Affinity of normorphine for its pharmacologic receptor in the naive and morphine-tolerant guinea-pig isolated ileum.

The affinity of normorphine for its pharmacologic receptor was determined using the longitudinal muscle-myenteric plexus preparation of naive and morphine-tolerant guinea pigs and the method of partial blockade of a fraction of the receptor population with the novel, irreversible opiate antagonist, beta-chlornaltrexamine. The normorphine concentration-response curve was antagonized by beta-chlornaltrexamine in a nonsurmountable fashion, at antagonist concentrations ranging from 6.0 to 15.0 nM in both naive and tolerant tissues. The dissociation constant (KA; reciprocal of affinity) of normorphine was found to be 1.54 (+/- 0.22) X 10(-6) M in naive and 2.30 (+/- 0.66) X 10(-6) M in morphine-tolerant ilea, values that did not differ significantly. The IC50 of normorphine was approximately one-sixth as large as KA in the naive, but was approximately equal to KA in the tolerant preparation. The stimulus-effect relation was nonlinear in both naive and tolerant ilea, but differed markedly in range in the two states. This study represents a direct pharmacological determination of the normorphine dissociation constant using concentration-response data; the values obtained agree well with those based on brain concentration in vivo and with data obtained in radioligand binding studies performed in the presence of sodium chloride. Furthermore, these results suggest that affinity changes may not be of major importance in the development of tolerance to opiates in this tissue and that the phenomenon of tolerance is probably related to changes in the postreceptor chain of events that lead to the measured effect.