Effects of Bradykinin on Permeability and Diameter of Pial Vessels In vivo

Abstract

The effect of bradykinin on the permeability and vasomotor response of pial vessels has been studied to enhance our understanding of the pathophysiological role of the kallikrein–kinin system in cerebral tissue. Intravital fluorescence microscopy of the pia arachnoidea was conducted using Na⁺-fluorescein, FITC-dextran, and FITC-albumin as low and high molecular weight blood–brain barrier indicators. Massive arterial dilatation evolved immediately upon administration of bradykinin by superfusion of the exposed cerebral surface. An increase of the arterial diameter by 40% was the maximal response found at bradykinin concentrations of 4 × 10⁻⁵ M. Arterial dilatation became attenuated with continuous superfusion of the preparation with bradykinin. In pial veins, a moderate reduction of the vessel diameter was observed, however, only after prolonged superfusion of the preparation. Bradykinin led to selective opening of the blood–brain barrier for Na⁺-fluorescein at super-fusate concentrations of ⩾4 × 10⁻⁷ M, but not for FITC-dextran or FITC-albumin. Topical administration of l-isoproterenol (10⁻⁴ M) was found to prevent extravasation of Na⁺-fluorescein in the presence of bradykinin concentrations of 4 × 10⁻⁶ M. Protection of the blood–brain barrier by isoproterenol was not observed when higher concentrations of bradykinin were employed. Intracarotid infusion of bradykinin led also to a selective opening of the blood–brain barrier for Na⁺-fluorescein, but not for FITC-dextran or FITC-albumin. In contrast to superfusion, this route of administration did not induce changes of the vasomotor behavior of the arteries or veins. Additional experiments with B₁-agonists and -antagonists suggest that bradykinin causes the opening of the blood–brain barrier through an interaction with B₂-receptors on endothelial cells, and arterial dilatation via interaction with B₂-receptors on vascular smooth muscle cells. Our findings support the concept that the release of kinins in the brain during an acute cerebral lesion mediates secondary damaging processes by the enhancement of blood–brain barrier dysfunction.