β-adrenergic receptors of human lymphocytes in physically active and immobilized subjects: Characterization by a polyethylene glycol precipitation assay

Abstract

To investigate the role of physical conditioning in the regulation of cellular β-adrenergic responsiveness in man, a simple and accurate method for the determination of lymphocytic β-adrenergic receptors was developed and subsequently utilized for the characterization of these receptors in subjects with varying degrees of physical fitness. This method combines the advantages of the use of [¹²⁵I]iodocyanopindolol as the radioligand and polyethylene glycol (PEG) precipitation for the separation of bound and unbound radioligand. The assay is quick and easy to perform and allows several samples to be processed simultaneously. The concentration of β-adrenergic receptors in lymphocytic membrane particulates in 16 healthy subjects was 58±4 fmol/mg protein (mean±SEM) and the K_d of the interaction between [¹²⁵I]iodocyanopindolol and the receptor was 4×10⁻¹¹ mol/l. The receptor level was similar in both sexes. As compared with the controls, the cellular concentration of β-adrenergic receptors was higher in eight regularly competing long-distance runners (73 ± 4 fmol/mg protein; p<0.025), whereas the corresponding receptor level in six sprinters (62±9 fmol/mg protein) did not differ from that of controls. The concentration of lymphocyte β-adrenergic receptors in seven patients undergoing supine immobilization for weeks because of neck fracture or dislocation showed a wide range from 44 to 155 fmol/mg protein. Differences in the receptor levels in the experimental groups were to some extent reflected in the cAMP content of the lymphocytes from the same subjects: the basal cAMP concentration in the controls, immobilized subjects, sprinters and long-distance runners were 22±4 pmol/mg protein, 24±6 pmol/mg protein, 32±6 pmol/mg protein and 56± 12 pmol/mg protein (p<0.()25 compared with the controls), respectively, and the corresponding isoproterenol-stimulated lymphocyte cAMP concentrations were 69±8 pmol/mg protein, 86±7 pmol/mg protein, 101±10 pmol/mg protein and 141 ±27 pmol/mg protein (p<0.05 as compared with the controls), respectively. The results demonstrate the feasibility of the PEG precipitation assay for the human β-adrenergic receptor and suggest that endurance training is associated with an increased ability of the cells to respond to β-adrenergic stimulation.

Key words:

• competitive protein binding
• isoproterenol
• cyclic AMP
• endurance runners
• [¹²⁵I]iodocyanopindolol
• lymphocytic membranes
• paraplegia
• radio-ligand binding
• sprinters
• tetraplegia