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V L Lew

Publications and source records attributed to V L Lew.

104 records · Page 6Linked to original sources
PubMed1970

Short-circuit current and ionic fluxes in the isolated colonic mucosa of Bufo arenarum.

1. The unidirectional fluxes of (22)Na, (36)Cl and [(14)C]bicarbonate ions were measured in paired portions of the isolated and short-circuited colonic mucosa of Bufo arenarum, separated from its muscular layer. Pharmacological effects as well as effects of changes in the composition of the nutrient solutions on the electrical parameters of membrane activity (potential difference, short-circuit current and total membrane resistance) are described.2. The net fluxes of both Cl and bicarbonate ions were not significantly different from zero in the absence of electrochemical gradients across the membrane.3. The net Na flux from mucosa to serosa represented a variable proportion of the short-circuit current ranging from 62 to 100%.4. The proportion of membranes with high discrepancies between net Na flux and short-circuit current decreased with the duration of captivity of the toads.5. When Na was entirely replaced by choline in the mucosal bathing solution, the short-circuit current dropped by a variable amount within the range of 64 to 98% of its control values in different membranes. This effect was completely reversible. Similar changes in the serosal solution had no effect.6. The short-circuit current and potential difference were very sensitive to the serosal concentration of bicarbonate ions. In different membranes, 60-100% of the short-circuit current was reversibly abolished by bathing the serosal surface with a bicarbonate-free solution. The mucosal bicarbonate level had no effect on either the potential difference or the short-circuit current. 5 mM bicarbonate in the serosal solution restored at least 50% of the short-circuit control value and full recovery was attained by concentrations near 30 mM bicarbonate.7. Anoxia brought the potential difference and short-circuit current reversibly down to zero in about 50 min.8. Ouabain reduced the short-circuit current up to 80% in about 40 min when present in the serosal solution at a concentration of 10(-4)M. At this or lower concentrations the ouabain effect was reversible. Above this level ouabain produced 100% inhibition in 3-4 hr, but this was no longer reversible. Ouabain had no effect on the short-circuit current either when applied to the mucosal surface or in the absence of Na from the mucosal solution.9. Diamox produced a variable inhibition of the short-circuit current of up to 30% only at concentrations above 10 mM.10. Possible mechanisms are discussed for the appearance of the non-Na component of the short-circuit current. A theory concerning its nature is proposed.

Acetazolamide↗
PubMed1970

Bicarbonate secretion and non-Na component of the short-circuit current in the isolated colonic mucosa of Bufo arenarum.

1. In the isolated colonic mucosa of Bufo arenarum, under special circumstances, there is a variable fraction of the short-circuit current (0-38%) that is unaccounted for by either the Na or the Cl and bicarbonate transmembrane net fluxes.2. The hypothesis that a special kind of bicarbonate transport may account for the non-Na component of the short-circuit current was investigated. According to this, bicarbonate ions formed within the membrane await transport towards the mucosal solution within a compartment that does not undergo isotopic exchange with the serosal bathing solution. This kind of transport may be detected by a lowering of mucosal specific activity of bicarbonate but would not be revealed by the classic method of comparing the difference between the unidirectional fluxes with the short-circuit current.3. The specific activity of bicarbonate was determined in the inside solution (initially bicarbonate-free) of ten normal and four everted colonic sacs incubated in an external medium (reservoir) containing a constant specific activity of bicarbonate. Comparison between membrane-to-internal solution bicarbonate flux and non-Na component of the short-circuit current was carried out in two different ways: (a) by measuring the remaining short-circuit current in Na-free medium and (b) by determining simultaneously the Na net flux.4. Whatever the value of the short-circuit current and its non-Na component, there is no reduction of the specific activity of the bicarbonate appearing in the inside solution of the everted colonic sacs.5. In the normal sacs there is a reduction of the specific activity of bicarbonate which accounts for a membrane-to-mucosa bicarbonate flux which parallels the variations of the non-Na component of the short-circuit current although quantitatively representing only 68-87% of it.6. There is no systematic decrease in the rate of reduction of the mucosal specific activity of bicarbonate in successive experimental flux periods; this excludes a slow equilibration of the intracellular bicarbonate with serosal bicarbonate.7. Other possible explanations of the present results are discussed, as well as the availability and hydration rate of metabolic CO(2) necessary to account for this kind of bicarbonate transport.

Animals↗
PubMed1970

Reversal of the potassium entry mechanism in red cells, with and without reversal of the entire pump cycle.

