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

Abstract

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.

Figure 1

Effect of denudation and L-NAME on responses of α₁-adrenoceptors. Contractile responses of endothelium-intact (+E) and-denuded (−E) first-(MA1; A), second-(MA2; B), third-(MA3; C) and fourth-order (MA4; D) mesenteric arteries (MA) to phenylephrine (PHE). Concentration-response curves tophenylephrine (0.16–20 µmol·L⁻¹) were analysed in the absence (circles) and in the presence of the NO synthase inhibitor L-NAME (100 µmol·L⁻¹; squares). Data are expressed as percentage the response to 10 µmol·L⁻¹ noradrenaline (NA) in the absence of inhibitor and are shown as mean ± SEM. L-NAME, N^ω-nitro-L-arginine methyl ester; NO, nitric oxide.

Figure 2

Sensitivity to phenylephrine. Sensitivity (pEC₅₀) to phenylephrine (PHE) for endothelium-intact (+E) and endothelium-denuded (−E) first-(MA1; A), second-(MA2; B), third-(MA3; C) and fourth-order (MA4; D) mesenteric arteries (MA) in the absence and presence of 100 µmol·L⁻¹ L-NAME. pEC₅₀ values were calculated for control conditions (control), in the presence of the non-selective ET-receptor antagonist bosentan (10 µmol·L⁻¹) and of the gap junction uncoupler 18α-glycyrrhetinic acid (18α-GA, 20 µmol·L⁻¹). Data are shown as mean ± SEM. *P < 0.05 versus +E. L-NAME, N^ω-nitro-L-arginine methyl ester.

Figure 3

Effect of sodium nitroprusside on responses to α₁-adrenoceptor stimulation in the presence of L-NAME. Concentration-response curves for phenylephrine (PHE) in endothelium-denuded (−E) and endothelium-intact (+E) fourth-order mesenteric arteries (MA4) treated with the NO synthase inhibitor L-NAME (100 µmol·L⁻¹). Vessels were studied in the absence and presence of the NO donor sodium nitroprusside (SNP, 30 nmol·L⁻¹, squares). Values are shown as mean ± SEM. L-NAME, N^ω-nitro-L-arginine methyl ester; NO, nitric oxide.

Figure 4

Effects of BayK 8644, depolarization and nerve stimulation. Contractile responses to the L-type voltage-operated calcium channel activator BayK 8644 (A), depolarization with K⁺ (B) and electrical field stimulation (C) in endothelium-intact (+E) and endothelium-denuded (−E) fourth-order mesenteric arteries (MA4) in the absence and presence of the NO synthase antagonist L-NAME (100 µmol·L⁻¹). Values are shown as mean ± SEM. L-NAME, N^ω-nitro-L-arginine methyl ester; NO, nitric oxide.

Figure 5

The effect of L-NAME on contractions mediated by α₁-adrenoceptors depends on arterial diameter. The difference of the sensitivity to phenylephrine (ΔpEC50) in the absence and presence of L-NAME (100 µmol·L⁻¹) in endothelium-intact (+E), +E treated with 18α-glycyrrhetinic acid (18α-GA, 20 µmol·L⁻¹), +E treated with bosentan (BOS, 10 µmol·L⁻¹) and endothelium-denuded (−E) mesenteric arteries (MAs) are shown as a function of arterial lumen diameter at which the largest contractile response to 10 µmol·L⁻¹ noradrenaline could be obtained (optimal diameter). Values are shown as mean ± SEM. A linear regression line connects the data points. For the +E untreated MAs a R² value of 0.92 is shown. L-NAME, N^ω-nitro-L-arginine methyl ester.