Effect of captopril on lymphocytic beta-adrenergic receptors in normal and hypoxic conditions

Abstract

Background: During heart failure, due to increased level of circulating norepinephrine, the number of beta-adrenergic receptors (β-AR), both at the cardiac and the lymphocytic level, is reduced (down-regulation). Captopril, an ACE-inhibitor containing an SH group appears capable of resetting β-AR when used in patients with heart failure. Our study was aimed at checking whether captopril exerts a direct effect upon the β-AR, possibly through its SH group by disulphur binding with cysteine residues located at the binding sites for catecholamines. Methods: The study was carried out in vitro on human lymphocytes obtained from healthy volunteers: 10 males (mean age, 34 years; range, 25–45) and 10 females (mean age, 34 years; range, 26–48). Lymphocytes were randomly divided in two groups of equal size. Group I were controls; in Group II cells were incubated with three different doses of captopril: 1, 10, and 100 μM. Control lymphocytes and those treated with 10 μM of captopril were exposed to 1 μM isoproterenol. The number of total and surface β-AR, and the sequestration of β-AR from isoproterenol under normoxic conditions and after 20 h of hypoxia were checked. Furthermore, the content of cAMP was assayed both in basal conditions and after stimulation with 10 μM and 100 μM isoproterenol and forskolin, respectively. Results: Total β-AR: 1082 ± 133 (controls) vs. 1174 ± 94 (treatment with 1 μM captopril), vs. 1237 ± 88 (10 μM captopril), vs. 1092 ± 105 (100 μM captopril). Surface β-AR: 84 ± 4.41% (controls) vs. 90.5 ± 2.1% (10 μM captopril). Basal cAMP: 1.21 ± 0.4 (controls) vs. 1.23 ± 0.5 pmol/10 cells (1 μM captopril), 1.05 ± 0.6 pmol/10 cells (10 μM captopril), 1.15 ± 0.4 pmol/10 cells (100 μM captopril). After 10 μM isoproterenol: controls 4.10 ± 0.8 vs. 4.30 ± 0.9 pmol/10 cells (1 μM captopril), 4.15 ± 0.7 pmol/10 cells (10 μM captopril), 3.50 ± 1.0 pmol/10 cells (100 μM captopril). After 100 μM forskolin: controls 13.2 ± 3.1 vs. 11.2 ± 3.1 pmol/10 cells (1 μM captopril), 13.1 ± 4.2 pmol/10 cells (10 μM captopril), 12.6 ± 2.9 pmol/10 cells (100 μM captopril). Neither of these differences were significant. Lymphocytic β-AR exposed to hypoxia did not show any significant difference. Exposure to captopril did not cause any further alteration on β-AR sequestration. Conclusions: Captopril does not seem to exert any direct action upon lymphocyte β-AR from healthy volunteers. Moreover, captopril does not modify cAMP storage either in basal conditions or after stimulation with isoproterenol or forskolin. Therefore our data suggest that action of captopril on β-AR is probably due to the inhibition of both systemic and tissue ACE-system.