Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Thin lipid (optically black) membranes were made from sheep red cell lipids dissolved in n-decane. The flux of Br across these membranes was measured by the use of tracer (82)Br. The unidirectional flux of Br (in 50-100 mM NaBr) was 1-3 x 10(-12) mole/cm(2)sec. This flux is more than 1000 times the flux predicted from the membrane electrical resistance (>10(8) ohm-cm(2)) and the transference number for Br(-) (0.2-0.3), which was estimated from measurements of the zero current potential difference. The Br flux was not affected by changes in the potential difference imposed across the membrane (+/-60 mv) or by the ionic strength of the bathing solutions. However, the addition of a reducing agent, sodium thiosulfate (10(-3)M), to the NaBr solution bathing the membrane caused a 90% reduction in the Br flux. The inhibiting effect of S(2)O(3) (=) suggests that the Br flux is due chiefly to traces of Br(2) in NaBr solutions. As expected, the addition of Br(2) to the NaBr solutions greatly stimulated the Br flux. However, at constant Br(2) concentration, the Br flux was also stimulated by increasing the Br(-) concentration, in spite of the fact that the membrane was virtually impermeable to Br(-). Finally, the Br flux appeared to saturate at high Br(2) concentrations, and the saturation value was roughly proportional to the Br(-) concentration. These results can be explained by a model which assumes that Br crosses the membrane only as Br(2) but that rapid equilibration of Br between Br(2) and Br(-) occurs in the unstirred layers of aqueous solution bathing the two sides of the membrane. A consequence of the model is that Br(-) "facilitates" the diffusion of Br across the unstirred layers.
J Gen Physiol 1972 Apr
PMID:The permeability of thin lipid membranes to bromide and bromine. 506 46

The Ca(++) transport mechanism in the red cell membrane was studied in resealed ghost cells. It was found that the red cell membrane can transport Ca(++) from inside the cell into the medium against great concentration gradient ratios. Tracing the movement of (45)Ca infused inside red cells indicated that over 95% of all Ca(++) in the cells was transported into media in 20 min incubation under the optimum experimental conditions. The influence of temperature on the rate constant of transport indicated an activation energy of 13,500 cal per mole. The optimum pH range of media for the transport was between 7.5 and 8.5. As energy sources, ATP(1), CTP, and UTP were about equally effective, GTP somewhat less effective, and ITP least effective among the nucleotides tested. The Ca(++) transport does not appear to involve exchange of Ca(++) with any monovalent or divalent cations. Also, it is not influenced by oligomycin, sodium azide, or ouabain in high concentrations, which inhibit the Ca(++) transport in mitochondria or in sarcoplasmic reticulum. In these respects, the Ca(++) transport mechanism in the red cell membrane is different from those of mitochondria and the sarcoplasmic reticulum.
J Gen Physiol 1969 Dec
PMID:Studies on the active transport of calcium in human red cells. 535 89

The apparent activation energy for the water diffusion permeability coefficient, P(d), across the red cell membrane has been found to be 4.9 +/- 0.3 kcal/mole in the dog and 6.0 +/- 0.2 kcal/mole in the human being over the temperature range, 7 degrees to 37 degrees C. The apparent activation energy for the hydraulic conductivity, L(p), in dog red cells has been found to be 3.7 +/- 0.4 kcal/mole and in human red cells, 3.3 +/- 0.4 kcal/mole over the same temperature range. The product of L(p) and the bulk viscosity of water, eta, was independent of temperature for both dog and man which indicates that the geometry of the red cell membrane is not temperature-sensitive over our experimental temperature range in either species. In the case of the dog, the apparent activation energy for diffusion is the same as that for self-diffusion of water, 4.6-4.8 kcal/mole, which indicates that the process of water diffusion across the dog red cell membrane is the same as that in free solution. The slightly, but significantly, higher activation energy for water diffusion in human red cells is consonant with water-membrane interaction in the narrower equivalent pores characteristic of these cells. The observation that the apparent activation energy for hydraulic conductivity is less than that for water diffusion across the red cell membrane is characteristic of viscous flow and suggests that the flow of water across the membranes of these red cells under an osmotic pressure gradient is a viscous process.
J Gen Physiol 1970 Apr
PMID:The state of water in human and dog red cell membranes. 543 80

