Evidence for Involvement of Both IK _Ca and SK _Ca Channels in Hyperpolarizing Responses of the Rat Middle Cerebral Artery

Abstract

Background and Purpose-Endothelium-derived hyperpolarizing factor responses in the rat middle cerebral artery are blocked by inhibiting IK Ca channels alone, contrasting with peripheral vessels where block of both IK Ca and SK Ca is required. As the contribution of IK Ca and SK Ca to endothelium-dependent hyperpolarization differs in peripheral arteries, depending on the level of arterial constriction, we investigated the possibility that SK Ca might contribute to equivalent hyperpolarization in cerebral arteries under certain conditions. Methods-Rat middle cerebral arteries (Ϸ175 m) were mounted in a wire myograph. The effect of K Ca channel blockers on endothelium-dependent responses to the protease-activated receptor 2 agonist, SLIGRL (20 mol/L), were then assessed as simultaneous changes in tension and membrane potential. These data were correlated with the distribution of arterial K Ca channels revealed with immunohistochemistry. Results-SLIGRL hyperpolarized and relaxed cerebral arteries undergoing variable levels of stretch-induced tone. The relaxation was unaffected by specific inhibitors of IK Ca (TRAM-34, 1 mol/L) or SK Ca (apamin, 50 nmol/L) alone or in combination. In contrast, the associated smooth-muscle hyperpolarization was inhibited, but only with these blockers in combination. Blocking nitric oxide synthase (NOS) or guanylyl cyclase evoked smooth-muscle depolarization and constriction, with both hyperpolarization and relaxation to SLIGRL being abolished by TRAM-34 alone, whereas apamin had no effect. Immunolabeling showed SK Ca and IK Ca within the endothelium. Conclusions-In the absence of NO, IK Ca underpins endothelium-dependent hyperpolarization and relaxation in cerebral arteries. However, when NOS is active SK Ca contributes to hyperpolarization, whatever the extent of background contraction. These changes may have relevance in vascular disease states where NO release is compromised and when the levels of SK Ca expression may be altered.

Endothelial coordination of cerebral vasomotion via myoendothelial gap junctions containing connexins 37 and 40

“…While incubation with apamin and charybdotoxin did result in loss of regular contractions and appearance of asynchronous smooth muscle cell [Ca 2ϩ ] i oscillations, these effects were accompanied by significant depolarization and constriction, an effect similar to that found previously after individual inhibition of IK Ca but not BK Ca , SK Ca , or K v (13). Because apamin and charybdotoxin produced the same effect in the absence of the endothelium, an action at IK Ca in the smooth muscle is likely, and, indeed, IK Ca is present in the media of the basilar artery, as shown in the present study, and in other cerebral vessels (35). Thus the loss of basilar artery vasomotion after K Ca inhibition can be attributed simply to an increase in smooth muscle cell [Ca 2ϩ ] i after membrane depolarization, leading to augmented vasoconstriction and a reduction in the amplitude and synchronization of oscillatory behavior of smooth muscle cells.…”

“…The number of smooth muscle cell profiles was determined as the average along four linear plots 90°apart, from the outer edge of the internal elastic lamina to the adventitia. To detect myoendothelial gap junctions, the internal elastic lamina in each serial section was examined at ϫ10,000, and all myoendothelial gap junctions were photographed at high magnification (ϫ20,000 to ϫ40,000) and identified according to their characteristic pentalaminar membrane structure (35,46). The number of myoendothelial gap junctions per endothelial cell was determined from the surface area of the arterial segment at the level of the internal elastic lamina, the surface area of endothelial cells, and the number of myoendothelial gap junctions present in this region.…”

The potassium channel KCa3.1 constitutes a pharmacological target for neuroinflammation associated with ischemia/reperfusion stroke

“…Invariably, the infarcted areas showed intense KCa3.1 staining (Figure 3a), which, on closer examination, localized to activated microglia/macrophages (M) and vascular endothelial cells (E), where KCa3.1 is expressed and involved in fluid movement 14 and endothelium-derived hyperpolarization. 15,26 Immunofluorescence confirmed KCa3.1 expression on amoeboid-shaped microglia/macrophages (Figure 3b). We further stained brain sections for Kv1.3 and observed similar intense Kv1.3 staining in the infarct areas, localized to microglia/macrophages ( Figure 3c and 3d).…”

The Ca2+-Activated K+ChannelKCNN4/KCa3.1 Contributes to Microglia Activation and Nitric Oxide-Dependent Neurodegeneration

“…KCa3.1 blockade reduced inflammatory cytokine production and damage in the spinal cord in a murine model of multiple sclerosis (Reich et al, 2005), but potential contributions of CNS cells were not determined. Blocking KCa3.1 channels in endothelia and smooth muscle cells in the rat middle cerebral artery inhibited hyperpolarization and vessel relaxation (McNeish et al, 2006). Together with the roles of KCa3.1 in microglia-mediated neurodegeneration, this suggests a possible target for stroke.…”