Altered platelet alpha 2-adrenoceptors in patients with ischemic heart disease

We evaluated the characteristics of platelet alpha 2-adrenoceptors in 12 patients with effort angina pectoris, 11 patients with variant angina pectoris and 11 normal control subjects. alpha 2-Adrenoceptors were quantified using a radioligand binding assay with radiolabelled rauwolscine, an alpha 2-selective antagonist. In addition, plasma norepinephrine concentrations were measured by high performance liquid chromatography. The mean value of the maximal number of binding sites (Bmax) in patients with effort angina (205.1 +/- 11.3 fmol/mg protein) was significantly lower than that in control subjects (293.0 +/- 10.2 fmol/mg protein). Bmax did not differ between patients with variant angina (322.9 +/- 45.4 fmol/mg protein) and control subjects. There was no significant difference in the dissociation constant (Kd) among the 3 groups. The plasma norepinephrine concentration tended to be higher in patients with effort angina or variant angina than in normal controls, but this difference was not statistically significant. In addition, studies in another group of young volunteers (n = 20) revealed a negative correlation (r = -0.50, p < 0.05) between the Bmax of 3H-rauwolscine binding to platelets and the percent change in the plasma norepinephrine concentration when subjects moved from the supine to the standing position. This suggests a functional correlation between platelet alpha 2-adrenoceptors and those located at presynaptic sites. If platelet alpha 2-adrenoceptors correlate with presynaptic alpha 2-adrenoceptors, the current findings of decreased alpha 2-adrenoceptor density in platelets from patients with effort angina could represent attenuated negative feedback of norepinephrine by presynaptic alpha 2-adrenoceptors.