Potassium channel diversity in vascular smooth muscle cells

Several recent observations suggest that the vascular medium is a mosaic of functionally and morphologically unique cell types. This diversity includes differences in cell phenotype and expression of cytoskeletal and contractile proteins as well as heterogeneity of the number and activity of potassium (K+) channel types. K+ channels play a role in the regulation of arterial tone and in the control of cell proliferation. There is evidence for cell to cell, segment to segment, and vascular bed to bed diversity of K+ channels that could explain the varying responses of arterial segments or different arteries to stimuli such as hypoxia, vasoactive drugs, or arterial wall injury. Pulmonary artery vascular smooth muscle cells contain several types of K+ channels, including calcium sensitive (KCa), delayed rectifier (KDR), and ATP gated (KATP). Hypoxic pulmonary vasoconstriction (HPV) is more prominent in the resistance than in the conduit arteries. HPV is initiated by the inhibition of a KDR channel, resulting in membrane depolarization, increase in the intracellular calcium, and contraction. We have shown that some pulmonary artery smooth muscle cells are enriched in KDR channels whereas others have more KCa channels. These cells can be differentiated by their morphology (using light microscopy and electron microscopy) and their electrical properties (using patch-clamp techniques). Although present throughout the pulmonary artery, KDR-enriched cells are more prominent in the distal-resistance segments whereas KCa-enriched cells are more prominent in the proximal-conduit segments. Nitric oxide (NO) causes relaxation in part by activating a KCa channel, causing membrane hyperpolarization and inactivation of the voltage-gated calcium channels. NO is a slightly more potent vasodilator in the conduit than in the resistance pulmonary artery. In summary, the pulmonary artery may be thought of as a mosaic of cells that have different proportions of key proteins, such as K+ channel subtypes, which confer upon the cell an ability to respond to a stimulus (hypoxia or NO) differently than an adjacent cell exposed to the same stimulus. The prevalence of these cells differs from conduit to resistance arteries. Diversity of cell function may be important in physiology and pathophysiology, allowing responses to vasodilators, vasoconstrictors, and proliferative stimuli to vary within or between vascular beds.