Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats.

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

Myeloperoxidase evokes substantial vasomotor responses in isolated skeletal muscle arterioles of the rat.

AIMS: Myeloperoxidase (MPO) catalyses the formation of a wide variety of oxidants, including hypochlorous acid (HOCl), and contributes to cardiovascular disease progression. We hypothesized that during its action MPO evokes substantial vasomotor responses. METHODS: Following exposure to MPO (1.92 mU mL(-1)) in the presence of increasing concentrations of hydrogen peroxide (H2O2), changes in arteriolar diameter of isolated gracilis skeletal muscle arterioles (SMAs) and coronary arterioles (CAs) and in the isometric force in basilar arteries (BAs) of the rat were monitored. RESULTS: Myeloperoxidase increased vascular tone to different degrees in CAs, SMAs and BAs. The mechanism of increased vasoconstriction was studied in detail in SMAs. MPO-evoked vasoconstrictions were prevented by the MPO inhibitor 4-aminobenzhydrazide (50 µM), by endothelium removal in the SMAs. Surprisingly, the HOCl scavenger L-methionine (100 µM), the thromboxane A2 (TXA2) antagonist SQ-29548 (1 µM) or the non-specific cyclooxygenase (COX) antagonist indomethacin (1 µM) converted the MPO-evoked vasoconstrictions to pronounced vasodilations in SMAs, not seen in the presence of H2O2. In contrast to noradrenaline-induced vasoconstrictions, the MPO-evoked vasoconstrictions were not accompanied by significant increases in arteriolar [Ca(2+)] levels in SMAs. CONCLUSION: These data showed that H2O2 -derived HOCl to be a potent vasoconstrictor upon MPO application. HOCl activated the COX pathway, causing the synthesis and release of a TXA2-like substance to increase the Ca(2+) sensitivity of the contractile apparatus in vascular smooth muscle cells and thereby to augment H2 O2 -evoked vasoconstrictions. Nevertheless, inhibition of the HOCl-COX-TXA2 pathway unmasked the effects of additional MPO-derived radicals with a marked vasodilatory potential in SMAs.

Structure-activity relationships of vanilloid receptor agonists for arteriolar TRPV1.

BACKGROUND AND PURPOSE: The transient receptor potential vanilloid 1 (TRPV1) plays a role in the activation of sensory neurons by various painful stimuli and is a therapeutic target. However, functional TRPV1 that affect microvascular diameter are also expressed in peripheral arteries and we attempted to characterize this receptor. EXPERIMENTAL APPROACH: Sensory TRPV1 activation was measured in rats by use of an eye wiping assay. Arteriolar TRPV1-mediated smooth muscle specific responses (arteriolar diameter, changes in intracellular Ca(2+)) were determined in isolated, pressurized skeletal muscle arterioles obtained from the rat and wild-type or TRPV1(-/-) mice and in canine isolated smooth muscle cells. The vascular pharmacology of the TRPV1 agonists (potency, efficacy, kinetics of action and receptor desensitization) was determined in rat isolated skeletal muscle arteries. KEY RESULTS: Capsaicin evoked a constrictor response in isolated arteries similar to that mediated by noradrenaline, this was absent in arteries from TRPV1 knockout mice and competitively inhibited by TRPV1 antagonist AMG9810. Capsaicin increased intracellular Ca(2+) in the arteriolar wall and in isolated smooth muscle cells. The TRPV1 agonists evoked similar vascular constrictions (MSK-195 and JYL-79) or were without effect (resiniferatoxin and JYL-273), although all increased the number of responses (sensory activation) in the eye wiping assay. Maximal doses of all agonists induced complete desensitization (tachyphylaxis) of arteriolar TRPV1 (with the exception of capsaicin). Responses to the partial agonist JYL-1511 suggested 10% TRPV1 activation is sufficient to evoke vascular tachyphylaxis without sensory activation. CONCLUSIONS AND IMPLICATIONS: Arteriolar TRPV1 have different pharmacological properties from those located on sensory neurons in the rat.