Studies have been made on the temperature dependence of both the hydraulic conductivity, L(p), and the THO diffusion coefficient, omega, for a series of cellulose acetate membranes (CA) of varying porosity. A similar study was also made of a much less polar cellulose triacetate membrane (CTA). The apparent activation energies, E(a), for diffusion across CA membranes vary with porosity, being 7.8 kcal/mole for the nonporous membrane and 5.5 kcal/mole for the most porous one. E(a) for diffusion across the less polar CTA membrane is smaller than E(a) for the CA membrane of equivalent porosity. Classical viscous flow, in which the hydraulic conductivity is inversely related to bulk water viscosity, has been demonstrated across membranes with very small equivalent pores. Water-membrane interactions, which depend upon both chemical and geometrical factors are of particular importance in diffusion. The implication of these findings for the interpretation of water permeability experiments across biological membranes is discussed.
J Gen Physiol 1971 May
PMID:Effect of geometrical and chemical constraints on water flux across artificial membranes. 555 4

In an isometric tetanus in frog's sartorius muscle tension approaches the plateau exponentially with rate constant alpha. alpha a depends on sarcomere length, s, and temperature, T, according to the Arrhenius equation See PDF for Equation for temperatures between 1 and 20 degrees C and for sarcomere lengths 2.0-2.8 microm. The energy of activation, E, does not vary significantly with s; E = 13.9 +/- 2.4 kcal/mole. A(s) decreases monotonically with s; A(2.1 microm) is about three times greater than A(2.8 microm). Late in relaxation active tension approaches zero exponentially with rate constant r. r decreases exponentially with increasing duration of tetanus, D, from r(0) in a twitch to r(infinity) for large D. The rate constant for decrease of r with D increases with s and with T. r(0) and r(infinity) obey the Arrhenius equation and decrease with increasing s.
J Gen Physiol 1971 Jul
PMID:Transient phases of the isometric tetanus in Frog's striated muscle. 556 60

The transport of Na in the cat red cells has been studied under various experimental conditions. The unidirectional radioactive Na influx increased with increasing temperature until it reached a maximum value at 37 degrees C +/- 2 degrees C and then decreased with a further increase in temperature. Errors stated in this paper represent 1.0 standard errors of the mean. The apparent activation energy was calculated in the region between 25 and 37 degrees C and was found to be 4.9 +/- 0.5 kcal/mole. Copper at a concentration of 0.04 mM inhibited this influx by 65%. When cells were suspended in isosmotic KCl buffer, cell volume was found to decrease initially with time. This unusual behavior is discussed in terms of Na to K preference of the cell membrane. In cat red cells, Na influx was found to increase about 13-fold when cell volume was decreased from 1.16 normal to 0.87. This effect could not be reproduced when the medium osmolarity was changed only by the addition of urea, a permeating molecule. On the other hand, K influx was found to decrease from 0.24 +/- 0.03 mEq/liters RBC, hr at a relative cellular volume equal to 1.0 to 0.11 +/- 0.01 mEq/liters RBC, hr at a cell volume of 0.75. Na influx in human red cells did not show any significant dependence on cell volume. The properties of Na movement in the cat red cells are compared to those of human red cells.
J Gen Physiol 1971 Jun
PMID:Sodium movement in high sodium feline red cells. 557 66

Calcium appears to be an essential participant in axon excitation processes. Many other polyvalent metal ions have calcium-like actions on axons. We have used the voltage-clamped lobster giant axon to test the effect of several of these cations on the position of the peak initial (sodium) and steady-state (potassium) conductance vs. voltage curves on the voltage axis as well as on the rate parameters for excitation processes. Among the alkaline earth metals, Mg(+2) is a very poor substitute for Ca(+2), while Ba(+2) behaves like "high calcium" when substituted for Ca(+2) on a mole-for-mole basis. The transition metal ions, Ni(+2), Co(+2), and Cd(+2) also act like high calcium when substituted mole-for-mole. Among the trivalent ions, La(+3) is a very effective Ca(+2) replacement. Al(+3) and Fe(+3) are extremely active and seem to have some similar effects. Al(+3) is effective at concentrations as low as 10(-5)M. The data suggest that many of these ions may interact with the same cation-binding sites on the axon membrane, and that the relative effects on the membrane conductance and rate parameters depend on the relative binding constants of the ions. The total amount of Na(+) transferred during a large depolarizing transient is nearly independent of the kind or amount of polyvalent ion applied.
J Gen Physiol 1968 Mar
PMID:The action of certain polyvalent cations on the voltage-clamped lobster axon. 564 28

