KEY POINTS: Several different voltage-dependent K (K ) channel isoforms are expressed in arterial smooth muscle cells (myocytes). Vasoconstrictors inhibit K currents, but the isoform selectivity and mechanisms involved are unclear. We show that angiotensin II (Ang II), a vasoconstrictor, stimulates degradation of K 1.5, but not K 2.1, channels through a protein kinase C- and lysosome-dependent mechanism, reducing abundance at the surface of mesenteric artery myocytes. The Ang II-induced decrease in cell surface K 1.5 channels reduces whole-cell K 1.5 currents and attenuates K 1.5 function in pressurized arteries. We describe a mechanism by which Ang II stimulates protein kinase C-dependent K 1.5 channel degradation, reducing the abundance of functional channels at the myocyte surface.
ABSTRACT: Smooth muscle cells (myocytes) of resistance-size arteries express several different voltage-dependent K (K ) channels, including K 1.5 and K 2.1, which regulate contractility. Myocyte K currents are inhibited by vasoconstrictors, including angiotensin II (Ang II), but the mechanisms involved are unclear. Here, we tested the hypothesis that Ang II inhibits K currents by reducing the plasma membrane abundance of K channels in myocytes. Angiotensin II (applied for 2 h) reduced surface and total K 1.5 protein in rat mesenteric arteries. In contrast, Ang II did not alter total or surface K 2.1, or K 1.5 or K 2.1 cellular distribution, measured as the percentage of total protein at the surface. Bisindolylmaleimide (BIM; a protein kinase C blocker), a protein kinase C inhibitory peptide or bafilomycin A (a lysosomal degradation inhibitor) each blocked the Ang II-induced decrease in total and surface K 1.5. Immunofluorescence also suggested that Ang II reduced surface K 1.5 protein in isolated myocytes; an effect inhibited by BIM. Arteries were exposed to Ang II or Ang II plus BIM (for 2 h), after which these agents were removed and contractility measurements performed or myocytes isolated for patch-clamp electrophysiology. Angiotensin II reduced both whole-cell K currents and currents inhibited by Psora-4, a K 1.5 channel blocker. Angiotensin II also reduced vasoconstriction stimulated by Psora-4 or 4-aminopyridine, another K channel inhibitor. These data indicate that Ang II activates protein kinase C, which stimulates K 1.5 channel degradation, leading to a decrease in surface K 1.5, a reduction in whole-cell K 1.5 currents and a loss of functional K 1.5 channels in myocytes of pressurized arteries.
