Effects of ring fluorination on the adrenergic properties of phenylephrine

The adrenergic properties of 2-, 4- and 6-fluorophenylephrine (2-FPE, 4-FPE, 6-FPE) were compared to those of phenylephrine (PE). The order of affinities of these compounds for alpha 1-adrenoceptors as determined by displacement of [3H]prazosin and [3H]WB-4101 binding to brain membranes was the same as the order of potencies for eliciting two alpha 1-adrenergic metabolic responses in guinea-pig cerebral cortical synaptoneurosomes, namely the stimulation of phosphatidylinositol turnover and the potentiation of 2-chloroadenosine-induced accumulation of cyclic AMP. In all cases the order was 6-FPE greater than PE greater than 4-FPE greater than 2-FPE. The order of affinities for alpha 2-adrenoceptors as determined by displacement of binding of [3H]clonidine to brain membrane was 6-FPE greater than PE greater than or equal to 4-FPE = 2-FPE. In contrast, the order of potencies for inhibition of forskolin-stimulated adenylate cyclase activity in human platelet membranes via an alpha 2-adrenoceptor was 6-FPE approximately equal to PE greater than 4-FPE much greater than 2-FPE. The FPEs and PE were partial agonists compared to epinephrine in human platelets. The affinities of these compounds for beta-adrenoceptors as determined by displacement of binding of [3H]dihydroalprenolol to brain membranes are 2-FPE greater than PE greater than or equal to 4-FPE much greater than 6-FPE. The FPEs and PE had positive chronotropic and inotropic effects in isolated guinea-pig atria apparently through the activation of a beta-adrenoceptor, since pindolol blocked the response while prazosin did not. 6-FPE appeared less active than the other PEs in atria. In fat cell membranes, 2-FPE was more potent than PE in stimulating adenylate cyclase via a beta-adrenoceptor, while 4-FPE and 6-FPE were inactive. Both, 2-FPE and PE were partial agonists in fat cells compared to isoproterenol. Of the three FPEs, 6-FPE represents a more potent and more selective agonist for alpha-adrenoceptors compared to beta-adrenoceptors than PE, while 4-FPE and, in particular, 2-FPE are less potent and selective as alpha-agonists.