Epinephrine-Induced Sickle Red Cell Adhesion and Vaso-Occlusion In Vivo Is Inhibited by the β-Adrenoceptor Blocker Propranolol.

Painful vaso-occlusive episodes in SCD are commonly associated with infection and other less definable stressors. Since epinephrine activates sickle red cell (SS RBC) adhesion in vitro, we studied the physiologic effect of adrenoceptor signaling activation by epinephrine on SS RBC adhesion to endothelium in vivo. We also investigated whether β-adrenoceptor blockade by propranolol would reduce adhesion and vaso-occlusion. Boluses of washed fluorescently labeled human SS RBCs, treated with epinephrine or vehicle in vitro, were infused into anesthetized athymic nude mice with window chambers implanted into their dorsal skin. Intravital microscopy of contralateral subdermal microvasculature was then performed to observe the dynamic interactions between flowing human SS RBCs and non-activated endothelium. Epinephrine induced human SS RBC adhesion, with frequent postcapillary obstruction. In contrast, neither sham-treated SS RBCs nor epinephrine-treated normal RBCs adhered appreciably to endothelium. Blood flow rates in venules of mice infused with epinephrine-treated SS RBCs was dramatically decreased, with fluxes of 19667±9048 and 6622±1494 circulating RBC/min/μm² for sham-treated and epinephrine-treated cells, respectively (p=0.0074). SS RBC trapping in lung, spleen and kidney was assessed by fluorescence microscopy of frozen tissue sections collected 30 minutes post injection. Sham-treated SS RBCs were trapped to some degree in the lungs and spleen but only minimally in the kidney. However, epinephrine treatment markedly increased SS RBC trapping in all organs. To quantitate cell survival, sham-treated and epinephrine-treated SS RBCs labeled with different fluorescent dyes were co-infused into the same mouse. Blood samples were collected at intervals after infusion and analyzed by flow cytometry. Ten minutes after infusion, the percentage of circulating sham-treated SS RBCs was 3-fold higher than for epinephrine-treated cells, thus showing an inverse relationship between the percentage of circulating SS RBCs and the degree to which these cells were trapped in the organs studied. Finally, to determine whether propranolol can block epinephrine-induced SS RBC adhesion, SS RBCs were pretreated with propranolol, followed by treatment with epinephrine, then washed before infusion. Propranolol significantly inhibited the effect of epinephrine on SS RBC adhesion, resulting in markedly decreased obstruction of postcapillary vessels. Propranolol improved epinephrine-treated SS RBC circulation, with fluxes of 18809±7868, 3560±1443 and 16722±4985 circulating RBCs/min/μm² for propranolol-treated, epinephrine-treated, and propranolol+epinephrine-treated cells, respectively (p<0.0001, propranolol-treated vs epinephrine-treated; p<0.0001, epinephrine-treated vs epinephrine+propranolol-treated). Ten minutes after infusion, the percentage of propranolol+epinephrine-treated SS RBCs in the circulation was similar to the percentage of propranolol-treated SS RBCs. Intravenous propranolol administration also blocked epinephrine-treated SS RBC adhesion, so that epinephrine-treated SS RBCs showed no increased adhesion in animals who received IV propranolol. We have thus demonstrated that (1) the stress hormone epinephrine can induce vaso-occlusion in vivo via activation of SS RBC adhesion, and (2) propranolol can inhibit epinephrine-induced SS RBC adhesion and prevent vaso-occlusion in this setting. Thus, we theorize that β-blockers may be useful in preventing or treating vaso-occlusion in SCD.