The three subtypes of
calcium-activated potassium channels (K(Ca)) of large, intermediate and small conductance (BK(Ca), IK(Ca) and SK(Ca)) are present in the vascular wall. In healthy arteries, BK(Ca) channels are preferentially expressed in vascular smooth muscle cells, while IK(Ca) and SK(Ca) are preferentially located in endothelial cells. The activation of endothelial IK(Ca) and SK(Ca) contributes to
nitric oxide (NO) generation and is required to elicit endothelium-dependent hyperpolarizations. In the latter responses, the hyperpolarization of the smooth muscle cells is evoked either via electrical coupling through myo-endothelial gap junctions or by
potassium ions, which by accumulating in the intercellular space activate the
inwardly rectifying potassium channel Kir2.1 and/or the Na(+)/K(+)-
ATPase. Additionally, endothelium-derived factors such as
cytochrome P450-derived epoxyeicosatrienoic
acids and under some circumstances NO,
prostacyclin,
lipoxygenase products and
hydrogen peroxide (H(2)O(2)) hyperpolarize and relax the underlying smooth muscle cells by activating BK(Ca). In contrast,
cytochrome P450-derived
20-hydroxyeicosatetraenoic acid and various endothelium-derived contracting factors inhibit BK(Ca). Aging and
cardiovascular diseases are associated with endothelial dysfunctions that can involve a decrease in NO bioavailability, alterations of
EDHF-mediated responses and/or enhanced production of endothelium-derived contracting factors. Because
potassium channels are involved in these endothelium-dependent responses, activation of endothelial and/or smooth muscle K(Ca) could prevent the occurrence of endothelial dysfunction. Therefore, direct activators of these
potassium channels or compounds that regulate their activity or their expression may be of some therapeutic interest. Conversely, blockers of IK(Ca) may prevent restenosis and that of BK(Ca) channels
sepsis-dependent
hypotension.