The Journal of Membrane Biology

Iino, Yasuhiko; Troy, Julia; Brenner, Barry

doi: 10.1007/BF02007632pmid: 7277469

We examined the direct effects of isoproterenol (ISO) andl-norepinephrine (NE) on electrolyte transport in isolated rabbit cortical collecting tubules (CCT) perfusedin vitro. The addition of either ISO (10−6 m) or NE (10−6 m) to the bath decreased transepithelial potential difference (PD), on average by 51 and 25%, respectively. These effects of ISO and NE were abolished by prior addition of the β-adrenergic blocker,l-propranolol. ISO (10−5 m) had no effect from lumen. Also, osmotic water permeability was not influenced by ISO. Ouabain and ISO had additive effects on PD. Elimination of chloride from both perfusate and bath, or addition of acetazolamide, abolished the effect of ISO on PD. Although isotopic sodium flux from lumen to bath was not influenced by ISO, chemical net chloride absorption increased from 1.1±0.4 to 2.7±0.6 peq·cm−1·sec−1 (n=8,p<0.005). In conclusion, both ISO and NE are capable of decreasing PD in rabbit CCT perfusedin vitro. This effect is mediated by β-adrenergic receptors and is accompanied by the increase in net chloride absorption. Although the mechanism responsible for this decrease in PD with ISO is unclear, active chloride absorption, active hydrogen secretion, or membrane chloride permeability changes may account for the effects of ISO.

Smith, Brian; Celle, P.; Siefring, Gerald; Lowe-Krentz, Linda; Lorand, Laszlo

doi: 10.1007/BF02007633pmid: 6792358

Excess calcium binding affects the shape and dynamics of cellular deformation of human erythrocytes. It may be hypothesized that incorporation of calcium may modify cellular deformability by processes which include specific cross-linking of membrane proteins with resultant changes in cell shape and deformability. Since previous studies indicate that accumulation of calcium ions causes development of γ-glutamyl-ɛ-lysine bridges in membrane proteins, under control of a membrane transamidating enzyme which specifically requires calcium ions for activation, experiments were devised to examine the relationship between cross-linking and deformability and to determine the effects of specific inhibitor of membrane protein cross-linking on the calcium-dependent modification of erythrocyte to the echinocytic shape. The elastic shear modulus of the membrane was not significantly affected by calcium-induced cross-linking, indicating that induced shape change, not altered elasticity, causes the observed reduction in cellular deformability. These findings support the interpretation that Ca++-induced and transamidase-catalyzed cross-linking of membrane proteins contributes to fixation of altered cellular shape and decreased cellular deformability.

Persechini, Pedro; Araujo, Elizabeth; Oliveira-Castro, Gilberto

doi: 10.1007/BF02007634pmid: 7024552

Some electrophysiological characteristics of macrophages and macrophage polykaryons of foreign body granuloma have been investigated. Cells were obtained from implants of small coverslips in the subcutaneous tissue or in the peritoneal cavity of rats and mice. Transmembrane potentials ranged from −5 to −40 mV. Input resistances ranged from 5 to 120 MΩ, being significantly higher in mice polykaryons. Approximately 10% of the cells exhibited spontaneous slow membrane hyperpolarizations (SH) indistinguishable from those observed in macrophages. SH responses were invariably evoked by iontophoretic injection of calcium ions into the cytoplasm of mice macrophages or macrophage polykaryons. The amplitude of these responses increased with the amount of current carried by calcium ions into the cells. The maximum amplitude of the calcium-induced SH responses is a linear function of the logarithm of [K+] 0 (from 3 to 40mm). The slope of the regression line is 43 mV for a 10-fold increase in [K+] 0 . Substituting sodium chloride by sodium isethionate or by choline chloride does not interfere with the occurrence of SH. The assumption that the SH is solely a consequence of an increase in the membrane conductance to K+ was used to calculate the potassium equilibrium potential (E K). TheE K value is also a linear function of the logarithm of [K+] 0 (from 3 to 40mm). The slope of the regression line is 46 mV for a 10-fold increase in [K+] 0 . These results constitute evidence of the calcium dependence of K+ permeability during SH both in macrophages and macrophage polykaryons. Macrophage polykaryons are a more convenient model than macrophages for the study of the mechanisms underlying the SH responses and their possible physiological implications.

Greenwell, J.; Low, H.

doi: 10.1007/BF02007635pmid: 6974244

Frog skin was mounted in an Ussing chamber and the actions of caerulein, gastrin, pentagastrin, and secretin on the active transport of sodium were studied using the short-circuit current method. All polypeptides exerted their effect when placed in the solution bathing the outside surface of the skin. The response was a transient dose-related increase in the transepithelial electrical potential difference and in the short-circuit current. Analysis of the response indicated that at submaximal doses the effect was due to an increase in the rate of entry of sodium through the outer barrier to active sodium transport. At supramaximal doses the passive permeability of the skin was also increased. Th ED50 concentrations of the hormones were: caerulein, 50pm; gastrin, 53pm; pentagastrin, 440pm; and secretin, 30pm. It is argued that the large quantity of caerulein or caerulein-like peptides stored in the skin may be required either to control the entry of sodium when the amphibian is undergoing maximum stress in a freshwater environment, or that it may have a protective function for the amphibian as it could elicit a noxious hypersecretion in the gastrointestinal tract of the predator together with a marked hypotension.

