CATION BINDING TO ANIONIC BIOPOLYMERS OF VASCULAR CONNECTIVE TISSUE

Résumé
Des variations de concentrations externes en protons et/ou en cations produisent des modifications des propriétés de liaison des macromolécules polyanioniques dans le tissu conjonctif vasculaire. Ainsi, la concentration extracellulaire des différentes espèces cationiques peut varier de façon rapide et importante dans les mailles du réseau conjonctif et au voisinage de la membrane des cellules vasculaires lisses. En particulier, l'adsorption potassique par acidification de la solution extracellulaire comme la liaison par coopération du potassium (consécutive aux changements conformationnels induits par les ions Mg²⁺ dans les polyanions) présentent une grande importance dans l'hyperpolarisation membranaire et la vasodilatation.

Abstract
The pH-dependent binding of monovalent cations to vascular connective tissue is dependent on the concentration and affinity constant of the ion species in question. The mode of interaction is competition. Divalent cation binding to vascular connective tissue and its proteoglycans is additionally dependent on conformational changes. For example, competition experiments in physiological Krebs solutions using multi-chain chondroitin sulphate and Mg²⁺ at pH 7.38 showed increasing ²³Na⁺ excess transverse relaxation rates during the initial phase [cf.5]. Above 4 mM Mg²⁺ simple exchange of bound Na⁺ with Mg²⁺ was observed. The increased excess transverse relaxation rate seen upon Mg²⁺ additions was referred to an increase in the correlation time. In the presence of Mg²⁺, cross-linking of chains, clustered together on the protein core, was promoted. Thus, physiological concentrations of Ma²⁺ ions can induce a specific change in the configuration of the anionic biopolymers, which enables K⁺ ions to bind cooperatively (n_H = 6.8), while the adsorption of all the other cations is competitively inhibited. This means that with extracellular Mg²⁺ excess not only more Mg²⁺ ions are bound to vascular connective tissue but also more K⁺ ions. [K⁺]₀ would decrease near the cell membrane, hyperpolarization and vasodilatation would occur.