Specific binding and clearance of [3H]dynorphin (1-13) in the perfused rat lung: an application of the multiple-indicator dilution method.

The clearance and binding of a kappa-selective opioid peptide, dynorphin (1-13), in the perfused rat lung has been examined, using the multiple indicator dilution method. More than 50% of [3H]dynorphin (1-13) entering the pulmonary circulation was eliminated by the lung during a single passage of a tracer dose. By contrast, when a high dose (100 microM) of dynorphin (1-13) was concomitantly injected, [3H]dynorphin (1-13) behaved like [14C]sucrose, an extracellular marker. The kinetic analyses of the pulmonary venous outflow curves of [3H]dynorphin (1-13) and [14C]sucrose at low and high doses of dynorphin (1-13) indicated that the initial uptake rate constant, extraction ratio and distribution volume of [3H]dynorphin (1-13) decreased significantly in the presence of a high concentration of unlabelled dynorphin (1-13). These results suggest that [3H]dynorphin (1-13) is eliminated by a saturable process and binds to a specific binding site in the perfused lung, which may be the kappa-type binding site. The multiple indicator dilution technique, in combination with a moment analysis, was successfully applied to demonstrate the specific binding and clearance of dynorphin (1-13) in the perfused lung.