Estimation of endothelial receptor sites with the mean transit time approach

A new procedure for quantitating endothelial receptor sites is described. The mean transit time of the passage of an appropriate radio-labeled ligand from the artery to vein of an organ is compared to the mean transit times of a vascular indicator and a water label. The concentration of the unlabeled ligand is progressively increased to define that concentration at which half of the enzyme sites are occupied. Values are calculated for the number of receptor sites accessible during a single circulation in each ml of exchangeable tissue water. This approach is illustrated by estimating the sites of carbonic anhydrase present on the pulmonary endothelium, utilizing labeled acetazolamide as a ligand. Preliminary studies of the receptor sites on isolated endothelial cells suspended in an elutriator are also presented.