Regulation of mesenteric resistance artery diameter by pharmacological modulators of KCa channels

• Author / Creator

  Lunn, Stephanie

• Background: The diameter of resistance arteries, and thus, tissue perfusion and blood pressure, is tightly regulated through the integrated activity of endothelial and smooth muscle cells, and sympathetic nerves. The endothelium regulates the contractility of smooth muscle cells by releasing diffusible factors such as nitric oxide (NO) and via gap junction-mediated electrical coupling; opening of endothelial Ca2+-activated K+ (KCa) channels causes hyperpolarization which spreads to underlying smooth muscle cells to reduce opening of voltage-dependent Ca2+ channels, decrease Ca2+ influx and so limit contraction. The bioavailability and therefore, biological activity, of NO is determined by its interaction with the free radical superoxide anion (O2-), elevated levels of which are associated with risk factors for cardiovascular disease.
  Traditionally, NO and endothelium-dependent smooth muscle hyperpolarization have been regarded as two separate mechanisms for regulation of arterial diameter. However, several lines of recent evidence support the proposal that NO bioavailability and KCa channel activity may be linked: 1. Exposure of endothelial cells to shear stress results in activation of both small conductance KCa channels and increased NO production. 2. Agonist-evoked NO production and NO-mediated relaxations can be inhibited by blockers of endothelial KCa channels. 3. Activators of endothelial KCa channels can evoke NO-mediated relaxation. 4. Stimulation of smooth muscle cells by 1-adrenoceptor agonists engages both endothelial intermediate conductance KCa channels and NO production via a process termed myoendothelial feedback. 5. O2- production by voltage-sensitive NADPH oxidase is reduced by membrane hyperpolarization which may lead to increased bioavailability of NO.
  Thus, my over-arching goal is to further explore the relationship between endothelial KCa channels and NO in regulating resistance artery diameter by testing three hypotheses:

  1. Activation of small conductance KCa channels can enhance NO-mediated inhibition of sympathetic vasoconstriction evoked by increases in shear stress.
  2. Intermediate conductance KCa channel-mediated myoendothelial feedback plays a role in NO-dependent modulation of sympathetic vasoconstriction.
  3. Pharmacological activators of endothelial KCa channels can reduce vascular O2- production and enhance NO-mediated modulation of vasoconstriction To test these hypotheses, I have addressed two major aims:
  4. To investigate the role of endothelial KCa channels in NO-mediated modulation of nerve-evoked vasoconstriction in the perfused mesenteric bed.
  5. To investigate whether pharmacological activators of endothelial KCa channels can modulate vascular O2- production and vasoconstriction stimulated by the 1-adrenoceptor agonist phenylephrine. Methods: To address these aims I have used a combination of functional and biochemical techniques to investigate the effects of modulators of endothelial KCa channels on diameter and O2- production in rat mesenteric resistance arteries. Results/Discussion: My data show that although myoendothelial feedback limits contractile responses to phenylephrine in isolated arteries, this pathway does not appear to contribute to endothelial modulation of sympathetic vasoconstriction at the level of the intact bed. Instead, shear stress-induced activation of small conductance KCa channels and release of NO provides the dominant mechanism for engagement of the endothelium to inhibit sympathetic vasoconstriction. Furthermore, activators of endothelial KCa channels can significantly limit nerve-evoked vasoconstriction. CyPPA (N-cyclohexyl-N-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-4-pyrimidinamine), an activator of small conductance KCa channels, enhances NO-dependent, shear stress-mediated inhibition of sympathetic vasoconstriction whereas SKA-31 (naphtho[1,2-d]thiazol-2-ylamine), an activator of intermediate conductance KCa channels, can directly inhibit release of noradrenaline from perivascular sympathetic nerves. Both CyPPA and SKA-31 can significantly reduce acute increases in O2- production stimulated by phenylephrine in isolated arteries but this effect is not associated with enhancement of NO-mediated endothelial modulation of vasoconstriction. Conclusion: To conclude, I have demonstrated that small and intermediate conductance KCa channels play different functional roles in modulation of nerve-evoked vasoconstriction; endothelial small conductance KCa channels mediate shear stress-induced, NO-dependent inhibition of vasoconstriction whereas the activity of neuronal IKCa channels can directly inhibit release of noradrenaline from sympathetic nerves. These functional roles reflect the differing locations of the channels within endothelial cells and the artery wall. Pharmacological activators of KCa channels can limit vascular O2- production supporting the proposal that the endothelial cell membrane potential may play a key role in vascular health and that targeting these channels could provide a novel approach to reducing O2- levels in disease states.

• Subjects / Keywords

  • vasoconstriction
  • NO
  • nitric oxide
  • ROS
  • shear stress
  • KCa channel
  • nerve
  • endothelium
  • superoxide anions
  • mesenteric arteries

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VD6PM68

• License

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