Inhibition of Brain Swelling after Ischemia-Reperfusion by β-Adrenergic Antagonists: Correlation with Increased K⁺ and Decreased Ca²⁺ Concentrations in Extracellular Fluid

Figure 1

Schematic illustration of stimulation of ERK phosphorylation by -adrenergic receptors in astrocytes. Isoproterenol (ISO) binds to -adrenergic receptors. At high concentrations (>1 μM), the activation of the receptors induces -adrenergic (red arrows), PKA-dependent “G_s/G_i switching,” which in turn induces an enhancement of intracellular Ca²⁺ concentration by Ca²⁺ release from intracellular stores. The latter activates Zn-dependent metalloproteinases (MMPs) and leads to shedding of growth factor(s). The released epidermal growth factor (EGF) receptor ligand stimulates phosphorylation of the EGF receptor in the same and adjacent cells. The downstream target of the EGF receptor, extracellular regulated kinase (ERK), shown in blue, is phosphorylated via the Ras/Raf/MEK pathway. As shown in yellow ovals the ERK phosphorylation by isoproterenol at high concentration can be inhibited by H-89, an inhibitor of protein-kinase A (PKA); by PTX, an inhibitor of G_i protein; by BAPTA/AM, an intracellular Ca²⁺ chelator; by GM6001, an inhibitor of Zn-dependent metalloproteinases; by AG1478, an inhibitor of the receptor-tyrosine kinase of the EGF receptor; by siRNA against -arrestin 1 and less completely by siRNA against -arrestin 2; and by U0126, an inhibitor of MEK, which directly phosphorylates ERK. In contrast, at low concentration (100 nM) -adrenergic (green arrows) activation of the receptors activates Src via recruitment of -arrestin 2. Src in turn stimulates ERK phosphorylation and phosphorylates EGF receptors at different sites than -adrenergic stimulation, without involvement of the receptor-tyrosine kinase. Its ERK_1/2 phosphorylation is secondary to MEK activation, which may be induced by direct activation of Raf or Ras by Src. The ERK phosphorylation by isoproterenol at low concentration can be inhibited by siRNA against -arrestin 2; by PP1, a Src inhibitor; and by the MEK inhibitor U0126. The effect of most of these inhibitors on MCAO-induced edema was investigated and tabulated in Table 3.