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Abstract
Catecholamines play an essential role in the activation of the cardiovascular system and in the regulation of energy metabolism in a variety of physiological conditions. Many of these effects are mediated through β-adrenoceptors located on cell membranes. Binding of catecholamines to β-adrenoceptor increases the concentration of intracellular cyclic AMP which in turn activates protein kinase A. This enzyme phosphorylates a number of other intracellular enzymes influencing cell metabolism and functions. The primary structures of the receptor and its topography in the cell membrane as well as its binding domains have been partially clarified.
In studies of the human β-adrenergic receptors blood lymphocytes have mostly been used as model cells. These cells carry receptors of mainly the β2-subtype. The adequacy of this model system has been demonstrated in several studies. In clinical work receptor assays have had limited use until now. However, studies on the pathophysiology of the adrenergic system in several diseases have revealed that receptor alterations may constitute an important factor in the disease process. Measurements of adrenergic receptors may also have increasing usefulness in determining optimal drug concentrations.
Our own studies have primarily focused on physiological adjustments in the β-adrenergic system during acute or prolonged physical exercise as well as receptor changes in heart failure, muscle diseases and the alcohol withdrawal syndrome. We have also explored receptor dynamics during therapy with beta-blocking agents. These studies, briefly reviewed in this communication, have led to the following conclusions: (1) High aerobic capacity is associated with an increased density and ability of lymphocytic β-adrenoceptors to respond to catecholamines. (2) Both short-and longterm physical exercise induce a rapid up-regulation and more effective functioning of lymphocytic β-adrenoceptors, (3) Administration of beta-blocking drugs is associated with a subnormal exercise-induced up-regulation and decreased functioning of the lymphocytic β-adrenoceptors. (4) The exercise-provoked up-regulation and improved functioning of β-adrenoceptors is blunted in heart failure patients. (5) Patients with Duchenne-type of muscular dystrophy have a reduced number of lymphocytic β-adrenoceptors. (6) In chronic alcoholics the lymphocytic β-adrenoceptor level is subnormal but during abrupt ethanol withdrawal a rapid increase in the number and functioning of the receptors to a normal level takes place. This sequence of events may lead to a condition of relative adrenergic hypersensitivity.