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Biomedical subjects

R Latorre

Publications and source records attributed to R Latorre.

At least 127 records · Page 7Linked to original sources
PubMed1981

Voltage-dependent conductance induced by alamethicin-phospholipid conjugates in lipid bilayers.

Alamethicin, a linear 20-amino acid antibiotic, forms voltage-dependent channels in lipid bilayer membranes. We show here that alamethicin-phospholipid conjugates can be prepared by photolysis of unilamellar vesicles containing alamethicin and a phosphatidylcholine analogue with a carbene precursor at the end of the C-2 fatty acyl chain. This result indicates that at least a portion of the alamethicin molecule is in contact with the hydrocarbon moiety of the membrane in the absence of an applied voltage. Furthermore, the alamethicin-phospholipid photoproduct is able to induce a voltage-gated conductance similar to that of natural alamethicin. The importance of these results in terms of mechanisms for channel gating is discussed.

Alamethicin↗
PubMed1981

Ion transport mediated by the valinomycin analogue cyclo(L-Lac-L-Val-D-Pro-D-Val)3 in lipid bilayer membranes.

Cyclo(L-Lac-L-Val-D-Pro-D-Val)3 (PV-Lac) a structural analogue of the ion-carrier valinomycin, increases the cation permeability of lipid bilayer membranes by forming a 1:1 ion-carrier complex. The selectively sequence for PV-Lac is identical to that of valinomycin; i.e., Rb+ greater than K+ greater than Cs+ greater than or equal to NH+4 greater than Na+ greater than Li+. The steady-state zero-voltage conductance, G(0), is a saturating function of KCl concentration. A similar behavior was found for Rb+, Cs+, and NH+4. However, the ion concentration at which G(0) reaches a plateau strongly depends on membrane composition. The current-voltage curves present saturating characteristics, except at low ion concentrations of Rb+, K+, or Cs+. The ion concentration at which the saturating characteristics appear depends on membrane composition. These and other results presented in this paper agree with a model that assumes complexation between carrier and ion at the membrane-water interface. Current relaxation after voltage-jump studies were also performed for PV-Lac. Both the time constant and the amplitude of the current after a voltage jump strongly depend on ion concentration and membrane composition. These results, together with the stationary conductance data, were used to evaluate the rate constants of the PV-Lac-mediated K+ transport. In glycerolmonooleate they are: association rate constant, 2 x 10(6) M-1 s-1; dissociation rate constant, 4 x 10(5) s-1; translocation rate constant for complex, 5 x 10(4) s-1; and the rate of translocation of the free carrier (ks), 55 s-1. ks is much smaller for PV-Lac than for valinomycin and thus limits the efficiency with which the carrier is able to translocate cations across the membrane.

Biological Transport↗
PubMed1981

A three-barrier model for the hemocyanin channel.

Keyhole limpet hemocyanin forms ion-conducting channels in planar lipid bilayer membranes. Ionic current through the open hemocyanin channel presents the following characteristics: (a) it is carried mainly by cations; (b) it is a nonlinear function of membrane potential; (c) channel conductance is a saturating function of ion activity; (d) it shows ionic competition. A model for the open hemocyanin channel is developed from absolute reaction rate theory. The model calls for three energy barriers in the channel. Two energy barriers represent the entrance and exit of the ion into and out of the channel. The third barrier separates two energy minima that represent two binding sites. Furthermore, only one ion is allowed inside the channel at a given time. This model is able to recreate all the hemocyanin characteristics found experimentally in negatively charged and neutral membranes.

Hemocyanins↗
PubMed1981

Surface potentials and sodium entry in frog skin epithelium.

1. The effects which alterations in the surface potential of the apical membrane of isolated Rana catesbeiana skin have on Na entry were examined. 2. Changes in the external ionic strength have little effect upon the rate of Na transport across the frog skin epithelium. 3. Uranyl ion (UO2(2+), 2.5 mM) induces a +145 mV change in the surface potential of phosphatidylserine monolayers, and a +60mV change in the surface potential of monolayers made from phosphatidylcholine. 4. UO2(2+) inhibits the short-circuit current (Isc) by a maximum of 20% in R. catesbeiana skin, while stimulating Isc by 40% in R. temporaria skin. Neither Isc stimulation nor inhibition by UO2(2+) can be seen in the presence of 10(-4) M-amiloride. 5. From points 1 and 2 above, we conclude that the surface charge density in the neighbourhood of the Na-selective entry site located in the apical membrane is small (greater than 1e-/600 A2). The results obtained using UO2(2+) suggest that Na entry is not affected by changes in the membrane surface potential.

