An enzymatic ion exchange model for active sodium transport

Evaluation on the reliability of the permeability coefficient (Kp) to assess the percutaneous penetration property of chemicals on the basis of Flynn's dataset

PURPOSE

The permeability coefficient (K_p) is often used for prediction of the dermal penetration of chemicals. Mathematical models have mostly been derived on K_p data basis. However, confusing K_p values are reported, questioning the general reliability of this parameter. In this study, we tested the plausibility of K_p values expressing the dermal penetration velocity (cm h^-1) of chemicals on a larger dataset from literature.

METHODS

K_p was applied for the calculation of the time for penetration through skin membranes of defined thickness (t_CrossSkin). K_p values were obtained from Flynn's dataset (1990), containing data determined mostly under similar experimental conditions using diffusion cells. Further skin penetration parameters, e.g., times at which the chemicals were firstly measured in the receptor phase, lag times, steady-state times, and exposure duration, where available, were related to K_p values. The data congruence was tested comparing K_p values from Flynn's dataset with those reported in the EDETOX database. Variables, which could bias the results, such as different experimental protocols and research groups were also considered.

RESULTS

K_p data for 94 chemicals matched the inclusion criteria were evaluated. According to the K_p values, 21 (22%) compounds would require longer than 100 h, and 20 (21%) further compounds longer than 10 h of exposure to penetrate skin membranes of ~ 0.01-2.5 mm thickness. Obviously, erroneous K_p were found in studies of almost all research groups in Flynn's database, indicating that neither the observer nor the experimental conditions alone biased the values.

CONCLUSIONS

Our evaluation demonstrates high implausibility of K_p values to represent the dermal penetration velocity and supports general invalidity of the parameter for implementation in studies using skin membranes. The K_p should not be used to characterize the percutaneous penetration of chemicals or in risk assessment without verification.

Permeability of frog skeletal muscle cells to choline

Using choline-methyl-C¹⁴ as a tracer, it has been shown that choline⁺ penetrates into the cells of resting frog skeletal muscle at a rate similar to that of Na⁺, and that it escapes from these cells much more slowly than does Na⁺. Some implications of these findings are discussed.

Effect of external cation and anion substitutions on sodium transport in isolated frog skin

Effects of changes in external ionic strength, external cation and/or anion substitution on transepithelial influx and efflux of sodium, short-circuit current and on transepithelial potential difference and resistance were studied in isolated frog skin. Active transport of Na was found to be highly dependent on both anionic and cationic composition of external medium. Relative abilities of external monovalent cations to inhibit active Na transport were H greater than Li greater than K greater than Rb greater than Cs greater than choline. Relative abilities of external monovalent anions to stimulate active Na transport were I greater than Br greater than Cl. Sequences of anion interaction and of resistance changes suggest that anionic stimulation of Na transport is not due to electrical coupling across outer cell membrane. The ability of different anions and cations to alter Na transport suggests that externally located charged groups act as important barriers or filters to ion movement. In addition, the experiments suggest that an increase in ionic strength of external medium has an effect on active transport of Na, a finding that indicates interference of surface charges with Na entry. Directional changes in efflux of Na due to changes in ionic composition of external medium usually paralleled changes in active Na transport. It is possible that the observed relationship between influx and efflux of Na is the result of common pathways and of interaction of the active transport system with Na efflux.

Acetylcholinesterase staining in subdivisions of the cat's inferior olive

The present paper describes a unique distribution of true AChE activity in the IO. In the dorsal accessory olive three areas with high AChE activity can be distinguished. The medial accessory olive can be subdivided into a caudal part which shows rostro-caudally directed bands with different enzymatic activity, and a rostral part which shows a more uniform, medium activity. In the nucleus beta and the dorso-medial cell column, AChE activity is low. The ventral and dorsal lamellae of the principal olive contain areas with high, medium, and low activity. The dorsal cap is strongly positive, while the ventrolateral outgrowth is negative for AChE. Enzyme distribution cannot be fully explained on the base of the known afferent and efferent connections with the IO. However, histochemical results provide evidence that generally supports a subdivision of the IO that mirrors these connections (Brodal, '40; Armstrong et al., '74' Boesten and Voogd, '75; Groenewegen et al., '75).

The action of antidiuretic hormone on cell membranes. Voltage transient studies

1. The instantaneous impedance method has been used to study the effects of antidiuretic hormone (ADH) on frog skin.2. The resting skin may be represented by a parallel RC network with a single time constant.3. Antidiuretic hormone causes an increase in conductance and capacitance and in some cases the appearance of a polarization angle.4. The structures in the skin responsible for the transients are located in the outermost membranes.5. The effects of ADH have been interpreted in terms of the formation of water-filled sodium-permselective pores in the outer facing membranes which occupy, at most, 0.3% of the skin surface. These pores constitute a parallel, and hence additive, capacitance with that of the normally ion impermeable parts of the cell surface, and in addition are responsible for the increase in conductance. The polarization angle is due to the polydisperse nature of the skin after hormone treatment.

Permeability of Frog Skin to Choline

An inward movement of choline can be demonstrated across isolated skins of Rana pipiens when the outside is bathed by a saline made with choline chloride. This is in contrast to an earlier report indicating that skins of R. esculenta are impermeable to choline. The flux was very small and independent of the active transport of sodium by the skins. For most experiments choline can be used as an "inert" replacement for sodium in this prepration.