Myoendothelial coupling in the mesenteric arterial bed; segmental differences and interplay between nitric oxide and endothelin-1

BACKGROUND AND PURPOSE

We tested the hypothesis that activated arterial smooth muscle (ASM) stimulates endothelial vasomotor influences via gap junctions and that the significance of this myoendothelial coupling increases with decreasing arterial diameter.

EXPERIMENTAL APPROACH

From WKY rats, first-, second-, third- and fourth-order branches of the superior mesenteric artery (MA1, MA2, MA3 and MA4 respectively) were isolated and mounted in wire-myographs to record vasomotor responses to 0.16-20 micromol x L(-1) phenylephrine.

KEY RESULTS

Removal of endothelium increased the sensitivity (pEC(50)) to phenylephrine in all arteries. The nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) (100 micromol x L(-1)) did not modify pEC(50) to phenylephrine in all denuded arteries, and increased it in intact MA1, MA2 and MA3 to the same extent as denudation. However, in intact MA4, the effect of L-NAME was significantly larger (DeltapEC(50) 0.57 +/- 0.02) than the effect of endothelium removal (DeltapEC(50) 0.20 +/- 0.06). This endothelium-dependent effect of L-NAME in MA4 was inhibited by (i) steroidal and peptidergic uncouplers of gap junctions; (ii) a low concentration of the NO donor sodium nitroprusside; and (iii) by the endothelin-receptor antagonist bosentan. It was also observed during contractions induced by (i) calcium channel activation (BayK 8644, 0.001-1 micromol x L(-1)); (ii) depolarization (10-40 mmol x L(-1) K(+)); and (iii) sympathetic nerve stimulation (0.25-32 Hz).

CONCLUSIONS AND IMPLICATIONS

These pharmacological observations indicated feedback control by endothelium of ASM reactivity involving gap junctions and a balance between endothelium-derived NO and endothelin-1. This myoendothelial coupling was most prominent in distal resistance arteries.

Two-photon microscopy of vital murine elastic and muscular arteries. Combined structural and functional imaging with subcellular resolution

Understanding vascular pathologies requires insight in the structure and function, and, hence, an imaging technique combining subcellular resolution, large penetration depth, and optical sectioning. We evaluated the applicability of two-photon laser-scanning microscopy (TPLSM) in large elastic and small muscular arteries under physiological conditions. Elastic (carotid) and muscular (uterine, mesenteric) arteries of C57BL/6 mice were mounted in a perfusion chamber. TPLSM was used to assess the viability of arteries and to visualize the structural components elastin, collagen, nuclei, and endothelial glycocalyx (EG). Functionality was determined using diameter changes in response to noradrenaline and acetylcholine. Viability and functionality were maintained up to 4 h, enabling the assessment of structure-function relationships. Structural vessel wall components differed between elastic and muscular arteries: size (1.3 vs. 2.1 microm) and density (0.045 vs. 0.57 microm(-2)) of internal elastic lamina fenestrae, smooth muscle cell density (3.50 vs. 1.53 microm(-3)), number of elastic laminae (3 vs. 2), and adventitial collagen structure (tortuous vs. straight). EG in elastic arteries was 4.5 microm thick, covering 66% of the endothelial surface. TPLSM enables visualization and quantification of subcellular structures in vital and functional elastic and muscular murine arteries, allowing unraveling of structure-function relationships in healthy and diseased arteries.

Changes in Vascular Distensibility during Angiotensin-Converting Enzyme Inhibition Involve Bradykinin Type 2 Receptors

Changes in arterial stiffness and structure occur during cardiovascular diseases and can be modified by angiotensin-converting enzyme (ACE) inhibitors. In the present study we investigated the role of membrane-bound ACE (t-ACE) in the regulation of arterial structure and mechanics. Large and small arteries of t-ACE-/- mice were isolated to determine the passive pressure-diameter relationship. We observed that t-ACE-/- mice exhibit a reduced arterial distensibility compared to t-ACE+/+ mice. This reduced arterial distensibility was also observed after 9 weeks of captopril treatment (80 mg/kg/ day). We hypothesized that bradykinin type 2 receptor (BK(2)) stimulation might be involved in the regulation of arterial stiffness. t-ACE-/- and t-ACE+/+ mice were treated with Hoe 140 (1 mg/kg/day) for 14 days. After Hoe 140 treatment, both the structural and mechanical changes observed in the t-ACE-/- carotid artery were abolished. Although Hoe 140 administration increased blood pressure in both groups by approximately 10 mm Hg, the pressure difference between the two groups did not change. Thus, t-ACE is involved in the regulation of arterial distensibility. The changes observed in t-ACE-/- mice are not caused by an altered fetal development. Moreover, it is likely that the regulation of arterial distensibility by ACE involves stimulation of the BK(2) receptor.