RESUMO
Free hemoglobin (Hb) induces a potent vasoconstrictor response that may limit its therapeutic application as a red blood cell replacement. We have investigated whether encapsulation of stroma-free Hb (SFHb) or cross-linked Hb (alpha alpha-Hb) in liposomes modulates Hb vasoactivity in isolated blood vessels. Relaxation of rabbit thoracic vessels was measured before and after exposure to acellular SFHb, alpha alpha-Hb, and liposome-encapsulated SFHb or alpha alpha-Hb. SFHb and alpha alpha-Hb caused significant inhibition of carbachol-induced relaxation at 0.5 mg/dl, whereas encapsulation inhibited vessel relaxation at 30- to 60-fold higher Hb concentrations. The contractile response of rabbit ear arterial segments to electrical stimulation in the presence of acellular alpha alpha-Hb resulted in a 150% increase (EC150) in contractile amplitude at 0.23 mg/dl, whereas the EC150 for encapsulated alpha alpha-Hb was 13.7 mg/dl. Mechanistic studies of the vasoconstrictor activity of Hb demonstrated that acellular alpha alpha-Hb had no effect on norepinephrine release in the rabbit ear artery. In addition, neither acellular nor encapsulated alpha alpha-Hb preparations inhibited endothelial nitric oxide (NO) synthase activity isolated from bovine pulmonary artery. However, inhibition of vessel relaxation by acellular or encapsulated alpha alpha-Hb was reversed by the NO donor S-nitrosylpenacillamine, implicating Hb-NO binding as a possible mechanism for the vasoconstrictor response. In vitro stopped-flow kinetic studies of Hb-NO binding showed similar rates of reaction for conversion of oxyhemoglobin to methemoglobin (metHb; < 2 ms), followed by rapid conversion of metHb to NO-Hb (300 ms) for both acellular and encapsulated alpha alpha-Hb, demonstrating that liposome encapsulation does not retard NO-Hb binding. The attenuated vasoactivity of encapsulated Hb may, therefore, result from the limited access of encapsulated Hb to NO imposed by the physical size of the liposome and reduced penetration of Hb across the vascular endothelium.
Assuntos
Artérias/efeitos dos fármacos , Artérias/fisiologia , Hemoglobinas/administração & dosagem , Vasoconstrição , Animais , Aorta Torácica/efeitos dos fármacos , Cápsulas , Bovinos , Interações Medicamentosas , Orelha/irrigação sanguínea , Estimulação Elétrica , Indução Enzimática , Hemoglobinas/farmacologia , Cinética , Lipossomos , Óxido Nítrico Sintase/metabolismo , Norepinefrina/metabolismo , CoelhosRESUMO
Human adrenomedullin (AM) and human calcitonin gene-related peptide (CGRP) produced a concentration-dependent relaxation in mouse aorta, precontracted with noradrenaline. EC(50) values for AM and CGRP were 9.8 +/- 2.4 and 4.2 +/- 0.1 nmol/l, respectively. AM-mediated vasorelaxation was partially (3-fold) shifted by AM(22-52), the C-terminal AM fragment, but not by CGRP(8-37), a selective CGRP1 antagonist. Both AM(22-52) and CGRP(8-37) failed to inhibit CGRP-mediated vasorelaxation of mouse aorta rings. Binding of rat [(125)I]AM to these membranes was specific. Both human AM and AM(22-52) displaced rat [(125)I]AM binding in a concentration-dependent manner with IC(50) values of 12.0 +/- 4 and 19.4 +/- 8 nmol/l, respectively. In contrast, both human CGRP and CGRP(8-37) were weak in displacing [(125)I]AM binding. Very little specific binding was observed with [(125)I]CGRP. In conclusion, the data presented here demonstrate that the mouse aorta displays AM receptors that mediate vasorelaxation.
Assuntos
Aorta/efeitos dos fármacos , Peptídeos/farmacologia , Vasodilatadores/farmacologia , Adrenomedulina , Animais , Aorta/fisiologia , Ligação Competitiva , Relação Dose-Resposta a Droga , Humanos , Técnicas In Vitro , Masculino , Camundongos , Ensaio Radioligante , Vasodilatação/efeitos dos fármacosRESUMO
The chiral synthesis of the potent and selective alpha-2A antagonist, BRL 48962, is described. Evaluation of BRL 48962 at cloned human alpha-adrenoceptors indicates that this antagonist has a selectivity in the order of 30-fold for the alpha-2A subtype.