REGULATION OF ENDOTHELIAL PERMEABILITY BY BETA-ADRENOCEPTOR AGONISTS - CONTRIBUTION OF BETA(1)-ADRENOCEPTOR AND BETA(2)-ADRENOCEPTORS

The barrier function of cultured, macrovascular endothelial cells derived from bovine aorta was analyzed using confluent monolayers of cells and measuring the exchange of fluorescein dextrans of different molecular masses. The effects of beta-adrenoceptor agonists with different selectivity for beta1- and beta2-adrenoceptors (AR) were investigated. Formoterol, a novel high-affinity agonist for beta2AR recently introduced in the treatment of bronchial asthma, showed a significant reduction of cell permeability with subnanomolar concentrations, whereas the catecholamines (-)-isoproterenol and (-)-norepinephrine only showed significant effects with micromolar concentrations. In order to elucidate if this difference in potential to regulate cell permeability is related to appropriate changes in the selectivity and affinity of the agonists for beta2AR, we investigated the betaAR-coupled adenylate cyclase (AC) in membranes from endothelial cells and compared AC stimulation with the binding of agonists to the receptors using [I-125](-)-iodopindolol as radioligand. Beta-Adrenoceptors revealed to be closely coupled to AC as assessed by a similar magnitude of effects by receptor agonists in comparison to GTP analogues and direct stimulants of AC activity. AC activity was increased by formoterol in parallel to its receptor occupancy of beta2AR with nanomolar concentrations which were 50-fold higher than those used for the regulation of cell permeability indicating the existence of spare receptors. In contrast to formoterol, the catecholamines (-)-isoproterenol and (-)-norepinephrine stimulated AC activity through both beta1AR and beta2AR. From the overproportional high contribution of beta1AR to AC stimulation (42%) in comparison to its low fraction (13%) in receptor binding we calculated that beta1AR is 3-4-fold more effectively coupled to AC than beta2AR.