The relative rate of penetration of the lower saturated monocarboxylic acids into mammalian erythrocytes

Monocarboxylic acid permeation through lipid bilayer membranes

The membrane permeability coefficients for the homologous monocarboxylic acids, formic through hexanoic, as well as benzoic and salicylic, were determined for egg phosphatidylcholine-decane planar bilayer membranes. The permeabilities of formic, acetic and propionic acid were also determined for "solvent-free" phosphatidylethanolamine bilayers. Permeability coefficients were calculated from tracer fluxes measured under otherwise symmetrical conditions, and precautions were taken to ensure that the values were not underestimated due to unstirred layer effects. The relation between the nonionic (HA) permeability (Pm) and the hexadecane/water partition coefficient (Kp) was: log Pm = 0.90 log Kp + 0.87 (correlation coefficient = 0.996). Formic acid was excluded from the analysis because its permeability was sixfold higher than predicted by the other acids. The permeabilities for "solvent-free" membranes were similar to those for decane-containing membranes. The exceptionally high permeability of formic acid and the high correlation of the other permeabilities to the hexadecane/water partition coefficient is a pattern that conforms with other nonelectrolyte permeabilities through bilayers. Similarly, the mean incremental free energy change per methylene group (delta delta G/-CH2-) was -764 cal mol-1, similar to other homologous solutes in other membrane systems. However, much less negative delta delta G values (-120 to -400 cal mol-1) were previously reported for fatty acids permeating bilayers and biological membranes. These values are due primarily to unstirred layer effects, metabolism and binding to membranes and other cell components.

Asymmetric proton block of inward rectifier K channels in skeletal muscle

Inward rectifier and delayed rectifier K currents were measured in frog skeletal muscle fibers with the vaseline gap voltage clamp technique as internal or external pH were lowered. Inward rectifier currents were only slightly reduced by low external pH but were completely and reversibly blocked when the internal pH was reduced to below 5.5 either by cutting the fiber ends in low pH solutions or by bathing the fiber exterior with permeant acetate buffers at low pH. The steepness of the pH dependence of this block suggests that more than one and perhaps three hydrogen ions are required to bind to the blocking site. The voltage dependence of inward rectifier gating was not shifted by low external pH. Either these channels are not located near the fixed negatively charged groups which apparently alter the voltage sensed by many other ionic channels or the membrane potential drop and the external [K+] are altered in a compensating manner such that the driving force on K+ (upon which inward rectification depends) remains unchanged. In contrast, delayed rectifier channels were blocked and their kinetics dramatically shifted by lowering external pH.

Block of inward rectification by intracellular H+ in immature oocytes of the starfish Mediaster aequalis

Intracellular pH was recorded in immature starfish oocytes using pH-sensitive microelectrodes, and inwardly rectifying potassium currents were measured under voltage clamp. When the intracellular pH was lowered using acetate-buffered artificial sea water from the normal value of 7.09 to 5.9, inward rectification was completely blocked. The relationship between inward rectification and internal pH between 7.09 and 5.9 could be fit by a titration curve for the binding of three H ions to a site with a pK of 6.26 to block the channel. The H+ block showed no voltage dependence, and the activation kinetics of the inwardly rectifying currents were not affected by the changes in internal pH.

Erythrocyte permeability to lipophilic solutes changes with temperature

Studies of permeability coefficients of biological barriers to members of homologous series can provide information of value in assessing barrier characteristics. To this end, we have determined the linear diffusion coefficients of tracer water (THO), [14C]antipyrine, [14C]acetamide, and n-[14C]alcohols over the range of 10-37 degrees C for dog erythrocytes (D), hemoglobin (D2), and plasma (D1). Permeability coefficients (Po) calculated with the series-parallel pathway model are higher than Po for water at 37 degrees C for all the alcohols except hexanol. Po for acetamide and antipyrine are considerably lower than Po for water at 37 degrees C. The apparent activation energies (Ea) for Po of acetamide and the C1, C2, and C3 alcohols are 10.5 kcal.mol-1, similar to the values obtained with epithelial or lipid bilayers. The Ea's for the larger alcohols are 2-4 kcal.mol-1. The Ea for Po of water is 5.3 kcal.mol-1, similar to the Ea for self-diffusion of water; Ea for antipyrine is 25 kcal.mol-1. We interpret these results to indicate heterogeneity of membrane-solute interactions or of membrane pathways in the erythrocyte for lipid-soluble molecules that is related to both lipid solubility and solute size, as we have suggested in an earlier study and confirmed experimentally in the present one.