Animals↗
PubMed1980

Modulation of alamethicin-induced conductance by membrane composition.

The effect of cholesterol on the opening and closing of the alamethicin-channel was studied varying the cholesterol content of glycerolmonooleate membranes. Increasing the cholesterol mole fraction in the membrane shifted the log of the steady-state conductance-voltage curve to the right along the voltage axis. The shift was found to be 80-100 mV when the cholesterol mole fraction was increased from 0 to 0.5. The kinetic for the approach to steady-state conductance during voltage clamp was also studied in membranes under these conditions. At all the different cholesterol contents the on kinetic is well described by a single exponential. The off kinetic, on the other hand, is described by a double exponential time course. At the same voltage the time constant describing the on current relaxation is the same as the one found for the slow relaxation present in the off relaxation. Increasing the membrane cholesterol content increases the magnitude of the time constant describing the fast and slow process by several orders of magnitude. Cholesterol also increases the voltage dependence of the slow time constant. The effect of cholesterol on the steady-state conductance can be explained, but not in a unique way, by an increase in the membrane dipole potential. Phenomenological comparison of the fast and slow kinetic processes seen in multi-channel membranes with single channel characteristics indicates that: (i) cholesterol increases the mean life of the single channel and (ii) cholesterol increases the mean life of the different conductance levels.

Alamethicin↗
PubMed1979

Effect of the anesthetics benzyl alcohol and chloroform on bilayers made from monolayers.

The neutral anesthetics chloroform and benzyl alcohol, at concentrations that block the nerve impulse, greatly modify the transport parameters of positive and negative ions in lipid bilayers made from monolayers. Both chloroform and benzyl alcohol increase the membrane permeability to these ions and increase the translocation rate for tetraphenylborate. It was found that both anesthetics increase the membrane permeability to positive ions more markedly than to negative ions. It was also found that the membrane capacitance increases lineary with the concentration of benzyl alcohol. At 51 mM benzyl alcohol, the increase in capacitance is approximately 6%. Chloroform also increases the membrane capacitance; the increase in capacitance was found to be 6% at 18 mM chloroform. An analysis of the changes in the transport parameters of the lipophilic ions, together with the changes in membrane capacitance, suggests that benzyl alcohol and chloroform modify the dipole potential and dielectric constant of the membrane. Benzyl alcohol may also increase the "fluidity" of the lipid bilayer membranes. At 36 mM benzyl alcohol, the membrane permeability to acetamide increases by 38%.

Benzyl Alcohol↗
PubMed1979

Pancuronium inactivates alamethicin-induced conductance in artificial membranes.

The effect of pancuronium on alamethicin-induced currents was studied in negatively charged lipid bilayer membranes. Pancuronium induces inactivation of the alamethicin-induced current. Inactivation is only observed if this compound is added to the compartment containing alamethicin. Moreover, the process of inactivation is reduced or abolished if pancuronium is added to the alamethicin-free side of the membrane. The time needed to recover from inactivation is greatly reduced if the aqueous solution in the alamethicin-free compartment is stirred. These data suggest that pancuronium permeates through the membrane when the alamethicin-induced conductance is "turned on," binds to the other membrane surface, and changes the surface potential.

Alamethicin↗
PubMed1979

Inactivation of the alamethicin-induced conductance caused by quaternary ammonium ions and local anesthetics.