1. (42)K has been used to study the ouabain-sensitive component of potassium efflux from intact human red cells.2. Ouabain-sensitive efflux of potassium was observed only in media containing either sodium ions in moderate or high concentration or potassium ions.3. The effects of sodium and potassium in the medium were not additive. Potassium ions always increased the ouabain-sensitive potassium efflux, but in media containing 4.2 mM-K an increase in sodium concentration from 4.5 to 131 mM had little effect.4. In potassium-free media, ouabain-sensitive potassium efflux increased roughly linearly as the external sodium concentration was increased from 35 to 155 mM.5. The sensitivity of potassium efflux to external potassium depended on the concentration of sodium in the medium. In a 5 mM-Na (choline) medium, ouabain-sensitive potassium efflux was half-maximal at about 0.27 mM-K. In a 150 mM-Na medium the effect of potassium was half-maximal at about 1 mM-K. The relation between external potassium concentration and ouabain-sensitive sodium efflux was similarly influenced by the concentration of sodium ions in the medium.6. Inosine greatly reduced the ouabain-sensitive efflux of potassium into both 5 mM-K and potassium-free media. It probably acted by reducing the intracellular phosphate concentration to a low level.7. Ouabain-sensitive potassium efflux was not affected by the concentration of inorganic phosphate outside the cells and was not associated with a ouabain-sensitive efflux of phosphate of comparable magnitude. A small associated efflux could not be excluded.8. Simultaneous measurements of sodium efflux and of potassium efflux from identical batches of cells incubated in potassium-free media showed that inosine reduced ouabain-sensitive potassium efflux at the same time as it increased ouabain-sensitive sodium efflux.9. When cells that had been largely depleted of energy stores by pre-incubation with 2-deoxyglucose were incubated in high-sodium media, with and without potassium, it was observed that potassium in the medium increased the ouabain-sensitive potassium efflux but reduced the ouabain-sensitive efflux of sodium.10. Simultaneous measurements of the ouabain-sensitive efflux of potassium and influx and efflux of sodium across the membranes of starved cells incubated in potassium-free media, with and without inosine, suggested that ouabain-sensitive potassium efflux was associated with a net ouabain-sensitive entry of sodium.11. The results are best explained by supposing that the ouabain-sensitive efflux of potassium does not reflect lack of discrimination by the mechanism responsible for sodium expulsion, but is brought about by the reversal of steps in the pump cycle normally responsible for potassium entry.

Biological Transport, Active↗
PubMed1970

Synthesis of adenosine triphosphate at the expense of downhill cation movements in intact human red cells.

1. When red cells that have been starved for about 6 hr are loaded with inorganic phosphate and incubated in high-sodium potassium-free media, the ouabain-sensitive efflux of potassium from the cells is accompanied by a ouabain-sensitive incorporation of inorganic phosphate into ATP.2. The magnitude of the incorporation varies roughly linearly with the concentration of sodium in the medium. The ratio (ouabain-sensitive potassium efflux)/(ouabain-sensitive ATP synthesis) is probably not much less than 2 nor much greater than 3.3. Potassium in the medium inhibits the ouabain-sensitive incorporation of phosphate. The concentration of potassium necessary for half-maximal inhibition is about the same as the concentration at which, under similar conditions, ouabain-sensitive potassium influx and the stimulation of ouabain-sensitive potassium efflux are both half-maximal.4. These observations suggest that the ouabain-sensitive efflux of potassium from red cells incubated in high-sodium potassium-free media is associated with a reversal of the entire pump cycle. In media containing sufficient potassium to saturate the pump, the efflux appears to involve the reversal of only part of the cycle.

Adenosine Triphosphate↗
PubMedDate not supplied

Compartmentalization of sickle-cell calcium in endocytic inside-out vesicles.

Much recent interest in the mechanism of dehydration of the dense subpopulation of sickle-cell anaemia (SS) red cells, including the 'irreversibly sickled cells' (ISCs), stems from the view that these relatively rigid cells have a major role in the two main clinical features of the disease, namely haemolytic anaemia and microvascular occlusion. The discovery that SS red cells have an elevated calcium content and accumulate Ca2+ during deoxygenation-induced sickling suggested a working hypothesis of wide appeal for the mechanism of cell dehydration: retained calcium would activate the red cell Ca2+-sensitive K+ channels, causing progressive net loss of KCl and water. However, retained calcium, which seemed as weakly bound to cytoplasmic buffers as in normal red cells, failed to show any measurable activation of K+ channels or Ca2+ pumps in metabolically normal SS cells, despite the apparent functional normality or near-normality of these transport systems. We now offer a possible explanation for this failure. We show that, contrary to the traditional views, SS cells, and to a lesser extent normal human red cells, possess intracellular vesicles with ATP-dependent Ca2+-accumulating capacity, and that nearly all the measurable calcium of fresh SS cells is contained within such vesicles, probably in the form of precipitates with inorganic or organic phosphates.

Adenosine Triphosphate↗