A potent and selective CGRP(2) agonist, [Cys(Et)(2,7)]hCGRP alpha: comparison in prototypical CGRP(1) and CGRP(2) in vitro bioassays

The development of highly selective and potent agonists and antagonists is critical in evaluating the physiological role(s) of each receptor subtype in a peptide family. The existence of at least two calcitonin gene related peptide (CGRP) receptor subtypes has been proposed based on the potency of CGRP(8-37) to antagonize the effect of hCGRP alpha in the guinea pig atrium and the agonistic properties of the linear analogue [Cys(Acm)(2,7)]hCGRP alpha to mimic the effect of hCGRP alpha in the rat vas deferens, However, the rather low potency of [Cys(Acm)2,7]hCGRP alpha (ED50 = 82 +/- 7.5 nM) to activate the CGRP(2) receptor subtype limits its usefulness. Accordingly, we investigated various structural modifications of this linear analogue in prototypical CGRP(1) and CGRP(2) in vitro bioassays. Among them, replacing the acetaminomethyl moiety (Acm) by an ethylamide group, [Cys(Et)(2,7)]hCGRP alpha demonstrated a high potency to inhibit the rat vas deferens twitch response (ED50 = 3.4 +/- 1.2 nM), whereas in the guinea pig atrium, this analogue induced only a slight inotropic effect at very high concentrations (1 mu M). Moreover, [Cys(Et)(2,7)]hCGRP alpha as well as the addition of a tyrosine residue at the N-terminal, [Tyr(0),Cys(Et)(2,7)]hCGRP alpha, competed with high affinities for [I-125]hCGRP binding in rat brain homogenates (IC50 = 0.3 and 0.1 nM, respectively). Taken together, these results suggest that [Cys(Et)(2,7)]hCGRP alpha is a new potent analogue that could prove valuable in addressing the functional relevance ofthe CGRP(2) receptor class.