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 deri ved from bovine aorta was analyzed using confluent monolayers of cells and measuring the exchange of fluorescein dextrans of different molec ular 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 introd uced in the treatment of bronchial asthma, showed a significant reduct ion of cell permeability with subnanomolar concentrations, whereas the catecholamines (-)-isoproterenol and (-)-norepinephrine only showed s ignificant effects with micromolar concentrations. In order to elucida te if this difference in potential to regulate cell permeability is re lated to appropriate changes in the selectivity and affinity of the ag onists for beta2AR, we investigated the betaAR-coupled adenylate cycla se (AC) in membranes from endothelial cells and compared AC stimulatio n with the binding of agonists to the receptors using [I-125](-)-iodop indolol as radioligand. Beta-Adrenoceptors revealed to be closely coup led to AC as assessed by a similar magnitude of effects by receptor ag onists in comparison to GTP analogues and direct stimulants of AC acti vity. AC activity was increased by formoterol in parallel to its recep tor occupancy of beta2AR with nanomolar concentrations which were 50-f old higher than those used for the regulation of cell permeability ind icating the existence of spare receptors. In contrast to formoterol, t he catecholamines (-)-isoproterenol and (-)-norepinephrine stimulated AC activity through both beta1AR and beta2AR. From the overproportiona l high contribution of beta1AR to AC stimulation (42%) in comparison t o its low fraction (13%) in receptor binding we calculated that beta1A R is 3-4-fold more effectively coupled to AC than beta2AR.