On plasma protein-mediated transport of steroid and thyroid hormones

Erythrocytes and the transport of drugs and endogenous compounds

This review considers the significance and measurement of endogenous compounds and drugs on erythrocytes. Part I examines literature examples where a direct measurement of hydrocortisone, phenytoin and valproate was performed on unwashed red cells in vitro and in vivo, showing a consistent contribution of the erythrocyte fraction to the transport of these compounds. In vitro partition experiments using systems composed of plasma water, plasma proteins and erythrocytes are discussed. When spiked blood is diluted with blank autologous plasma water, erythrocytes always discharge the compound over-proportionally compared to plasma proteins. In vivo, during the distribution phase, the elimination half-life from the erythrocyte is the same as or shorter than that from plasma water, and substantial amounts of drug leaving the circulation originate from erythrocytes. In Part II, the transfer of compounds is considered and evidence for the facilitated exchange of red cell associated substances between the erythrocyte and capillary endothelium presented. Situations where a failure to analyse the erythrocyte compartment leads to the loss of vital information are identified. Part III explores methods for analysing erythrocyte associated substances, most commonly indirect calculation, or analysis of washed erythrocytes. A direct determination is rarely performed, but one such method, allowing concurrent plasma analysis, is discussed. An instrument collects a fixed and known quantity of a maximally compressed cell mass, without disturbing the equilibrium between cells and plasma. To isolate compounds associated with the mass of erythrocytes, the red cell sediment can often be extracted quantitatively into a blank protein solution.

Plasma binding and transport of diazepam across the blood-brain barrier. No evidence for in vivo enhanced dissociation

The tissue uptake of extensively plasma-bound compounds is reportedly inconsistent with the conventional free-drug hypothesis limiting transport to unbound moiety in rapid intracapillary equilibrium with bound complex. Instead, protein-mediated/cell surface enhancement of dissociation has been postulated to occur in the microvasculature. This possibility was investigated by studying the passive transport of diazepam across the blood-brain barrier. Microdialysis probes placed within the vena cava and brain cortex were used to directly compare steady-state, interstitial unbound diazepam levels in both Wistar and genetically analbuminemic rats. The absence of albumin in the latter increased the unbound fraction of diazepam by almost fivefold; however, in both groups, the ratio of unbound concentrations in brain and blood at equilibrium was equal to unity. If enhanced dissociation occurred in the microvasculature, then the unbound brain level should have been greater than that in the systemic circulation. It is probable that earlier findings suggestive of protein-mediated transport reflect a nonequilibrium phenomenon. Comparison of the extent of diazepam's in vivo binding in blood by microdialysis to that estimated in vitro using conventional equilibrium dialysis with microcells showed good agreement, thus validating a widely accepted assumption of equivalency of these two values.

Thyroxine transport and distribution in Nagase analbuminemic rats

The postulate that thyroxine (T4) in plasma enters tissues by protein-mediated transport or enhanced dissociation from plasma-binding proteins leads to the conclusion that almost all T4 uptake by tissues in the rat occurs via the pool of albumin-bound T4 (Pardridge, W. M., B. N. Premachandra, and G. Fierer. 1985. Am. J. Physiol. 248:G545-G550). To directly test this postulate, and to test more generally whether albumin might play a special role in T4 transport in the rat, we performed in vivo kinetics studies in six Nagase analbuminemic rats and in six control rats, all of whom had similar serum T4 concentrations and percent free T4 values. Evaluation of the plasma disappearance curves of simultaneously injected 125I-T4 and 131I-albumin indicated that the flux of T4 from the extracellular compartment into the rapidly exchangeable intracellular compartment was similar in the analbuminemic rats (51 +/- 21 ng/min, mean +/- SD) and in the control rats (54 +/- 15 ng/min), as was the size of the rapidly exchangeable intracellular pool of T4 (1.13 +/- 0.53 vs. 1.22 +/- 0.36 micrograms). This latter finding was confirmed by direct analysis of tissue samples (liver, kidney, and brain). We also performed in vitro kinetics studies using the isolated perfused rat liver. The single-pass fractional extraction by normal rat liver of T4 in pooled analbuminemic rat serum was indistinguishable from that of T4 in pooled control rat serum (10.9 +/- 3.3%, n = 3, vs. 11.4 +/- 3.4%). When greater than 98% of the albumin was removed from normal rat serum by chromatography with Affi-Gel blue, the single-pass fractional extraction of T4 (measured by a bolus injection method) did not change (16.3 +/- 2.1%, n = 5, vs. 15.2 +/- 2.5%). These data provide the first valid experimental test of the enhanced dissociation hypothesis and indicate that there is no special, substantive role for albumin in T4 transport in the rat.