Density and agonist‐promoted high and low affinity states of the β‐adrenoceptor on human B‐ and T‐cells

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Abstract
β‐Adrenoceptor binding on peripheral blood mononuclear cells (PBMC) of healthy adult volunteers was investigated using the radioligand 125iodo‐cyanopindolol (ICYP). Saturation binding studies were performed with nine different concentrations of ICYP. Receptor density and affinity were calculated by Scatchard plots. Resolution of β‐adrenoceptors into those with high and low affinity state of the β‐adrenoceptor was obtained from inhibition curves with salbutamol using Hofstee plots. Receptor density on enriched B‐cells (‘B‐cells’) was two‐fold higher than on enriched T‐cells (‘T‐cells’) (P < 0025). Affinity (KD values) of β‐adrenoceptors did not differ for B‐ and T‐cells. However, when two distinct binding states for β‐adrenoceptor agonists were identified using salbutamol displacement curves, β‐adrenoceptors on T‐cells presented more receptors in a high affinity state than those on B‐cells (P < 001). Since the ability of an agonist to activate adenylate cyclase is closely correlated with the ratio of low to high affinity states formed in the presence of the agonist, increased intrinsic activity for the β‐adrenoceptor agonist on T‐cells may be postulated. In conclusion, determination of the B/T ratio is a prerequisite for interpretation of β‐adrenoceptor changes on peripheral lymphocytes in various diseases.