Irreversible blockade of the kappa-receptor by a newly synthesized dynorphin derivative containing a 3-nitro-2-pyridinesulfenyl group.

The interactions of the dynorphin derivative containing a 3-nitro-2-pyridinesulfenyl group (dynorphin derivative) with kappa-receptors were studied. The dynorphin derivative inhibited, in a concentration-dependent manner, the twitch response to electrical stimulation of the longitudinal muscle of the guinea-pig ileum which contains both mu- and kappa-receptors. It also inhibited the twitch response to electrical stimulation of the rabbit vas deferens, reported to contain only kappa-receptors; its pD2 value was 11.57 +/- 0.09, suggesting that the agonistic activity of the dynorphin derivative is mainly mediated through kappa-receptors. The KD and Bmax values of [3H]U-69593 were calculated from a Scatchard plot of the specific binding. After a 30 min treatment with 10(-5) M of this dynorphin derivative, the KD value did not alter but the Bmax value decreased. However, this decrease recovered after treatment with 2 x 10(-3) M of dithiothreitol. These results suggest that the dynorphin derivative binds to kappa-receptors through the disulfide bound. The dissociation constant of nalorphine-7,8-oxide (nalorphine-epoxide) was determined according to the method of Furchgott, by irreversibly binding a fraction of the kappa-receptors with the dynorphin derivative. The negative logarithms of the dissociation constants (pKA values) for nalorphine-epoxide did not differ in the guinea-pig ileum and rabbit vas deferens, suggesting that the kappa-receptors in the two preparations are identical. However, the pD2 value and efficacy for nalorphine-epoxide in the guinea-pig ileum were significantly greater than those in the rabbit vas deferens. These results indicate that the kappa-receptor reserve in the longitudinal muscle of the guinea-pig ileum is greater than in the rabbit vas deferens. It is thought that the dynorphin derivative, containing a 3-nitro-2-pyridinesulfenyl group, is useful to clarify the kappa-receptor mechanisms.