Discussion a

W. M. PARDRIDGE (University of California. Los Angeles, School of Medicine, Los Angeles, CA): Dr. Ekins illustrates the extent to which one must go to save the free hormone hypothesis. He implies that my data could be explained within the context of the free hormone hypothesis, if there is a dissociation limited uptake of hormone. What Dr. Ekins fails to point out is for that to be true for thyroid hormones one has to postulate membrane permeability that is 3 log orders of magnitude, a thousandfold higher, than is experimentally observed. If you do not carry out in vivo experiments I guess that you do not have to worry if you have a 3 log orders difference in magnitude. Without tremendously high membrane permeability you do not have dissociation limitation and the in vitro measurements of free hormones should predict what you measure with our techniques. Our techniques show that the exchangeable fraction is much higher; it is coming from the protein-bound pool. The second point is that Dr. Oppenheimer has another explanation for the discrepancies in the free hypothesis. Dr. Oppenheimer's data indicates that the T3 receptor in the liver nucleus is 50% occupied in vivo and that the dissociation constant (K,) for that receptor is 1 nmolar. To get a 50% occupancy you have to have a free T3 in the nucleus equal to the K,, and this is 200-fold higher than the free T3, which is 5pM. Dr. Oppenheimer just postulates that there is an active T4 pump at the nuclear membrane. When he isolated the nucleus he could not find the pump experimentally. The third major point of Dr. Ekins is that I have two hypotheses. I have from the beginning said that free hormones are trivial in most cases compared to the amount of hormone coming into the tissue from the bound pool. Those three parameters are important: capillary transit time; membrane permeability; and plasma protein dissociation rates. Since the 1981 review, we have studied a number of different organs and we now realize that organ-specific enhanced dissociation is more important than the difference in capillary transit time, in terms of explaining organ differences, but it is still the Same basic hypothesis. R. P. EKINS (Middlesex Hospital Medico1 School, University of London, London, England): The hypotheses of Dr. Pardridge are rather incorrect. They work, indeed, but they are incorrect. My data demonstrate that they are mathematically incorrect. Indeed they are. PARDRIDGE: That is not true. Your equation is just as wrong as mine was. Your equation predicts the extraction goes to 0 when albumin goes to 0. P. K. SIITERI (University of California, Sun Francisco, San Francisco, CA): I do not think we are going to resolve this issue today. The assumption that only free hormone that exits the capillary, regardless of the mechanism involved, is important for cells applies to all of the models as far as I can determine. It is really important to know what is happening on the other side of the capillary wall since in fact there must be reestablishment of equilibrium with proteins that exist there. Since intra and extravascular systems are likewise in equilibrium, we must consider transport proteinbound hormones interacting with cells, as is the case with protein hormones such as FSH, LH or insulin. No one has addressed this problem, because it is quite difficult to know what the protein concentrations are outside of the capillary wall, i.e., in the interstitial fluid.