The development of specific binding techniques for the study of adrenergic receptors on circulating human blood cells has allowed a better understanding of the physiological alterations of adrenergic receptors and changes of adrenergic receptors in pathological conditions such as hypertension. Alpha adrenoceptors play an important part in blood pressure regulation at several sites. There are contradictory and conflicting reports on whether alpha receptor mechanisms are altered in essential hypertension. To address further the role of alpha 2 adrenoceptors in human essential hypertension the number and the affinity of alpha 2 adrenergic receptors and plasma catecholamine levels were measured in 20 normotensive and 24 hypertensive subjects. The median number of receptors (Bmax) was 159.10 +/- 14.38 fmol/mg protein for controls versus 179.09 +/- 13.26 fmol/mg protein for hypertensives. The median dissociation constant (KD) of the receptors for 3H-Yohimbine was 1.43 +/- 0.17 nmol/l for controls and 1.85 +/- 0.19 nmol/l for hypertensives patients. There were no differences in catecholamine plasma levels between the two groups. In controls platelet alpha 2 receptor number correlated with age (p less than 0.003) but not with blood pressure values. Our results show that measurement of platelet alpha 2 receptor levels and affinity is unable to differentiate a group of hypertensives from normotensives. Nevertheless, we cannot exclude a possible role of peripheral alpha 2 adrenergic receptors in the pathogenesis of high blood pressure.
