Permeability and transport systems in living cells

A note on the diffusion of drugs through artificial phospholipid membranes

A relatively simple method for the assessment of the passive diffusion of drugs through artificial phospholipid membranes composed of lecithin and collodion yields data on the permeability of some pharmaceutical substances through lipid barriers. Although there are inherent limitations in using artificial phospholipid membranes as substitutes for more complex biological membranes, this model system can furnish information on permeability kinetics and on the effect of structural groups on biological activity.

Tubular transport maxima of PAH and diodrast measured individually in the aglomerular kidney of Lophius, and simultaneously as competitors under conditions of equimolar loading

The maximal tubular transfer rates (Tm) of both p-aminohippurate (PAH) and diodrast (3,5-diiodo-4-pyridone-N-acetic acid or iodopyracet) were found to be fixed and reproducible when measured separately in Lophius (goosefish) under standard laboratory conditions. Expressed on a molar basis Tm_PAH was four times Tm_D. However, when these transport competitors were presented simultaneously in equimolar concentrations with the plasma levels of each sufficiently high enough to saturate the carrier system, the relative rates of excretion were reversed with the diodrast transfer rate then four times that of PAH. The combined rate of excretion was far below Tm_PAH alone, and roughly equal to Tm_D. Interaction with a common carrier was indicated by the gradations in degree of inhibition which resulted when plasma concentration ratios of diodrast to PAH were extended from 0.1 to 3.2, and PAH transfer rates expressed as percentage of Tm_PAH were correspondingly depressed from 17 to 1.0 per cent respectively. These observations again point up the inverse relationship between transfer rate and competitive effectiveness which exists for members of a series of substances actively transported by a common mechanism. It appears that carrier affinity and dissociation characteristics may be quite different for various compounds in a series, and also that these parameters may vary significantly from species to species.

THE ROLE OF ADENOSINE TRIPHOSPHATE IN TRANSPORT REACTIONS

The evidence bearing on the participation of adenosine triphosphate (ATP) in transport reactions is reviewed. The connection may take the form of a strict quantitative relationship as observed in potassium uptake by red blood cells or as in amino acid uptake by slices of rat brain cortex. There may also be a qualitative relationship such as that observed in studies of amino acid uptake by Ehrlich ascites carcinoma cells. In other tissues (e.g. slices of the Ehrlich carcinoma) amino acid uptake may only have an uncertain relationship to the ATP content of the tissue. Some of the schemes which have been put forward to account for the involvement of ATP are discussed.

The interaction between steroid hormones and lipid monolayers on water

The interaction of progesterone, testosterone, androsterone, and etiocholanolone with insoluble lipid films (cholesterol and saturated hydrocarbons containing either alcohol, ester, acetamide, phosphate, amine, or carboxyl groups) was studied. In addition to surface pressure and surface potential measurements of the surface films, radioactive tracers were used to measure the concentration of adsorbed steroid in the lipid films. In general, steroids form mixed films with the insoluble lipid films. Compression of the insoluble lipid films to their most condensed state leads to complete ejection of adsorbed steroid from the surface in all cases except with the amine, for which a small amount of steroid is still retained in the surface. Interactions between the steroids and insoluble lipids are primarily due to van der Waals or dispersion forces; there were no significant contributions from dipole-dipole interactions (except possibly with the amine). Specific interactions between cholesterol and the soluble steroids were not observed. Evidence suggests that low steroid concentrations influence structure of lipid films by altering the hydration layer in the surface film. In contrast to a specific site of action, it is proposed that steroid hormones initiate structural changes in a variety of biological sites; this model of steroid action is consistent with the ubiquity of many steroid hormones.

AMINO ACID TRANSPORT IN BRAIN CORTEX SLICES: III. THE UTILIZATION OF ENERGY FOR TRANSPORT

When rat brain cortex slices are incubated with amino acids in the presence of various sugars, the ratio of amino acid transported to the steady-state level of ATP (termed the transport quotient) is constant. The actual amount of ATP maintained is greatest with glucose and mannose but somewhat less with fructose and galactose. Certain α-keto acids (pyruvate, oxalacetate, α-ketoglutarate, or glutamate which can give rise to α-ketoglutarate) also support glycine transport but with a lesser efficiency. Fumarate, malate, and succinate have little ability to support amino acid transport.The transport supported by pyruvate is particularly resistant to the action of agents which markedly reduce the transport supported by glucose as substrate. These agents include dinitrophenol, digoxin, and uranyl ions. Inhibitory effects are obtained with high concentrations of potassium ions whether the substrate is glucose or pyruvate.These results are interpreted in terms of pyruvate dismutation and acetate thiokinase leading to ATP production and in terms of intracellular localization of ATP.