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Background and Purpose — The structurally related, inwardly rectifying K + (K IR ) channel and the ATP-sensitive K + (K ATP ) channel are important modulators of cerebral artery tone. Although protein kinase C (PKC) activators have been shown to inhibit these channels with the use of patch-clamp electrophysiology, effects of PKC on K + channel function in intact cerebral blood vessels are unknown. We therefore tested whether pharmacological alteration of PKC activity affects cerebral vasodilator responses to K IR and/or K ATP channel activators in vivo. Methods — We measured changes in basilar artery diameter using a cranial window preparation in anesthetized rats. In addition, intracellular recordings of smooth muscle membrane potential were made in isolated basilar arteries. Results — K + (5 to 15 mmol/L) and aprikalim (1 to 10 μmol/L) each elicited reproducible vasodilatation. The PKC activator phorbol-12,13-dibutyrate (PdBu) (50 nmol/L) inhibited responses to K + (by 40% to 55%) and aprikalim (by 40% to 70%), whereas responses to papaverine were unaffected. The PKC inhibitor calphostin C (0.1 μmol/L) augmented responses to K + (by 2- to 3-fold) and aprikalim (2-fold) but not papaverine. In addition, K + (5 mmol/L) and aprikalim (3 μmol/L) each hyperpolarized the basilar artery. PdBu inhibited these responses to aprikalim by 45% but had no effect on K + -induced hyperpolarization. Conclusions — These data suggest that both basal and stimulated PKC activity inhibit K IR and K ATP channel–mediated cerebral vasodilatation in vivo. The inhibitory effect on K ATP channel–mediated vasodilatation occurs at least partly by inhibition of hyperpolarization mediated by K ATP channels. PKC inhibits K + -induced vasodilatation without affecting hyperpolarization, suggesting that the inhibitory effect of PKC on vasodilator responses to K + does not involve altered K IR channel function.

Potassium ion (K + ) normally causes cerebral vasodilatation by activating inwardly rectifying K + (K IR ) channels. We tested whether chronic hypertension affects the magnitude and/or mechanism of K + -induced cerebral vasodilatation in vivo. Basilar artery responses were examined in anesthetized Wistar-Kyoto (WKY; mean arterial pressure, 114±4 mm Hg) and spontaneously hypertensive (SHR; 176±3 mm Hg) rats. In WKY, elevating cerebrospinal fluid K + concentration from 3 mmol/L to 5 and 10 mmol/L caused vasodilatation (percent maximum, 12±1 and 48±7, respectively). The response to 5 mmol/L K + was greater in SHR (percent maximum, 17±2 [ P IR channel inhibitor, barium ion (Ba 2+ , 100 μmol/L) selectively inhibited dilator responses to 5 and 10 mmol/L K + by ≈75% in WKY. In SHR, Ba 2+ had no effect on the response to 5 mmol/L K + , and only partially inhibited (by ≈40%) the response to 10 mmol/L K + . The nonselective NO synthase (NOS) inhibitor Nω-nitro- l -arginine methyl ester, the neuronal NOS (nNOS) inhibitor 1-(2-trifluromethyl-phenyl)imidazole, and the N -type calcium channel inhibitor ω-conotoxin GVIA, were all without effect in WKY, but markedly inhibited the response to 5 mmol/L K + in SHR. When applied together with Ba 2+ , each of these inhibitors also profoundly reduced responses to 10 mmol/L K + in SHR. Immunostaining of basilar arteries revealed that the perivascular nNOS-containing nerve plexus was denser in SHR. Thus, K + dilates the normotensive basilar artery predominantly via K IR channel activation. During chronic hypertension, small physiological elevations in K + dilate the basilar artery by an nNOS-dependent mechanism that appears to be upregulated in a compensatory manner.

