Altered extracellular magnesium and variations in vascular smooth muscle responses to agonists

Abstract

Background: There is a paucity of information on the heterogeneity of vascular smooth muscles in their responses to agonists following exposure to varying extracellular magnesium, [Mg²⁺]0. The present study was designed to examine, comparatively, the influence of variations in [Mg²⁺]₀ on vascular smooth muscles of rabbit aortic, carotid and femoral arteries. Methods: Contractile responses induced by Phenylephrine (PE), Histamine (HIST) and 5-HydroxylTryptamine (5-HT) were examined on 2mm ring segments of the arteries which were suspended in 20 ml organ baths containing physiological salt solution (PSS), for measurement of isometric contractions, at 37^oC and pH 7.4. The medium was bubbled with 95% O₂, 5% CO₂₊, and rings were given an initial load of 2g. Cumulative concentration responses to the agonists were studied in normal PSS (control) and following 30 minutes exposure to Mg²⁺-free or high-Mg²⁺ (4.8mM) PSS. Contractile responses were expressed as percentage of 80 mM K⁺ contractions in normal PSS. Results: Maximal contractions (Emax) induced by PE, HIST and 5-HT compared with high K⁺ contraction in the various preparations were differentially altered (p<0.05) following exposure to varying [Mg²⁺]o. Conclusion: Based on the sensitivity (EC₃₀) and potency (EC₅₀) values for the dose-response curves of the agonists, we report that vascular smooth muscles of rabbit aortic, carotid and femoral arteries demonstrate considerable variability in their responses to altered [Mg²⁺]o.

Keywords: Vascular smooth muscle, Extracellular magnesium, Phenylephrine, Histamine, 5-Hydroxytryptamine