Functional cross-talk between endothelial muscarinic and α₂-adrenergic receptors in rabbit cerebral arteries

Abstract

1. Interactions between two classes of receptors have been observed in several cell lines and preparations. The aim of this work was to assess the impact of simultaneous stimulation of endothelial muscarinic and α₂-adrenergic receptors (α₂-AR) on vascular reactivity.

2. Rabbit middle cerebral arteries were isolated and changes in isometric tension were recorded in the presence of indomethacin.

3. Inhibition of nitric oxide (NO) synthase with N^ω-nitro-L-arginine (L-NOARG, 100 μmol l⁻¹) revealed α-AR-dependent contractions. Pre-addition of acetylcholine (ACH, 1 μmol l⁻¹) augmented oxymetazoline (OXY, 10 μmol l⁻¹, α₂-AR agonist)-, but decreased phenylephrine (PE, 10 μmol l⁻¹, α₁-AR agonist)-induced contraction (P<0.05). The effects of ACH were endothelium-dependent.

4. Vessels were precontracted with 40 mmol l⁻¹ KCl-physiological salt solution (PSS) in the absence of L-NOARG, or PE or OXY in the presence of L-NOARG. In the presence of high external K⁺ or PE, ACH induced a potent relaxation (P<0.05). In the presence of OXY, however, ACH mediated contraction (P<0.05).

5. After pertussis toxin (PTX, inactivator of Gα_i/o proteins) pre-treatment, α₂-AR-dependent contractions were abolished. Forty mmol l⁻¹ KCl-PSS induced contraction was not altered by PTX whereas ACH-induced relaxation was augmented (P<0.05).

6. To investigate if endothelin-1 (ET-1) intervened in the endothelium-dependent contractile response to ACH in the presence of OXY-dependent tone, vessels were incubated in the presence of BQ123 (1 μmol l⁻¹), an ET_A receptor antagonist. OXY-mediated tone was not affected by BQ123; however, ACH-induced contraction was reversed to a relaxation (P<0.05).

7. These data indicate that activation of endothelial α₂-AR triggers an endothelium-dependent, ET-1 mediated, contraction to ACH. This suggests that activation of α₂-AR affects muscarinic receptor/G protein coupling leading to an opposite biological effect.

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