Bradykinin Receptors of Cerebral Microvessels Stimulate Phosphoinositide Turnover

Abstract

We examined by ligand binding methods whether bradykinin (BK) receptors exist in rat and pig cerebral microvessels, and in the cerebral cortex from which the microvessels were isolated. We found a high-affinity and saturable BK receptor site in both rat and pig cerebral microvessels, but not in their cerebral cortex. The maximal density of binding and the dissociation constant were 8.0 ± 4.1 and 6.8 ± 1.5 fmol/mg of protein and 47 ± 24 and 150 ± 8 p M (mean ± SD) in cerebral microvessels of the pig and rat, respectively. The high-affinity specific binding of BK was effectively displaced by des-Arg⁰[Hyp³-Thi^5–8,D-Phe⁷]BK, a specific B₂ receptor antagonist, but not by des-Arg⁹[Leu⁸]BK, a specific B₁ antagonist. We also demonstrated that BK increases phosphatidylinositol hydrolysis in cerebral microvessels of the rat and pig. This effect was also blocked by the B₂, but not by the B₁, antagonist. Increased phosphatidylinositol hydrolysis was manifested by a rapid transient increase in inositol trisphosphate and the later slow accumulation of inositol bisphosphate and inositol monophosphate. Preincubation of microvessels with phorbol ester, stable GTP analogs, pertussis toxin, or in Ca²⁺-free buffer did not influence BK activation of phosphatidylinositol hydrolysis. These results demonstrate the existence of BK receptors of the B₂ subtype in brain microvessels, which may play an important role in modulation of the brain microcirculation, probably via increased phosphoinositide turnover.