Uterotonic activity and myometrial receptor affinity of 1-deamino-1-carba-2-tyrosine(O-methyl)-oxytocin

The contractile effect of the analogue 1-deamino-1-carba-2-tyrosine(O-methyl)-oxytocin (carbetocin) on isolated myometrial strips from rats was compared with that of oxytocin. The uterotonic activity of the analogue was 15 +/- 3 IU/mg as compared with 438 +/- 41 for oxytocin, however, the response was more prolonged and could not be abolished by washing of the preparation. Despite the low biological activity of carbetocin, its binding affinity to receptors of isolated myometrial plasma membranes was of the same order of magnitude as that of oxytocin. On the basis of the present results it is concluded that a longer half-life of the analogue at the receptor compartment may be a contributing factor to the prolonged uterotonic effect observed in vivo.