The effects of chloride substitution on intracellular pH in crab muscle

1. Intracellular pH (pHi) was measured in crab muscle fibres using pH-sensitive micro-electrodes. The mean stable pHi was 7.19 +/- 00.2 (S.E. of mean) and the corresponding mean membrane potential was -64.6 +/- 0.4 mV (S.E. of mean) at an external pH of 7.5. 2. The effects on pHi of replacing 20% (100 mM) of the external NaCl by the Na salts of various anions were examined. The anions of weak acid (pK'a greater than 4.5) caused large internal acidifications. The anions of strong acids (pK'a less than 2.6) caused little or no change in pHi. The anions of acids with an intermediate pK'a had varied effects on pHi. In particular salicylate (pK'a = 2.97) was found to cause a large fall in pHi. 3. Increasing the external pH reduced the effects of the anions of weak acids on pHi. It is argued that these effects are the result of the entry and subsequent dissociation of undissociated acid molecules. 4. The results with propionate were quantified by comparing them with the effects of 5% CO2 and were found to be smaller than expected. It is suggested that this is the result of substantial membrane permeability to the propionate anion.

Temperature dependence of nonelectrolyte permeation across red cell membranes

The temperature dependence of permeation across human red cell membranes has been determined for a series of hydrophilic and lipophilic solutes, including urea and two methyl substituted derivatives, all the straight-chain amides from formamide through valeramide and the two isomers, isobutyramide and isovaleramide. The temperature coefficient for permeation by all the hydrophilic solutes is 12 kcal mol(-1) or less, whereas that for all the lipophilic solutes is 19 kcal mol(-1) or greater. This difference is consonant with the view that hydrophilic molecules cross the membrane by a path different from that taken by the lipophilic ones. The thermodynamic parameters associated with lipophile permeation have been studied in detail. DeltaG is negative for adsorption of lipophilic amides onto an oil-water interface, whereas it is positive for transfer of the polar head from the aqueous medium to bulk lipid solvent. Application of absolute reaction rate theory makes it possible to make a clear distinction between diffusion across the water-red cell membrane interface and diffusion within the membrane. Diffusion coefficients and apparent activation enthalpies and entropies have been computed for each process. Transfer of the polar head from the solvent into the interface is characterized by DeltaG(double dagger) = 0 kcal mol(-1) and DeltaS(double dagger) negative, whereas both of these parameters have large positive values for diffusion within the membrane. Diffusion within the membrane is similar to what is expected for diffusion through a highly associated viscous fluid.

Hindered diffusion in microporous membranes with known pore geometry

The hindrance effect on the aqueous diffusion rate of solutes within membrane pores of molecular size has been accurately determined. Mica membranes, 3 to 5 micrometers thick, were prepared with uniform, straight pores from 90 to 600 angstroms in diameter. With these membranes a direct estimation was possible of the interaction between pore size and molecular diffusion rates. There were no uncertainties due to wide pore size distributions or nonuniform tortuous channels as in previously used model microporous materials such as dialysis tubing or gels. Aqueous diffusion rates through these mica membranes were measured for a series of compounds with molecular diameters from 5.2 to 43 angstroms and were corrected for "liquid film resistances" adjacent to the membrane-solution interface to obtain estimates of molecular diffusivities within the micropores of the membrane. Definite evidence is presented showing that, even when molecular size is a small fraction of pore size, diffusion rates decrease markedly. The apparent reduction in solute diffusivity in the microporous membrane can be quantitatively estimated by means of the Renkin equation for hindered diffusion.