Effects of full and partial beta-adrenergic agonists and antagonists on human lung adenylate cyclase

The beta-adrenergic stimulation of adenylate cyclase in membranes from human lung was compared to that of adenylate cyclase in membranes with a majority of beta 2-adrenergic receptors (from rat lung) and in membranes with a homogeneous population of beta 2-adrenergic receptors (from rat erythrocytes and reticulocytes). In terms of adenylate cyclase stimulation, three full agonists (isoproterenol, epinephrine and norepinephrine), four partial agonists (procaterol, salbutamol, fenoterol and zinterol), and four antagonists (propranolol, metoprolol, atenolol and practolol) were tested. The potency (Kact or Ki) of the eleven beta-adrenergic agents, and the Hill coefficient (of 1) for the four antagonists tested indicated that the activation of human lung adenylate cyclase occurred through receptors of the beta 2-subtype only. Partial beta-adrenergic agonists were efficiently discriminated by the human lung preparation, as shown by distinct intrinsic activities. The mediocre efficacy and the relatively low potency of all beta-adrenergic agonists on adenylate cyclase suggested a relatively low density of beta 2-adrenergic receptors, as compared to the enzyme density.