The Journal of Membrane Biology

Hillyard, Stanley; Sera, Ronald; Gonick, Harvey

doi: 10.1007/BF01868751pmid: N/A

Cadmium ion (Cd++) was found not to inhibit active sodium transport across the isolated frog skin when added in 10−3 m concentration to the basal-lateral surface. The same Cd++ concentration similarly had no effect on Na+ transport across the isolated epithelial cell layer from the frog skin, although this dose of Cd++ did inhibit Na+ transport across the frog urinary bladder and large intestine. When 10−3 m Cd++ was added to the apical surface of the isolated frog skin or to the isolated epithelial cells from the frog skin, sodium transport was reversibly stimulated, in contrast to the irreversible inhibition noted above. If equimolar cysteine was added with Cd++ to the apical surface of the skin, however, active Na+ transport was irreversibly inhibited. In conjunction with the inhibition produced by equimolar Cd++ and cysteine, isotopic Cd++ permeation into the tissue was three times higher when added with cysteine than in the absence of cysteine. Thus, the effects of Cd++ on epithelial Na+ transport is variable according to the epithelium studied and the presence of potential carrier molecules.

Sarracino, Susan; Dawson, David

doi: 10.1007/BF01868752pmid: N/A

If mucosal Na is completely replaced by Li the isolated turtle colon exhibits a steady-state short-circuit current (I sc) consistent with the transport of Li from mucosa to serosa.I sc persists in the absence of a transmural electrochemical gradient for Li and is abolished by amiloride or ouabain. In the presence of mucosal Li the amiloridesensitive, transepithelial cation transport path can be described as a constant emf in series with a variable conductance. A comparison of equivalent circuit parameters, however, reveals that in the presence of mucosal Li the apparent emf of the cation transport path is markedly reduced, but the conductance of the active path may be greater than, equal to, or less than that observed in the presence of Na. In tissues characterized by a relatively low cellular conductance in the presence of Na, Li substitution increases the amiloridesensitive conductance, whereas, in tissues characterized by an initially high active “Na-conductance”, the amiloride-sensitive conductance is reduced in the presence of Li. Thus, the approximate identity ofI sc in some tissues after cation substitution is a fortuitous consequence of an increased amiloride-sensitive conductance in the presence of mucosal Li coupled with adecreased apparent emf for Li transport. The addition of small amounts of Li to Na-containing mucosal or serosal solutions inhibits active Na transport, suggesting that Li exerts a “toxic” effect on the transport machinery apart from its ability to serve as a substitute cation.

Mobley, Bert

doi: 10.1007/BF01868753pmid: N/A

Single contractures were elicited in segments of skinned frog muscle fibers when the segments were moved from relaxing-loading solutions to various test solutions. The effective test solutions produced an increase in the concentration of chloride ions in the myofilament space, [Cl] ms , and/or presumably caused the sarcoplasmic reticulum to undergo a change in volume. The contractures were quantified in terms of their maximum tension and time-integral. Two outer segments from each fiber underwent a contracture in a control solution (chloride ions were substituted for all of the methanesulfonate ions in the relaxing solution). The mean values of tension and area in the control contractures of each fiber were divided into the corresponding values from a test contracture obtained in the central segment of the same fiber. Test contractures obtained upon increasing [Cl] ms and increasing the product, [K] ms ×[Cl] ms , were compared to contractures that were obtained by increasing [Cl] ms while keeping [K] ms ×[Cl] ms constant. The former contractures were greater in magnitude for a given [Cl] ms . Whereas the former solutions may have caused an increase in the volume of the sarcoplasmic reticulum and altered the electrical potential across the membranes of the sarcoplasmic reticulum as well, only a change in potential was presumed to have occurred in the latter solutions. Other types of contractures were investigated to show that both swelling of the sarcoplasmic reticulum and changes in the electrical potential of its membranes can cause release of calcium ions and elicit contractures in skinned fibers.

Holz, Ronald; Stratford, Carol

doi: 10.1007/BF01868754pmid: N/A

A new assay has been developed for vesicle-vesicle fusion based upon the mixing of intravesicular contents of two sets of vesicles. Purified firefly luciferase and MgCl2 were incorporated into one set of vesicles (LV) and ATP into the other (AV). Vesicles were prepared from soybean phospholipids. The luminescence that resulted from hydrolysis of ATP by luciferase was measured to determine the extent of mixing of the intravesicular contents. In the absence of divalent ions, incubation of a mixture of LV and AV did not produce luminescence. However, if Ca++ or other divalent ions were present at millimolar concentrations, luminescence occurred. The luminescence did not result from extravesicular reaction of vesicle contents that had leaked into the medium. Instead, luminescence resulted from the mixing of intravesicular spaces of AV and LV in fused vesicles. Optical density changes and negative stain electron microscopy indicated that Ca++ induced extensive aggregation of vesicles. However, quantitation of the maximum possible luminescence indicates that only a small percentage (less than 1%) of the vesicles actually fused in a fusion experiment.

Dilger, James; McLaughlin, Stuart

doi: 10.1007/BF01868755pmid: N/A

We report here a kinetic study of the mechanism by which the weak acid TTFB (4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazole) transports protons across phospholipid bilayer membranes. A previous kinetic study of the homologous dichloro compound, DTFB, revealed that the rate limiting step for proton translocation was the back diffusion of the neutral, HA, form of the weak acid; we conclude here that this is also the rate limiting step for proton translocation with TTFB. At high concentrations of either DTFB or TTFB the charged permeant species is an HA 2 − complex. The kinetic analysis and independent measurements reveal that the permeability of the membrane to HA and adsorption coefficients of A− and HA are an order of magnitude higher for TTFB than for DTFB. When either DTFB or TTFB was present in a solution where the pH was less than the pK of the weak acid, an unusual relaxation in the current was noted on application of a voltage step. The amplitude of the relaxation decreased as the voltage was increased. This relaxation is possibly due to a reorientation of the benzimidazole molecules at the membrane-solution interface. We also report experiments performed with DTFB on mitochondria. It was possible to reconcile these results with the bilayer data and, therefore, with the chemiosmotic hypothesis by postulating that the dielectric constant of the mitochondrial membrane is greater than that of a bilayer formed with decane as a solvent. To demonstrate the effect of dielectric constant on permeability, we replaced decane by 1-chlorodecane. This increased the capacitance of the artificial bilayer by a factor of two and the permeability of the bilayer to the A− form of DTFB by two orders of magnitude.