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Involvement of multiple receptors in the biological effects of calcitonin gene‐related peptide and amylin in rat and guinea‐pig preparations

1 The activity of rat α and β calcitonin gene‐related peptide (CGRP) as compared to the structurally related peptide, rat amylin, has been investigated in the guinea‐pig isolated left atrium (electrically driven), in mucosa‐free strips from the base of the guinea‐pig urinary bladder and in the rat isolated vas deferens (pars prostatica). The antagonist activity of the C‐terminal fragment of human αCGRP, αCGRP(8–37), was also investigated. 2 In the guinea‐pig isolated left atrium the three peptides produced a concentration‐related positive inotropic effect, amylin being about 16 and 31 times less potent than α or βCGRP, respectively. Human αCGRP(8–37) produced a rightward displacement of the log concentration‐response curve to the three agonists tested, without depression of maximal response attainable. Apparent pKB values calculated on the basis of the displacement produced by 1 μm human αCGRP(8–37) indicated an agonist‐independent affinity of the antagonist (6.66 ± 0.11 for αCGRP, 6.42 ± 0.17 for βCGRP and 6.95 ± 0.11 for amylin). 3 In the guinea‐pig isolated urinary bladder, α or βCGRP or amylin produced a concentration‐related inhibition of twitch contractions evoked by train electrical field stimulation (10 Hz frequency, 0.25 ms duration at 100 V for 0.5 s every 60 s). Amylin was about 100 times less potent than α or βCGRP. Human αCGRP(8–37) (3 μm) did not significantly affect the inhibitory action of the three agonists tested. 4 In the rat isolated vas deferens, α or βCGRP or amylin produced a concentration‐related inhibition of twitch contractions evoked by electrical field stimulation (0.2 Hz frequency, 0.5 ms duration at 60 volts). Amylin was about 100 times less potent than α or βCGRP. Human αCGRP(8–37) at 3 μm did not significantly affect the inhibitory action of amylin and at 3 μm antagonized the responses to rat α and βCGRP with apparent pKB values of 5.86 ± 0.15 and 6.11 ± 0.13, respectively. 5 These findings indicate that multiple receptors mediate the actions of peptides of the CGRP/amylin family in the preparations investigated. In the guinea‐pig atrium both α and β forms of rat CGRP as well as amylin act by stimulating a single class of receptors which are sensitive to the inhibitory action of human αCGRP(8–37). In rat isolated vas deferens, at least two receptors could be present, one activated by α and βCGRP and partially sensitive to human αCGRP(8–37) and another which is sensitive to amylin but not recognised by human αCGRP(8–37). This latter type of receptor could be entirely responsible for the action of the agonists in the guinea‐pig urinary bladder.