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Biomedical subjects

R Villegas

Publications and source records attributed to R Villegas.

At least 55 records · Page 3Linked to original sources
PubMed1972

Ionic channels and nerve membrane constituents. Tetrodotoxin-like interaction of saxitoxin with cholesterol monolayers.

Saxitoxin (STX) and tetrodotoxin (TTX) have the same striking property of blocking the Na(+) channels in the axolemma. Experiments with nerve plasma membrane components of the squid Dosidicus gigas have shown that TTX interacts with cholesterol monolayers. Similar experiments were carried out with STX. The effect of STX on the surface pressure-area diagrams of lipid monolayers and on the fluorescence emission spectra of sonicated nerve membranes was studied. The results indicate a TTX-like interaction of STX with cholesterol monolayers. The expansion of the monolayers caused by 10(-6)M STX was 2.2 A(2)/cholesterol molecule at 25 degrees C. From surface pressure measurements at constant cholesterol area (39 A(2)/molecule) in media with various STX concentrations, it was calculated that the STX/cholesterol surface concentration ratio is 0.54. The apparent dissociation constant of the STX-cholesterol monolayer complex is 4.0 x 10(-7)M. The STX/cholesterol ratio and the apparent dissociation constant are similar to those determined for TTX. The presence of other lipids in the monolayers affects the STX-cholesterol association. The interactions of STX and TTX with cholesterol monolayers suggest (a) that cholesterol molecules may be part of the nerve membrane Na(+) channels, or (b) that the toxin receptor at the nerve membrane shares similar chemical features with the cholesterol monolayers.

Acetates↗
PubMed1971

Nonelectrolyte permeability, sodium influx, electrical potentials, and axolemma ultrastructure in squid axons of various diameters.

The penetration of (14)C-labeled ethylene glycol, erythritol, mannitol, and sucrose was measured in giant axons of various diameters isolated from the hindmost stellar nerves of Doryteuthis plei squid. Axon diameter depends mainly on the age of the squid. The influx of (22)Na, some electrical properties, and the ultrastructure of the axolemma were also studied. The results confirm our previous observation that in medium sized axons of D. plei stimulation causes an increase in the permeability to the penetration of erythritol, mannitol, and sucrose. They also demonstrate that the magnitude of the increase in the penetration of these probing molecules diminishes progressively as the axon diameter increases. The diminution in permeability may be due to a reduction in size of the pathways used by nonelectrolytes to enter the axon. No effect of stimulation on the ethylene glycol permeability is observed. The sodium influx and electrical properties are independent of axon size. The ultrastructural study shows that the axolemma thickness increases with axon diameter. The present experiments indicate that the nonelectrolyte permeability of stimulated axons depends on nerve fiber properties related to axon diameter and on the size of the hydrophilic nonelectrolyte probe.

Action Potentials↗
PubMed1970

Ionic channels and nerve membrane lipids. Cholesterol-tetrodotoxin interaction.

Experiments were carried out to investigate possible interactions of tetrodotoxin (TTX) with lipid molecules isolated from nerve fiber plasma membranes of the squid Dosidicus gigas. TTX has a highly selective ability to block the channel normally used by Na(+) to cross the axolemma during nervous impulse conduction. In order to investigate the interaction each lipid sample was spread on 5 x 10(-7)M TTX and TTX-free 0.15 M NaCl solutions adjusted to pH 7.4 with 7 x 10(-3)M phosphate buffer. The surface pressure-area diagrams of the lipid monolayers revealed that TTX interacts only with cholesterol. The expansion of the cholesterol monolayers at 5 x 10(-7)M TTX was 2 A(2)/molecule at zero pressure for the experiments at 20 degrees C and 2.5 A(2)/molecule for those at 25 degrees C. Similar results were obtained in KCl subphases. The apparent dissociation constant of the cholesterol-TTX complex calculated from dose-response experiments is 2.6 x 10(-7)M. Experiments at pH 10.1 revealed that the zwitter ionic form of TTX is less active. Experiments with cholesterol derivatives (cholesteryl acetate, cholesterol methyl ether, cholestanol, and cholestanyl acetate) indicate that for the interaction with TTX a partial negatively charged group at C-3 and a double bond between C-5 and C-6 on the steroid nucleus are required. Tetrodonic acid, a biologically inactive derivative of TTX, does not interact with cholesterol. The results lead us to propose that cholesterol is part of the Na(+) channel.

Animals↗
PubMed1968

Adenosine triphosphatase activity in the membranes of the squid nerve fiber.

