A B S T R A C T Basic lipophilic drugs such as propranolol and lidocaine are strongly bound by a1-acid glycoprotein, also called orosomucoid. Although the liver is known to rapidly clear plasma protein-bound propranolol or lidocaine, it is generally regarded that peripheral tissues, such as brain or heart, are only exposed to the small fraction of drug that is free or dialyzable in vitro. The "free drug" hypothesis is subjected to direct empiric testing in the present studies using human sera and an in vivo rat brain paradigm.Serum from 27 human subjects (normal individuals, newborns, or patients with either metastatic cancer or rheumatoid arthritis) were found to have up to a sevenfold variation in orosomucoid concentrations. The free propranolol or lidocaine as determined in vitro by equilibrium dialysis at 370C varied inversely with the orosomucoid concentration. Similarly the rate of transport of propranolol or lidocaine through the blood-brain barrier (BBB) was inversely related to the existing serum concentration of orosomucoid. However, the inhibition of rat brain extraction of drug by orosomucoid in vivo was only about one-fifth of that predicted by free drug measurements in vitro. This large discrepancy suggested orosomucoid-bound drug was readily available for transport into brain in vivo.Studies using purified human orosomucoid in the rat brain extraction assay also showed that orosomucoidbound propranolol or lidocaine is readily transported through the BBB. Conversely, albumin-bound propranolol or lidocaine was not transported through the BBB. The studies using albumin provide evidence that

The literature regarding the blood--brain barrier (BBB) transport of butanol is conflicting as studies report both incomplete and complete extraction of butanol by the brain. In this work the BBB transport of both [14C]butanol and [3H]water was studied using the carotid injection technique in conscious and in ketamine- or pentobarbital-anesthetized rats employing N-isopropyl-p-[125I]iodoamphetamine ([125I]IMP) as the internal reference and as a fluid microsphere. The three isotopes (3H, 125I, 14C) were conveniently counted simultaneously in a liquid scintillation spectrometer. IMP is essentially completely sequestered by the brain for at least 1 min in conscious rats and for 2 min in anesthetized animals. Butanol extraction by rat forebrain is not flow limited but ranges between 77 +/- 1 and 87 +/- 1% for the three conditions. The incomplete extraction of butanol by the forebrain is due to diffusion restriction of butanol clearance in some regions (frontal cortex, colliculi) but not in others (caudate, hippocampus, olfactory bulb). The permeability-surface area product/cerebral blood flow ratio of butanol and water in rat forebrain remains relatively constant, 1.7 +/- 0.2 and 1.0 +/- 0.1, respectively, despite a twofold increase in cerebral blood flow in conscious relative to pentobarbital-anesthetized rats. The absence of an inverse relationship between flow and butanol or water extraction is consistent with capillary recruitment being the principal mechanism underlying changes in cerebral blood flow in anesthesia. The diffusion restriction of BBB transport of butanol in some regions, but not in others, necessitates a careful regional analysis of BBB permeability to butanol prior to usage of this compound as a cerebral blood flow marker.

Tryptophan is the only amino acid in the circulation that is bound by albumin, and previous studies have suggested that the brain tryptophan supply is a function of either the free or the albumin-bound pool of tryptophan in blood. Since the albumin molecule per se does not cross the brain capillary wall, i.e., the blood-brain barrier (BBB), the transport of tryptophan from the circulating albumin-bound pool may involve enhanced dissociation of tryptophan from the albumin binding sites within the cerebral microcirculation. This hypothesis was confirmed in the present studies wherein the dissociation constant (KaD) of albumin binding of tryptophan in the rat or rabbit brain microcirculation was measured in vivo. Brain extraction data for [14C]tryptophan determined with the carotid artery injection technique were fit to the Kety-Renkin-Crone equation modified for protein-bound solute. The KaD of albumin binding in the rat or rabbit brain microcirculation under pentobarbital anesthesia was 1.7 +/- 0.1 and 3.9 +/- 1.0 mM, respectively, as compared to the KD value measured in vitro with equilibrium dialysis, 0.13 +/- 0.03 mM. In contrast, the KaD value of albumin binding of tryptophan in vivo in the rabbit brain microcirculation was reduced by ether anesthesia to a value of 2.1 +/- 0.4 mM. This reduction in the KaD under ether anesthesia was associated with a 2.5-fold increase in cerebral blood flow. In addition, dialyzed rabbit serum caused a statistically significant inhibition in [14C]tryptophan influx during ether, but not pentobarbital, anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipophilic amine drugs such as propranolol and lidocaine are actively sequestered by tissues via saturable cytoplasmic binding systems. The present studies were designed to characterize the kinetics of drug transport and sequestration in rat brain in vivo by using the carotid injection technique. Both propranolol and lidocaine are sequestered by brain, and the half time (t 1/2) of clearance of the drugs from brain to blood is 6-7 min. The t 1/2 of propranolol association and dissociation reactions with the brain sequestration system are 0.38 +/- 0.03 and 1.33 +/- 0.20 min, respectively. The blood-brain barrier transport of propranolol and lidocaine is inhibited by acid pH, and drug transport is mediated by a low-affinity, high-capacity saturable transport system [propranolol half-saturation constant (Km) = 9.8 +/- 1.2 mM, maximal rate of saturable transport (Vmax) = 5.7 +/- 0.7 mumol X min-1 X g-1, and nonsaturable transport constant (KD) = 0.061 +/- 0.012 ml X min-1 X g-1). These studies indicate that in addition to cerebral blood flow, the distribution of lipophilic amines in brain is a function of plasma pH and of the activity of brain sequestration systems. The latter may represent high-capacity cytoplasmic drug-binding proteins that normally bind endogenous ligands in brain.

The transport into rat liver of thyroxine (T4) bound to human prealbumin was studied with the use of sera obtained from patients with thyroid hormone-binding globulin (TBG) deficiency and with purified human prealbumin. The unidirectional extraction of 125I-T4 by liver was measured after rapid injection of isotope mixed in human serum into the portal vein of ketamine-anesthetized rats. The percentage of total serum T4 transported into liver was 47.6 +/- 5.2% in subjects with TBG deficiency, and this represented a 50% increase in hepatic T4 transport relative to control human serum. Since T4 is bound to albumin and to prealbumin in complete TBG deficiency, these results suggested that T4 bound to human prealbumin was transported into rat liver. This was confirmed using portal vein injections of human prealbumin at physiological concentrations (0.1-0.3 mg/ml). At these concentrations, T4 bound to human prealbumin was readily transported into liver. These studies suggest factors present in the liver microcirculation inhibit the binding of T4 to human prealbumin such that T4 bound to human prealbumin is highly transportable in liver; conversely, T4 bound to rat prealbumin is not transportable in rat liver. The inability of human prealbumin to sequester T4 in plasma may provide the basis for the selective advantage in humans of TBG, which does sequester T4 in plasma.