Channels in Murine Endothelial Cells: Block by Intracellular Calcium and Magnesium

Blood fl ow is intimately linked to endothelial membrane potential and intracellular Ca 2+ levels ([Ca 2+ ] i ). Endothelial Ca 2+ infl ux appears to depend on the electrochemical gradient, and likely occurs through nonvoltage-dependent Ca 2+ entry pathways, possibly transient receptor potential (TRP) channels. Therefore, hyperpolarization of the endothelium membrane elevates [Ca 2+ ] i by an increase in Ca 2+ infl ux that induces relaxation of the underlying smooth muscle ( Luckhoff and Busse, 1990 ) through endothelium-derived hyperpolarizing factors (EDHFs) and the generation of nitric oxide and prostacyclin. Several types of potassium channels have been proposed to regulate endothelial membrane potential, including large conductance, calcium-sensitive potassium (BK Ca ) channels, inward rectifi er (K ir ) potassium channels, and small (SK Ca ) and intermediate (IK Ca ) conductance Ca 2+ -activated potassium channels ( Hoger et al., 2002 ; Shimoda et al., 2002 ; Fang et al., 2005 ). SK Ca and IK Ca channels in vascular endothelium appear to have particularly prominent roles, since inhibition of these channels prevents EDHFmediated vasodilation ( Eichler et al., 2003 ; Weston et al., 2005 ; Feletou and Vanhoutte, 2006 ). The current view is that endothelial-dependent vasodilators such as acetylcholine, bradykinin, or substance P elevate intracellular calcium through calcium infl ux and release; this in turn activates SK Ca and IK Ca channels, which cause membrane potential hyperpolarization and further elevation of intracellular calcium through increases in the calcium electrochemical gradient. The SK Ca and IK Ca -induced membrane hyperpolarization and elevation of intracellular calcium directly or indirectly induce membrane hyperpolarization and relaxation of the nearby vascular smooth muscle (Feletou and Vanhoutte , 2006). Therefore, SK Ca and IK Ca channels in vascular endothelial cells are thought to act as a positive feedback element, such that their activation causes membrane potential hyperpolarization and thus increased calcium entry ( Garland et al., 1995 ; Marchenko and Sage, 1996 ; Eichler et al., 2003 ; Weston et al., 2005 ; Feletou and Vanhoutte , 2006). However, the role of these channels in the regulation of endothelial membrane potential and intracellular calcium in the absence of endothelial agonists is not known. Although it is clear that SK Ca and IK Ca channels can affect endothelial membrane potential, it is unclear how membrane potential regulates currents through Ca 2+ activated K + Channels in Murine Endothelial Cells: Block by Intracellular Calcium and Magnesium