This investigation deals with the localization of sites of ATPase activity, especially of transport ATPase, in nerve fibers of the squid Doryteuthis plei, at the subcellular level. Splitting of ATP liberates inorganic phosphate which reacts with lead to form a precipitate in the tissue. The reaction was made on nerve fibers fixed with glutaraldehyde. Frozen slices were incubated in Wachstein-Meisel medium containing ATP and Pb(NO(3))(2). Deposits of reaction product were found in the axolemma (towards its axoplasmic side), Schwann cell membranes (mainly at the channels crossing the layer), and mitochondria. Control experiments revealed that no deposits were observed in nerve fibers fixed in osmium tetroxide prior to incubation in the medium containing ATP, or in nerve fibers incubated without substrate or with adenosine monophosphate, adenosine diphosphate, glycerophosphate, or guanosine triphosphate as substrate. For evaluation of transport ATPase activity, these findings were compared with results obtained with nerve fibers treated with G-strophanthin or K-strophanthoside before or after glutaraldehyde fixation. The cardiac glycosides produced a disappearance or diminution of the deposits. The largest inhibitory effect was observed in the axolemma. The findings indicate that the highest ATPase activity is localized in the axolemma and may be due primarily to transport ATPase.

Adenosine Triphosphatases↗
PubMed1968

Nature of the Schwann cell electrical potential. Effects of the external ionic concentrations and a cardiac glycoside.

The effects on the Schwann cell electrical potential of external ionic concentrations and of K-strophanthoside were investigated. Increasing (K)(o) depolarized the cell. The potential is related to the logarithm of (K)(o) in a quasi-linear fashion. The linear portion of the curve has a slope of 45 mv/ten-fold change in (K)(o). Diminutions of (Na)(o) and (Cl)(o) produced only small variations in the potential. Calcium and magnesium can be replaced by 44 mM calcium without altering the potential. Increase of (Ca)(o) to 88 mM produced about 10 mv hyperpolarization. The cell was hyperpolarized by 11 mv and 4 mv within 1 min after applying K-strophanthoside at concentrations of 10(-3) and 10(-5)M, respectively. No variations of cellular potassium, sodium, or chloride were observed 3 min after applying the glycoside. The hyperpolarization caused by 10(-3)M K-strophanthoside was not observed when (K)(o) was diminished to 1 or 0.1 mM or was increased to 30 mM. At a (K)(o) of 30 mM, 10(-2)M strophanthoside was required to produce the hyperpolarizing effect. In high calcium, the cell was further hyperpolarized by the glycoside. The initial hyperpolarization caused by the glycoside was followed by a gradual depolarization and a decrease of the cellular potassium concentration. The results indicate that the Schwann cell potential of about -40 mv is due to ionic diffusion, mainly of potassium, and to a cardiac glycoside-sensitive ion transport process.

Animals↗
PubMed1966

Nonelectrolyte penetration and sodium fluxes through the axolemma of resting and stimulated medium sized axons of the squid Doryteuthis plei.

The penetration of (14)C-labeled erythritol, mannitol, and sucrose through the axolemma was determined in medium sized paired axons, one at rest and the other stimulated 25 times per sec. The resting permeabilities, in 10(-7) cm/sec, are erythritol, 2.9 +/- 0.3 (mean +/- SEM); mannitol, 2.3 +/- 0.4; and sucrose 0.9 +/- 0.1. In the stimulated axons they are: erythritol, 5.2 +/- 0.3; mannitol, 4.0 +/- 0.5; and sucrose, 1.8 +/- 0.3. Thus, the calculated permeabilities during activity (1 msec per impulse), in the same units, are: 100, 75, and 38, respectively. These changes in permeability are reversible. The effects of external potassium and sodium concentrations on erythritol penetration were also studied. At rest, erythritol penetration is independent of potassium and sodium concentrations. In the stimulated axons, erythritol penetration decreases when the extracellular sodium is diminished. Sodium influx (not the efflux) decreases during rest and activity when the extracellular sodium is diminished. The diminution during activity of erythritol and sodium entries in low sodium solutions may be related to a decrease of a drag effect of sodium ions on the nonelectrolyte molecules or to independent effects of the sodium concentration on sodium influx and the nonelectrolyte pathways. The axolemma discriminates among erythritol, mannitol, sucrose, and the different ionic species during rest and activity.

Alcohols↗
PubMed1965

Sodium, potassium, and chloride concentrations in the Schwann cell and axon of the squid nerve fiber.

Sodium, potassium, and chloride concentrations were determined in the sheath cells and axoplasm of the nerve fiber of the squid Sepioteuthis sepioidea. The sheaths were obtained by slitting the nerve fiber, the extracellular electrolytes were washed out in isotonic sucrose solution, and the concentrations in the cells were determined after different soaking times in the sucrose solution. Values for the Schwann cell were calculated by extrapolation to zero time from the plots of the logarithms of the concentrations in the cells as a function of soaking time in sucrose solution. The Schwann cells made up 84 per cent of the sheath's total cellular volume. The Schwann cell concentrations in millimols per liter, are: 312 (404-241) for sodium, 220 (308-157) for potassium, and 167 (208-138) for chloride. The concentrations in the axoplasm (mean +/- SE), in millimols per liter are: 52 +/- 10 for sodium, 335 +/- 25 for potassium, and 135 +/- 14 for chloride. The possibility that some fraction of the Schwann cell electrolytes, especially of sodium, is bound, cannot be discarded.

Animals↗