Dilatation of cerebral arterioles in response to arachidonic acid is dependent on activity of cyclooxygenase. In this study, we examined mechanisms that mediate dilatation of the basilar artery in response to arachidonate. Diameter of the basilar artery (baseline diameter = 216 ± 7 μm) (means ± SE) was measured using a cranial window in anesthetized rats. Arachidonic acid (10 and 100 μM) produced concentration-dependent vasodilatation that was not inhibited by indomethacin (10 mg/kg iv) or NG-nitro-l-arginine (100 μM) but was inhibited markedly by baicalein (10 μM) or nordihydroguaiaretic acid (NDGA; 10 μM), inhibitors of the lipoxygenase pathway. Dilatation of the basilar artery was also inhibited markedly by tetraethylammonium ion (TEA; 1 mM) or iberiotoxin (50 nM), inhibitors of calcium-dependent potassium channels. For example, 10 μM arachidonate dilated the basilar artery by 19 ± 7 and 1 ± 1% in the absence and presence of iberiotoxin, respectively. Measurements of membrane potential indicated that arachidonate produced hyperpolarization of the basilar artery that was blocked completely by TEA. Incubation with [3H]arachidonic acid followed by reverse-phase and chiral HPLC indicated that the basilar artery produces relatively small quantities of prostanoids but large quantities of 12(S)-hydroxyeicosatetraenoic acid (12-S-HETE), a lipoxygenase product. Moreover, the production of 12-HETE was inhibited by baicalein or NDGA. These findings suggest that dilatation of the basilar artery in response to arachidonate is mediated by a product(s) of the lipoxygenase pathway, with activation of calcium-dependent potassium channels and hyperpolarization of vascular muscle.

We tested whether activation of inwardly rectifying K+ (Kir) channels, Na+-K+-ATPase, or nitric oxide synthase (NOS) play a role in K+-induced dilatation of the rat basilar artery in vivo. When cerebrospinal fluid [K+] was elevated from 3 to 5, 10, 15, 20, and 30 mM, a reproducible concentration-dependent vasodilator response was elicited (change in diameter = 9 ± 1, 27 ± 4, 35 ± 4, 43 ± 12, and 47 ± 16%, respectively). Responses to K+ were inhibited by ∼50% by the Kir channel inhibitor BaCl2 (30 and 100 μM). In contrast, neither ouabain (1–100 μM, a Na+-K+-ATPase inhibitor) nor N G-nitro-l-arginine (30 μM, a NOS inhibitor) had any effect on K+-induced vasodilatation. These concentrations of K+ also hyperpolarized smooth muscle in isolated segments of basilar artery, and these hyperpolarizations were virtually abolished by 30 μM BaCl2. RT-PCR experiments confirmed the presence of mRNA for Kir2.1 in the basilar artery. Thus K+-induced dilatation of the basilar artery in vivo appears to partly involve hyperpolarization mediated by Kir channel activity and possibly another mechanism that does not involve hyperpolarization, activation of Na+-K+-ATPase, or NOS.

Maintenance of vascular tone by the endothelium involves the production of endothelium-derived nitric oxide (NO). NO, produced from endothelial nitric oxide synthase diffuses to the underlying smooth muscle to stimulate soluble guanylate cyclase, resulting in increased cyclic GMP levels, and subsequent smooth muscle relaxation and blood vessel dilatation. Endothelial dysfunction, manifested as diminished NO bioavailability, is a common feature of a number of vascular-related diseases.. Oxidative stress can be defined as an imbalance between reactive oxygen species (ROS) production and/or impaired ROS metabolism that favours them being present in excess of physiological levels. Oxidative stress can negatively impact many cell types, including in the vasculature. There is now a wealth of evidence suggesting that oxidative stress is a major cause of endothelial dysfunction in the cerebral circulation. This review will summarize disease models in which both oxidative stress and endothelial dysfunction occur in the cerebral circulation, namely hypertension involving angiotensin II (Ang II), diabetes, subarachnoid hemorrhage, stroke and Alzheimer's disease. Molecular mechanisms by which oxidative stress occurs, (eg increased NADPH-oxidase activity) will also be discussed.