Lih Kuo

cAMP-Independent Dilation of Coronary Arterioles to Adenosine

Adenosine is known to play an important role in the regulation of coronary blood flow during metabolic stress. However, there is sparse information on the mechanism of adenosine-induced dilation at the microcirculatory levels. In the present study, we examined the role of endothelial nitric oxide (NO), G proteins, cyclic nucleotides, and potassium channels in coronary arteriolar dilation to adenosine. Pig subepicardial coronary arterioles (50 to 100 microm in diameter) were isolated, cannulated, and pressurized to 60 cm H(2)O without flow for in vitro study. The arterioles developed basal tone and dilated dose dependently to adenosine. Disruption of endothelium, blocking of endothelial ATP-sensitive potassium (K(ATP)) channels by glibenclamide, and inhibition of NO synthase by N(G)-nitro-L-arginine methyl ester and of soluble guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one produced identical attenuation of vasodilation to adenosine. Combined administration of these inhibitors did not further attenuate the vasodilatory response. Production of NO from coronary arterioles was significantly increased by adenosine. Pertussis toxin, but not cholera toxin, significantly inhibited vasodilation to adenosine, and this inhibitory effect was only evident in vessels with an intact endothelium. Tetraethylammonium, glibenclamide, and a high concentration of extraluminal KCl abolished vasodilation of denuded vessels to adenosine; however, inhibition of calcium-activated potassium channels by iberiotoxin had no effect on this dilation. Rp-8-Br-cAMPS, a cAMP antagonist, inhibited vasodilation to cAMP analog 8-Br-cAMP but failed to block adenosine-induced dilation. Furthermore, vasodilations to 8-Br-cAMP and sodium nitroprusside were not inhibited by glibenclamide, indicating that cAMP- and cGMP-induced dilations are not mediated by the activation of K(ATP) channels. These results suggest that adenosine activates both endothelial and smooth muscle pathways to exert its vasodilatory function. On one hand, adenosine opens endothelial K(ATP) channels through activation of pertussis toxin-sensitive G proteins. This signaling leads to the production and release of NO, which subsequently activates smooth muscle soluble guanylyl cyclase for vasodilation. On the other hand, adenosine activates smooth muscle K(ATP) channels and leads to vasodilation through hyperpolarization. It appears that the latter vasodilatory process is independent of G proteins and of cAMP/cGMP pathways.

Divergent Roles of Angiotensin II AT1 and AT2 Receptors in Modulating Coronary Microvascular Function

Abstract— Angiotensin II (Ang II) is a potent vasoconstrictor in the peripheral circulation and has been implicated in many cardiovascular diseases associated with elevated oxidative stress. However, its direct vasomotor action and its linkage to oxidative stress–induced vascular dysfunction in the coronary microcirculation remain elusive. In this study, we directly assessed the vasomotor action of Ang II in isolated porcine coronary arterioles and also examined whether Ang II can modulate endothelium-dependent nitric oxide (NO)-mediated dilation via superoxide production. Ang II evoked vasoconstriction at a low concentration (1 nmol/L) and dilations at higher concentrations (>10 nmol/L). Ang II type 1 (AT1) receptor antagonist losartan abolished vasoconstriction, whereas Ang II type 2 (AT2) receptor antagonist PD 123319 eliminated vasodilation. Adenosine stimulated a significant arteriolar NO production and dilation. NO synthase inhibitor NG-monomethyl-l-arginine (L-NMMA) abolished stimulated NO production and attenuated vasodilation. Pretreating vessels with a subvasomotor concentration of Ang II (0.1 nmol/L, 60 minutes) mimicked inhibitory effects of L-NMMA. Ang II–mediated inhibition was not observed in the presence of L-NMMA or after endothelial removal but was prevented by losartan, superoxide scavenger TEMPOL, or NADPH oxidase inhibitor apocynin. Dihydroethidium staining showed that Ang II elicited losartan- and TEMPOL-sensitive superoxide production in arterioles. These results demonstrate that Ang II evokes AT1 receptor–mediated vasoconstriction and AT2 receptor–mediated vasodilation of coronary arterioles. Ang II at a subvasomotor level impairs endothelium-dependent NO-mediated dilation attributable to elevated superoxide production via AT1 receptor activation of NADPH oxidase. These data may partly explain the impaired coronary flow regulation in heart diseases associated with an upregulated renin-angiotensin system.

Heterogeneous β2-Adrenoceptor Expression and Dilation in Coronary Arterioles Across the Left Ventricular Wall

Background—Previous in vivo studies have shown that &bgr;-adrenoceptor agonists cause a redistribution of coronary flow away from the subendocardium; however, the underlying mechanism remains uncertain. We tested the hypothesis that a heterogeneous distribution of &bgr;-adrenoceptors and their vasomotor responses exists in the coronary microcirculation across the left ventricular wall. Methods and Results—Porcine subepicardial and subendocardial arterioles (<100 &mgr;m) were isolated from the left ventricle and pressurized for in vitro study of vasodilation to the nonselective &bgr;-adrenoceptor agonist isoproterenol and the selective &bgr;2-adrenoceptor agonist procaterol. Both vessel types developed a similar level of basal tone and dilated to isoproterenol and procaterol. However, subepicardial arterioles exhibited a much higher sensitivity and greater dilation capacity to both agonists. The isoproterenol-induced vasodilations were inhibited by glibenclamide, an ATP-sensitive potassium (KATP) channel blocker. In contrast to isoproterenol, dilations of subepicardial and subendocardial arterioles to pinacidil, a KATP channel opener, were similar. In both vessel types, isoproterenol-induced dilation was inhibited by the &bgr;2-adrenoceptor blocker ICI-118,551 but was insensitive to the &bgr;1-adrenoceptor blocker atenolol. Reverse transcription–polymerase chain reaction and immunohistochemical data revealed that &bgr;2-adrenoceptor mRNA and protein expression, respectively, were markedly greater in subepicardial arterioles. Conclusions—This study demonstrates that selective activation of &bgr;2-adrenoceptors elicits dilation of both subepicardial and subendocardial arterioles through opening of KATP channels. The higher &bgr;2-adrenoceptor expression in subepicardial arterioles may contribute to the greater dilation of these vessels to &bgr;2-adrenoceptor activation.