Abstract

Guinea pig ilea were incubated in vitro with naloxazone, an irreversible opioid receptor antagonist, in an attempt to determine a dissociation constant (KA) value for the opioid agonist, BW 942C, by the method of partial receptor alkylation. However, concentrations of naloxazone up to 3 X 10(-4) M (120 min incubation-60 min washout) did not depress the maximal response or the slope of the concentration-response curve for BW 942C and, therefore, did not allow calculation of KA value. To analyze the residual activity of BW 942C, tissues were incubated with naloxone for 60 min after naloxazone treatment. Schild analysis of naloxone antagonism after 3 X 10(-6) M naloxazone produced a pKB of 8.0 +/- 0.06 and a slope of 0.88 +/- 0.03, suggesting simple competitive antagonism at a single site. In the absence of naloxazone treatment, naloxone 10(-8) to 10(-5) M antagonized the effects of BW 942C yielding a pKB value and slope of 8.34 +/- 0.08 and 1.0 +/- 0.04, respectively. Schild analysis of naloxone antagonism after 10(-4) M naloxazone yielded a pA2 of 6.23 +/- 0.20 and a slope of 0.55 +/- 0.08. These values were not consistent with simple competitive antagonism at a single receptor site. Modeled curves showed that the data for naloxone antagonism after 10(-4) M naloxazone were consistent with action at two receptor sites with naloxone pKB values of approximately 8.3 (experimentally determined) and approximately 6.0. An alternative explanation of altered affinity for BW 942C and naloxone at a single site produced by naloxazone treatment cannot be excluded.