Enhanced Phenylephrine Contractions in Rabbit Carotid Arteries Following Exposure to Haemoglobin from Subjects with Sickle Cell Trait

We have previously reported raised resting diastolic blood pressure (BP) in subjects with single S-gene inheritance – although the mechanism was unclear. The goal of this study was to characterize, in vitro, the modulatory role of erythrocyte components from subjects with different Hemoglobin (Hb) genotypes on contractile responses induced by phenylephrine (PE) in isolated rabbit carotid arterial smooth muscle. Carotid arteries were isolated from rabbits and cut into 2mm rings, suspended in 20ml organ baths and bubbled with 95% O2, 5% CO2 and isometric contractions examined under an initial load of 1g, at 37oC and pH 7.4. Contractile responses to EC70 (M) PE in arterial rings exposed to various erythrocyte components obtained from subjects of different Hb genotypes (AA, AS and SS) were examined in control rings as well as in rings exposed for 30 minutes to (a) intact washed erythrocytes (b) erythrocyte ghosts and (c) haemoglobin solution. Arterial rings were exposed to 50μl of each of the erythrocyte constituents at an adjusted haematocrit of 0.6. The magnitudes of the PE-induced contractions with intact erythrocytes were: 1180±202, 1700±260 and 900±302 for Hb AA, Hb AS and Hb SS respectively (n=13); these values were significantly increased following exposure to various erythrocyte components in the order: RBC> HB > Ghost (P<0.05, respectively). There were no significant differences in PE contractions following exposure to intact erythrocytes and ghosts from subjects with different Hb genotypes; however, exposure to haemoglobin solution significantly enhanced PE contractions in Hb AS subjects than in Hb AA and Hb SS (P<0.05 respectively). In conclusion, we reason that the haemoglobin content of Hb AS erythrocytes may be responsible for the enhanced contractile responses to PE and may explain in part, why diastolic BP values remain high in Hb AS subjects.