Characterization of the Thoracodorsal Artery: Morphology and Reactivity : Characterization of the Thoracodorsal Artery

Perivascular adipose tissue secretes an adipocyte-derived relaxing factor(s) (ADRF) that opens K(v) channels in rat arteries. Visceral fat accumulation causes adipocyte dysfunction, including hyposecretion of adiponectin. We tested the hypothesis that ADRF might be adiponectin and that adiponectin plays a role in the paracrine control of vascular tone by perivascular adipose tissue. We studied Sprague-Dawley rats, wild-type and adiponectin gene-deficient (Apn 1-/-) mice, and New Zealand obese (NZO) mice. In rat aortas, recombinant adiponectin at serum levels (2-5 microg/ml) inhibited serotonin-induced contractions. The effects were abolished by K(v) channel inhibition with 4-aminopyridine (4-AP, 2 mM). Similar effects were observed in NZO mouse mesenteric arteries. To study vascular function in Apn 1-/- mice, the mesenteric vascular bed was isolated, cannulated, and perfused at a constant 4-5-ml/min flow in the absence and presence of serotonin. 4-AP (2 mM) induced a similar increase in perfusion pressure in the Apn 1-/- perfused isolated mesenteric vascular bed, compared to wild-type mice. Removal of perivascular fat increased the vasoconstrictor responses, but abolished the 4-AP effects. The anti-contractile effects of perivascular fat were similar in mesenteric artery and aortic rings from Apn 1-/- and wild-type mice. Despite high adiponectin levels, the anti-contractile effects of perivascular fat were diminished in mesenteric arteries of NZO mice with age. Adiponectin is a novel humoral vasodilator that relaxes aortic and mesenteric rings by opening K(v) channels. Similar to the rat, perivascular adipose tissue of the mouse harbors an ADRF, which is malfunctional in NZO mice and is not adiponectin.

It has been proposed that activation of endothelial SK3 (K(Ca)2.3) and IK1 (K(Ca)3.1) K+ channels plays a role in the arteriolar dilation attributed to an endothelium-derived hyperpolarizing factor (EDHF). However, our understanding of the precise function of SK3 and IK1 in the EDHF dilator response and in blood pressure control remains incomplete. To clarify the roles of SK3 and IK1 channels in the EDHF dilator response and their contribution to blood pressure control in vivo, we generated mice deficient for both channels. Expression and function of endothelial SK3 and IK1 in IK1(-/-)/SK3(T/T) mice was characterized by patch-clamp, membrane potential measurements, pressure myography, and intravital microscopy. Blood pressure was measured in conscious mice by telemetry. Combined IK1/SK3 deficiency in IK1(-/-)/SK3(T/T) (+doxycycline) mice abolished endothelial K(Ca) currents and impaired acetylcholine-induced smooth muscle hyperpolarization and EDHF-mediated dilation in conduit arteries and in resistance arterioles in vivo. IK1 deficiency had a severe impact on acetylcholine-induced EDHF-mediated vasodilation, whereas SK3 deficiency impaired NO-mediated dilation to acetylcholine and to shear stress stimulation. As a consequence, SK3/IK1-deficient mice exhibited an elevated arterial blood pressure, which was most prominent during physical activity. Overexpression of SK3 in IK1(-/-)/SK3(T/T) mice partially restored EDHF- and nitric oxide-mediated vasodilation and lowered elevated blood pressure. The IK1-opener SKA-31 enhanced EDHF-mediated vasodilation and lowered blood pressure in SK3-deficient IK1(+/+)/SK3(T/T) (+doxycycline) mice to normotensive levels. Our study demonstrates that endothelial SK3 and IK1 channels have distinct stimulus-dependent functions, are major players in the EDHF pathway, and significantly contribute to arterial blood pressure regulation. Endothelial K(Ca) channels may represent novel therapeutic targets for the treatment of hypertension.

The coordination of vascular smooth muscle cell constriction plays an important role in vascular function, such as regulation of blood pressure; however, the mechanism responsible for vascular smooth muscle cell communication is not clear in the resistance vasculature. Pannexins (Panx) are purine-releasing channels permeable to the vasoconstrictor ATP and thus may play a role in the coordination of vascular smooth muscle cell constriction. We investigated the role of pannexins in phenylephrine- and KCl-mediated constriction of resistance arteries. Western blot, immunohistochemistry, and immunogold labeling coupled to scanning and transmission electron microscopy revealed the presence of Panx1 but not Panx2 or Panx3 in thoracodorsal resistance arteries. Functionally, the contractile response of pressurized thoracodorsal resistance arteries to phenylephrine was decreased significantly by multiple Panx inhibitors (mefloquine, probenecid, and (10)Panx1), ectonucleotidase (apyrase), and purinergic receptor inhibitors (suramin and reactive blue-2). Electroporation of thoracodorsal resistance arteries with either Panx1-green fluorescent protein or Panx1 small interfering RNA showed enhanced and decreased constriction, respectively, in response to phenylephrine. Lastly, the Panx inhibitors did not alter constriction in response to KCl. This result is consistent with coimmunoprecipitation experiments from thoracodorsal resistance arteries, which suggested an association between Panx1 and α1D-adrenergic receptor. Our data demonstrate for the first time a key role for Panx1 in resistance arteries by contributing to the coordination of vascular smooth muscle cell constriction and possibly to the regulation of blood pressure.