THE USE OF TRITIATED PHENOXYBENZAMINE FOR INVESTIGATING RECEPTORS

¹ Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota

Attempts were made to label the adrenergic alpha receptor with phenoxybenzamine-H³ in experiments based on the concept of receptor protection. Radioactivity measurements were made of chloroform-methanol extracts and the remaining nonlipid residues of both protected and nonprotected preparations of the seminal vesicle of the rat. These results were then compared with those obtained from cumulative dose-response curve studies of other preparations. The results obtained with several protecting agents (norepinephrine, phentolamine, cocaine and MJ-1999) led to the conclusion that the radioactivity associated with the nonlipid residue is probably concerned with alpha receptor interaction. It was further concluded that no significant "receptor reserve" for norepinephrine exists in the seminal vesicle. From the values of experiments employing phentolamine, which afforded nearly 100% protection from blockade, it was estimated that there may be as many as 55,000 alpha receptors per smooth muscle cell, and that only a very small portion (0.004%) of the muscle cell surface would be involved in receptor interactions.