Characterisation of a high-affinity VIP receptor in human lung parenchyma

A method is described for preparing human lung parenchymal membranes essentially free of carbon contamination. Using this technique, a high-affinity 125I-VIP-binding site has been characterised. The receptor density is approx. 200 fmol/mg protein, and the Kd of 125I-VIP by saturation binding is 200 pM. The dissociation kinetics are complex and cannot be described by first-order kinetics. Several VIP-related peptides displace 125I-VIP from this binding site with a rank order of potency: VIP greater than rat GRF greater than PHM greater than PHI greater than human GRF greater than secretin greater than glucagon. Displacement curves of these peptides exhibited slope factors significantly less than unity with the exception of human GRF.