SELECTIVITY OF AGONISTS FOR CLONED ALPHA(1)-ADRENERGIC RECEPTOR SUBTYPES

The potencies and intrinsic activities of agonists in activating clone d alpha(1)-adrenergic receptor (AR) subtypes were compared. The hamste r alpha(1B)-, bovine alpha(1C)-, or rat alpha(1A/D)-ARs were expressed at high levels in human embryonic kidney 293 cells. Catecholamines an d phenylethylamines, but not lower efficacy agonists, were more potent in inhibiting radioligand binding to the expressed alpha(1A/D) subtyp e than to the alpha(1B) or alpha(1C) subtypes; this selectivity remain ed in the presence of different buffers, nucleotides, and cations. Act ivation of all three subtypes caused substantial increases in [H-3]ino sitol phosphate formation in cells grown in 96-well plates. Pretreatme nt with phenoxybenzamine decreased maximal responses to norepinephrine (NE) with only smalt decreases in apparent potency, suggesting simila r small receptor reserves for all three subtypes. The catecholamines N E, epinephrine, and 6-fluoro-NE were full agonists with similar potenc ies at the three subtypes; alpha-methyl-NE was also a full agonist but was about 20-fold less potent at alpha(1B)-ARs than at alpha(1C)- or alpha(1A/D)-ARs. Phenylephrine had similar potencies at all three subt ypes but gave a submaximal response at alpha(1B)-ARs. Methoxamine was a full agonist at alpha(1C)- and alpha(1A/D)-ARs, with about 20-fold g reater potency at the alpha(1C) subtype, but showed lower intrinsic ac tivity at alpha(1B)-ARs. A number of imidazolines, amide-phrine, and S KF 89748 had substantial intrinsic activity at alpha(1C)-ARs but littl e or no intrinsic activity at the other two subtypes. We conclude that the potencies of many agonists in competing for radioligand binding s ites are related to their potencies in activating functional responses but that this relationship is not the same for all subtypes. NE and e pinephrine activate all three cloned alpha(1)-AR subtypes With similar potencies and intrinsic activities, but many widely used agonists sho w significant selectivity for different alpha(1)-AR subtypes.