A comparison is made between the quantitative predictions of equilibrium ionic distribution in living cells according to the membrane theory (Donnan equilibrium) and according to the association-induction hypothesis. This comparison shows that both theories predict competitive effects of one permeant ion on the equilibrium concentration of another permeant ion; but within the limit of experimental accuracy only the association-induction model predicts quantitatively significant specific competition of one specified ion with the accumulation of another specified ion. The equilibrium distributions of K(+), Rb(+), and Cs(+) ions in frog sartorius muscle were studied and quantitatively significant specific competition was demonstrated; these results favor the association-induction hypothesis (adsorption on cell proteins and protein complexes and partial exclusion from cell water). Based on this model we estimated that at 257deg;C, the apparent association constants for K(+), Rb(+), and Cs(+) ion are 665, 756, and 488 (mole/liter)(-1). We found that the total concentration of adsorption sites (no less than 240 mmole/kg of fresh cells) agrees with the analytically determined concentrations of beta- and gamma-carboxyl groups of muscle cell proteins (260 to 288 mmole/kg).
J Gen Physiol 1966 Mar
PMID:Studies on ion accumulation in muscle cells. 594 17

Calcium accumulation by rod disks was studied in excised bullfrog retinas with 45Ca tracer-exchange methods. Ca uptake by disks is a necessary requirement if light-induced Ca releases from disks mediate photoreceptor excitation. In an hour-long incubation, disks exchanged less than or equal to 0.01 mole of Ca per mole of rhodopsin, or less than or equal to 10% of their total Ca. This corresponds to a unidirectional flux of less than or equal to 0.01 fmol/cm2 S, or less than or equal to 5 ions/disk-second across the disk membrane. Neither incubation in 10 mM Ca (which increases cytoplasmic activity 10--100-fold) nor photostimulation (which photoactivated up to 50% rhodopsin/h) had measurable effect on exchange rate, though an increase of several orders of magnitude would have been expected according to the hypothesis. The observed exchange could not be explained by: (a) 45Ca losses from disks before measurement (neither the net efflux nor the Ca-Ca exchange property of disks adequately explains such losses), (b) a limited pool of exchangeables Ca from strongly binding intradiskal sites, or (c) rate-limiting flux across the plasma membrane during incubation. For the study of the Ca efflux properties of disks, separate experiments were performed with 45Ca-loaded disks. Intradiskal activity could be estimated from the disks' hyperosmotically sensitive 45Ca pool and from their intradiskal volume (indirectly assayed by density). Ca-Ca exchange was undetectable (less than or equal to 0.1 fmol/cm2 S) in disks whose intradiskal activity was at least 0.3 mM. Net efflux was 0.2 fmol/cm2 S for an intradiskal activity of approximately 1 mM and is comparable to published fluxes for phospholipid vesicles. These results seem to exclude the internal space of disks as the source of Ca for photoreceptor excitation.
J Gen Physiol 1980 Sep
PMID:Calcium flux across disk membranes. Studies with intact rod photoreceptors and purified disks. 625 76

In the presence of 10(-9) M calcium, rod outer segments freshly detached from dark-adapted frog retinas contain between 0.01 and 0.02 moles of guanosine 3',5'-cyclic monophosphate (cyclic GMP) per mole of rhodopsin. The dark level of cyclic GMP is reduced approximately 50% by illumination that bleaches 5 x 10(5) rhodopsin molecules/outer segments. The dark levels of cyclic GMP also can be suppressed to approximately 0.007 mol/mol of rhodopsin by increasing the concentration of calcium from 10(-9) M to 2 x 10(-9) M, and they remain at this level as calcium concentration is raised to 10(-3) M. The final level to which illumination reduces cyclic GMP in unaffected by the calcium concentration between 10(-9) and 10(-3) M. The maximal light-induced decrease in cyclic GMP occurs within 1 s from the onset of illumination at all calcium concentrations. The magnitude and time-course of the light-induced decrease in cyclic GMP measured in these experiments are comparable to values obtained previously (Woodruff et al. 1977. J. Gen. Physiol. 69:677-679; Woodruff and Bownds. 1979. J. Gen. Physiol. 73:629-653). The data are consistent with a role for cyclic GMP in visual transduction irrespective of the calcium concentration.
J Gen Physiol 1981 Jan
PMID:Influence of calcium on guanosine 3',5'-cyclic monophosphate levels in frog rod outer segments. 625 73


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>