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Biomedical subjects

M Claret

Publications and source records attributed to M Claret.

At least 109 records · Page 6Linked to original sources
PubMed1973

Ionic transport and membrane potential of rat liver cells in normal and low-chloride solutions.

1. The ouabain-sensitive component of Na efflux and K influx amount to 58 and 72% respectively. Taking into account also (a) the diffusional passive fluxes of Na (5%) and K (13%), as estimated by Ussing's equation and (b) the ouabain-insensitive Na-Na exchange (28%), 85% (K) and 90% (Na) of the measured total fluxes can be accounted for.2. Na efflux is diminished when K is partially or totally removed from the medium. This effect is reversible, indicating probably activation of the Na pump by external K.3. The coupling ratio of Na and K ouabain-sensitive fluxes is equal to 1.58, suggesting that three Na ions are removed from and two K ions are carried into the cell in one cycle of the pump. Hence, in liver cell membranes, the Na pump must be electrogenic.4. A tenfold decrease in [Cl](o) by substitution with an impermeant anion results in a membrane hyperpolarization and a decrease in [Cl](i). Cl loss from the liver is compensated by an equivalent loss of intracellular K to preserve electroneutrality.5. The measurement of passive fluxes indicates that Cl removal from the perfusing solutions increases P(K) but does not alter P(Na).6. Addition of ouabain brings about a depolarization which is three times greater in low-Cl solutions (21.9 mV) than in normal-Cl solutions (6.8 mV).7. It is concluded that hyperpolarization which develops when Cl ions are removed can be accounted for entirely by (a) the increase in P(K), (b) the increase of the contribution of the electrogenic pump to membrane potential.

Animals↗
PubMed1972

Ionic fluxes and permeabilities of cell membranes in rat liver.

1. Intracellular ion concentrations, measured in rat liver perfused with saline solutions were, at steady state:[K](1) = 113; [Na](1) = 16.4; [Cl](1) = 25.5 m-mole l.(-1) of cells.2. Intracellular Cl concentration was measured when both [Cl](o) and membrane potential were changed. The experimental values were close to the predicted ones by the Nernst equation, indicating a passive distribution of this ion across the cell membrane.3. Fluxes were determined by means of radioactive tracers and had the following values:m(K) = 6.6; m(Na) = 12.4 and m(Cl) = 8 x 10(-12) mole cm(-2) sec(-1).4. When Na was replaced by Li in the perfusing solutions, the Na efflux was decreased by 3.3 x 10(-12) mole cm(-2) sec(-1). This was attributed to a Na-for-Na exchange (exchange-diffusion).5. A mathematical model was applied to the perfused liver. It allowed estimation of the actual fluxes across the membrane. Corrections resulting from the application of the model remain small.6. The permeability coefficients were calculated from the passive fluxes and were:P(K) = 7.6; P(Na) = 4.0; P(Cl) = 12.3 x 10(-8) cm sec(-1), corresponding to relative permeabilities of P(Na)/P(K) = 0.52 and P(Cl)/P(K) = 1.6.7. The membrane potential calculated from the Goldman equation was significantly different from the measured one. This may be accounted for by an electrogenic activity of the Na-K pump. Applying the Mullins & Noda equation, the ratio of active Na flux to active K flux becomes 3/2.

Animals↗
PubMedDate not supplied

[Calcium and liver].

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

Animals↗