Die Bedeutung der Bindung von Pharmaka an Albumin und Erythrocyten eines Perfusionsmediums der isolierten Rattenleber

Summary

The binding of lipophilic drugs to albumin and erythrocytes and its importance for the uptake of the drugs into the isolated rat liver were investigated. Promazine and chlorpromazine were taken as lipophilic drugs, which were compared in some experiments with 3,3′-dimethyl-4,4′-diacetyldiphenylbisguanylhydrazone (BG 60), a much more hydrophilic drug.

The perfusion medium contained 2% albumin, about 30% bovine erythrocytes and the drug in a concentration of 10⁻⁴ M respectively 1.3×l0⁻⁴ M in Krebs-Henseleit-solution. In this solution promazine was bound to erythrocytes (69%) and to albumin (20%) whereas the buffer solution contained 11%; 77% of chlorpromazine were bound to erythrocytes, 19% to albumin, whereas the buffer solution contained 4% (Table 1). The distribution of chlorpromazine in the perfusion medium did not depend on the time of incubation (Fig. 1). Increasing the albumin concentration of the perfusion medium caused a rise of the chlorpromazine fraction bound to albumin and a decrease of the fraction bound to the erythrocytes and the fraction of the free, unbound substance (Table 3).

The distribution of BG 60 in the perfusion medium markedly depended on the time of incubation. Without incubation 7.5% of BG 60 were bound to erythrocytes, 63% were bound after an incubation time of 2 h (Fig. 1).

When suramine was added to the perfusion medium the fraction of promazine bound to albumin increased and the fraction bound to erythrocytes decreased, whereas the fraction of the free substance in the aqueous phase was not significantly changed. In a perfusion medium lacking erythrocytes the fraction of free promazine was considerably higher (Table 1).

After the perfusion medium had passed the liver one times, the total chlorpromazine (the fractions of the free and the bound substance) of the first part of the perfusion medium (50 ml) was extracted by the liver (Fig.2). This led to the suggestion that the binding of the phenothiazine derivatives to the membranes of erythrocytes was reversible.

In the experiments with BG 60 no fraction of the perfusate of the liver was found, which did not contain BG 60. Beyond that the uptake of BG 60 into the liver depended on the time of incubation of the perfusion medium (Fig. 2).

Chlorpromazine was taken up more rapidly than promazine into the recirculating perfused liver (Fig. 3). By suramine the uptake of promazine into the liver was enlarged. KCN did not markedly change the uptake of promazine into the liver (Table 4). If there were no erythrocytes in the perfusion medium the initial uptake of promazine into the liver was increased (Fig.4).

There seems to be no difference in the biotransformation of promazine and chlorpromazine as the examination of the perfusate and liver homogenate by thin layer chromatography pointed out. In the experiments with a perfusion medium lacking erythrocytes promazine was much less metabolized (Table 5).

The binding of lipophilic substances to erythrocytes may be of similar pharmacokinetic importance as the binding of substances to plasma proteins, as the experiments demonstrated.