Neville, Margaret; Selker, Frank; Semple, Kathleen; Watters, Christopher

doi: 10.1007/BF02007636pmid: 6268790

Crude particulate preparations from the mammary glands of lactating mice were shown to transport calcium against a concentration gradient in the presence of ATP and mitochondrial inhibitors. Density gradient centrifugation with both sucrose and Percoll gradients indicated the presence of ATP-dependent transport in more than one membrane fraction. A Golgi-enriched membrane fraction possessed the highest specific activity of calcium transport. Digitonin, which increases the permeability of plasma membranes to calcium, did not affect this process. The Golgi fraction contained a 100,000 Dalton protein whose phosphorylation by γ-[32P]-ATP was enhanced by a micromolar concentrations of free calcium. The phosphorylation was acid-stable and hydroxylamine-sensitive. These properties suggest that Golgi membranes in an actively secreting mammary epithelium possess a calcium transport system which resembles the calcium ATPase present in the sarcoplasmic reticulum of skeletal muscle.

Brodie, D.; Podesta, R.

doi: 10.1007/BF02007637pmid: 7277467

Ultrastructural changes associated with osmotically-induced water transport and water permeability were examined in two flatworm species,Schistosoma mansoni andHymenolepis diminuta. The structure of the surface layer of these parasites is unusual in that it is a syncytial epithelial layer that lacks tight junctions and lateral extracellular spaces. The permeability coefficients observed in this study are therefore necessarily associated only with the transcellular route of transepithelial transport. The ultrastructural changes associated with volume transport across the epithelial syncytium were also unusual in that the basally located channels extending distally from the inward-facing membrane into the syncytial layer remained open regardless of the direction of water flow.

Lea, T.; Ashley, C.

doi: 10.1007/BF02007638pmid: 6456353

The paper describes an investigation into the increase in intracellular free Ca2+ and resting tension of barnacle muscle fibers when exposed to CO2. Isometric tension was recorded in isolated myofibrillar bundles prepared from barnacles and crabs. On replacement of a low relaxing bathing solution (free Ca2+∶20nm) at pH 7.1 with a similar one containing 100% CO2 and 130mm HCO 3 − , also at pH 7.1, the bundles developed a phasic contraction, which aequorin experiments confirmed was due to a release of Ca2+ from a store within the bundles. The source of this Ca2+ is tentatively identified as the sarcoplasmic reticulum (SR) for the following reasons: (1) prior exposure to 20mm caffeine depleted this Ca2+ store, (2) procaine (10mm) inhibited the response, and (3) the extracellular space or “clefts” and the mitochondria could be eliminated as possible sources. An effect of the CO2+HCO 3 t- on the free Ca2+/Mg2+ ratio in the bathing solution was excluded as a possible mechanism. The diuretic furosemide (1mm) enhanced the response to CO2+HCO 3 t- . Both furosemide and SITS (1–10mm), by themselves, also released Ca2+ in myofibrillar bundles. A scheme is put forward to explain these results: it is suggested that diffusion of dissolved CO2 into the SR produces an acidification of the SR lumen, which modifies either the Ca2+/-ATPase or the Ca2+-induced release process in such a way to release Ca2+.

Nagel, W.; Garcia-Diaz, J.; Armstrong, W.

doi: 10.1007/BF02007639pmid: 6974243

Intracellular Na+, K+, and Cl− activities (a Na i ,a K i ,a Cl i ) and transapical membrane potentials (V o) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23°C in normal amphibian Ringer's solution. Under control conditionsa Na i =14±3mm,a K i =132±10mm anda Cl i =18±3mm (sd). The value ofa Cl i is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl− is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2–5 μm) to the apical bathing medium,a K i ,a Na i , anda Cl i were essentially unchanged althoughV o had hyperpolarized by about 30–40 mV. During long-term exposure to amiloridea K i anda Cl i did not change significantly,V o depolarized by about 16 mV from the maximal value anda Na i decreased to 8±3mm. Immediately after exposure to amiloride the transmembrane driving force for Na+ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in bothV o anda Na i , this driving force remained virtually constant. SinceI sc during this period was close to zero, it is suggested that the observed driving force for Na+ under these conditions approximates the maximal driving force generated by the Na+−K+ ATP-ase pump in the basolateral cell membrane.

Levine, Sherman; Kachadorian, William

doi: 10.1007/BF02007640pmid: 6792357

Unstirred layers of water complicate the measurement of water permeability across epithelia. In the toad urinary bladder, the hormone vasopressin increases the osmotic water permeability of the granular epithelial cell's luminal membrane, and also leads to the appearance of aggregates of particles within this membrane. The aggregates appear to be markers for luminal membrane osmotic water permeability. This report analyzes the relationship between transbladder osmotic water flow and aggregate frequency, and demonstrates that flow across the bladder is significantly attenuated by unstirred layers of water or by structural barriers other than the luminal membrane when the luminal membrane is made permeable by vasopressin. This analysis in addition yields unique values for the permeabilities of both the luminal membrane and the barriers to water flow which lie in series with it.

Durbin, R.; Hill, Adrian

doi: 10.1007/BF02007641pmid: 7277468