Long alkyl chain quaternary ammonium ions (QA), the local anesthetics (LA) tetracaine and lidocaine, imipramine, and pancuronium cause inactivation of the alamethicin-induced conductance in lipid bilayer membranes. The alamethicin-induced conductance undergoes inactivation only when these amphipathic compounds are added to the side containing alamethicin. The concentration of QA required to cause a given amount of inactivation depends on the length of the hydrocarbon chain and follows the sequence C9 greater than C10 greater than C12 greater than C16. LA and imipramine, in contrast to QA or pancuronium, are able to promote appreciable inactivation only if the pH of the alamethicin-free side is equal to or lower than the pK of these compounds. The membrane permeability to QA, LA, or imipramine is directly proportional to the alamethicin-induced conductance and is larger than the one for potassium. The observed steady state and time-course of the inactivation are well described by a model similar to that proposed by Heyer et al. (1976. J. Gen. Physiol. 67:703--729) and extended for any value of the diffuse double layer potential and for LA and imipramine. In this model QA, LA, or imipramine are able to permeate through the membrane only when the alamethicin-induced conductance is turned on. The amphipathic compounds then bind to the other membrane surface, changing the transmembrane potential and turning the conductance off. For a given concentration of QA, LA, or imipramine the extent of inactivation depends on two factors: first, the binding characteristics of these compounds to the membrane surface and second, their ability to permeate through the membrane when the alamethicin-induced conductance is turned on. The several possible mechanisms of permeation of the amphipathic molecules tested are discussed.

Alamethicin↗
PubMed1978

Voltage-dependent capacitance in lipid bilayers made from monolayers.

Electrocompression has been measured in lipid bilayers made by apposition of two monolayers. The capacitance C(V), as a function of membrane potential, V, was found to be well described by C(V) = C(O) [1 + alpha(V + delta psi)2] where C(O) is the capacitance at V = O, alpha is the fractional increase in capacitance per square volt, and delta psi is the surface potential difference. In lipid bilayers made from monolayers alpha has a value of 0.02 V-2, which is ca. 500-fold smaller than the value found in solvent containing membranes. In asymmetric bilayers made of one neutral and one negatively charged monolayer, delta psi values were found to be those expected from independent measurements of surface charge density. If the fractional increase in capacitance found here is a good approximation to that of biological membranes, nonlinear capacitative charge displacement derived from electrostriction is expected to be less than 1% of the total gating charge displacement found in squid axons.

Electrochemistry↗
PubMed1977

Phloretin-induced changes in ion transport across lipid bilayer membranes.

Phloretin, the aglucone derivative of phlorizin, increases cation conductance and decreases anion conductance in lipid bilayer membranes. In this paper we present evidence that phloretin acts almost exclusively by altering the permeability of the membrane interior and not by modifying the partition of the permanent species between the membrane and the bulk aqueous phases. We base our conclusion on an analysis of the current responses to a senylborate, and the cation complex, peptide PV-K+. These results are consistent with the hypothesis that phloretin decreases the intrinsic positive internal membrane potential but does not modify to a great extent the potential energy minima at the membrane interfaces. Phloretin increases the conductance for the nonactin-K+ complex, but above 10(-5) M the steady-state nonactin-K+ voltage-current curve changes from superlinear to sublinear. These results imply that, above 10(-5) M phloretin, the nonactin-5+ transport across the membrane becomes interfacially limited.

Anti-Bacterial Agents↗
PubMed1975

The nature of the voltage-dependent conductance of the hemocyanin channel.

The electrical responses of individual hemocyanain channels in oxidized cholesterol membranes demonstrate that the voltage-dependent conductance of many-chanel membranes arises from two different mechanisms. These are the voltage-dependent redistribution of channels among several discrete single-channel conductance states themselves. The relaxation time for the discrete conductance changes is of the order of seconds nd the relaxation time of the continuous conductance changes is of the order 10(-4) seconds. As salt concentration in the bathing medium is increased, the single-channel conductance first increases lineary and then saturates. The characteristics of the saturation curves suggest that the continuous conductance changes occur at the edges of the channel and that the mean time an ion spends in the channel is 4 nanoseconds...

Cholesterol↗
PubMed1975

Voltage-dependent conductance induced by hemocyanin in black lipid films.

When hemocyanin is added to a black lipid film, the conductance increases in discrete steps. For negative potentials the single step conductance is constant, but for positive potentials the step conductance appears to decrease as the potential increases. At high positive potentials the conductance fluctuates between several levels. These data suggest that, in lipid membranes, hemocyanin conducts ions through discrete channels. The voltage-dependent conductance observed at high levels of conductance seems to be a consequence of the properties of the conductance of the single channel.

Binding Sites↗