channels) impaired dilation to AA (19+/-9% [KCI] versus 76+/-5% [control] and 47+/-6% [charybdotoxin] versus 91+/-3% [control]; P<0.05 for both). After depolarization with endothelin-1 (-26+/-1 mV from -48+/-3 mV [before endothelin]), AA (10(-5)mol/L) in the presence of INDO and L-NAME induced hyperpolarization of VSMCs (-57+/-5 mV). In the presence of 17-ODYA together with INDO and L-NAME, endothelin produced similar depolarization (-26+/-2 mV from - 48+/- 3 mV), but hyperpolarization to AA was reduced (-33+/-2 mV; P<0.05 versus absence of 17-ODYA). AA metabolites formed primarily by P450 produce potent endothelium-dependent dilation of human coronary arterioles via opening of Ca2+-activated K+ channels and hyperpolarization of VSMCs. These findings support an important role for P450 metabolites in the regulation of human coronary arteriolar tone.

Endothelial Dysfunction of Coronary Resistance Arteries Is Improved by Tetrahydrobiopterin in Atherosclerosis

“…Because impairment of coronary blood flow due to atherosclerosis is a very important clinical issue, it is of great significance to perform experiments in human vessels. However, there are only few data available from experiments in isolated perfused human vessels 21 ; this situation is most likely due to the difficult preparation and the difficulty in obtaining true control vessels. In the present study, we used as relative controls the vessels from patients without significant atherosclerosis, ie, vessels from patients undergoing open heart surgery for reasons other than atherosclerosis, such as valve replacement or repair of cardiac defects, in whom the presence of significant atherosclerosis had preoperatively been excluded via coronary angiography.…”

There^|^rsquo;s More to Flow-Mediated Dilation Than Nitric Oxide

“…The contributions of NO, PGI2, and EDHF to endothelium-dependent dilation are difficult to define as their importance may vary by vessel type and size, by disease state, and by the agonist chosen to stimulate the endothelium [140][141][142][143] . Isolating the predominant mechanism is challenging, since a redundancy in endothelial dilator mechanisms, or interaction, has been demonstrated.…”

Potentiation of EDHF-mediated relaxation by chloride channel blockers

“…The potentiating effect on EDHF response by blockade of Cl -channels may also be relevant to the action of epoxyeicosatrienoic acids (EETs). It has been reported that EETs hyperpolarize and relax coronary arteries by activating BK Ca channels in vascular smooth muscle cells [39][40][41][42] , probably through a G-protein-depending signaling pathway [43] . In addition, EETs may also stimulate vanilloid transient receptor potential channels 4 [44] , which increase Ca 2+ influx, and subsequently activate BK Ca channels in vascular smooth muscle cells.…”