Effect of exercise duration on density and coupling of β-adrenergic receptors on human mononuclear cells

The effect of maximal exercise on lymphocyte β-adrenergic receptors was examined in 26 normal subjects. Exercise increased O₂ consumption (V̇O₂) from 5 ± 1 to 50 ± 4 ml · min^-1 · kg^-1, plasma norepinephrine level from 188 ± 28 to 2,682 ± 160 pg/ml, and plasma epinephrine level from 94 ± 72 to 857 ± 180 pg/ml. The density of β-adrenergic receptors on lymphocytes obtained at rest was 31 ± 3.7 fmol/mg protein; exercise increased the density of receptors by 86 ± 33% (range 0-257%) to 58.3 ± 1.5 fmol/mg protein but did not alter the affinity of the receptor for [¹²⁵I]iodopindolol or the coupling of the receptor to the guanine nucleotide-binding regulatory protein. The density of β-adrenergic receptors increased progressively throughout exercise and paralleled the increase in heart rate. The magnitude of the change in the density of β-adrenergic receptors did not correlate with the magnitude of the increase in heart rate, V̇O₂, or plasma levels of catecholamines. The density of receptors was still elevated 15 min after completion of exercise but fell below base line 1 h after peak exercise to 18.2 ± 6.7 fmol/mg protein (P < 0.05 vs. base-line levels). These results demonstrate that exhaustive exercise results in a progressive increase in the number of β-adrenergic receptors on lymphocyte membranes, followed by a reduction in the density of receptors during the recovery phase of exercise. Despite a significant increase in the level of plasma catecholamines, the receptor remains coupled to the guanine nucleotide-binding regulatory protein.