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Biomedical subjects

M Chabre

Publications and source records attributed to M Chabre.

80 records · Page 5Linked to original sources
PubMed1975

X-ray diffraction studies of retinal rods. I. Structure of the disc membrane, effect of illumination.

The structure of the retinal rod disc membrane and its modifications upon bleaching have been studied by X-ray diffraction. Three types of preparations are used: functioning isolated from retina, isolated rods from frog retina, oriented by a magnetic field, and stacked discs from cattle retina. X-rays are detected by a position-sensitive linear counter. Diffraction spectra are obtained in 10-100 s. The electron density profile favors models where the rhodopsin molecule spans the whole thickness of the membrane. Upon bleaching, a small increase of electron density appears instantly at the cytoplasmic edge of the membrane. In the intact retina this structural change is accompanied by disorder and slow swelling reactions which are not observed in the isolated rod outer segment. The diffraction signal arising from the protein distribution in the plane of the membrane has been reinvestigated carefully. Patterns identical to those of Blasie (Blaise (1969) J. Mol. Biol. 39, 407 and Blaise (1972) Biophys. J. 12, 191) can be obtained but these are shown to be dominated by artefacts. The actual signal is a single broad band around (55 A)-1, upon which bleaching has a negligible effect. No measurable displacement of rhodopsin in the thickness of the membrane occurs upon bleaching. Temperature effects on the protein distribution are found to be large only for disc membranes from cattle retina. In this material from a warm-blooded animal those effects are correlated with the occurrence, upon lowering the temperature, of a partial phase transition of the paraffin chains of the lipids. The position and the slope of the transition are not sensitive to bleaching.

Animals↗
PubMed1975

X-ray diffraction studies of retinal rods. II. Light effect on the osmotic properties.

A fast X-ray diffraction technique has been used to study the osmotic reaction of frog rod outer segments to bleaching and to changes in the osmolarity and composition of the outer medium. Dissected excitable retina and isolated outer segments have been used. Upon bleaching in isotonic Ringer only a small transient diminution of the disc repeat distance is observed in isolated rods. Disorder and slow swelling reactions are also observed in the intact cells. In calcium-free Ringer the light-induced shrinkage is considerably enhanced. In the intact cell this is interpretable as due to the switching off of a large sodium dark current. The persistence of the effect in isolated outer segment suggests the existence of an active ionic efflux from this part of the cell. Upon hypotonic shock, bleached rods swell more than dark-adapted ones. The difference, however, appears only in a slow component of the osmotic kinetics, a few minutes after the shock. Upon hyertonic shock, part of the rods, even in the "intact" excitable retina, appear to be leaking. Those cells which are intact are impermeable to all the solutes added to increase the osmolarity: NaC1, KC1, Sucrose, Melezitose. No light dependence of the response to a hyperosmotic NaC1 shock is detectable. The discs are osmotically reactive, even when the outer cell membrane of the rods is leaking. Assuming the discs to be perfect osmometers, a thickness of 20 plus or minus 5 A is estimated for the liquid layer inside the discs.

Animals↗
PubMedDate not supplied

Millisecond activation of transducin in the cyclic nucleotide cascade of vision.

Cyclic GMP has been implicated as a messenger molecule involved in visual transduction. Photoexcited rhodopsin (R*) binds to a multisubunit membrane protein called transducin (T) and stimulates the exchange of a bound GDP molecule for GTP. This leads to the release of the alpha-subunit of T with bound GTP (T alpha-GTP), which activates a cyclic GMP phosphodiesterase. The question arises as to whether the hydrolysis of cyclic GMP that results from activation of the phosphodiesterase is sufficiently rapid to be involved in visual excitation, which occurs on a time scale of approximately 2 s in the single-photon limit. Previous studies have suggested that the cyclic GMP phosphodiesterase is activated in less than 100 ms at moderate light levels. We report here light scattering studies of magnetically orientated frog rod outer segments which show that a molecule of R* catalyses the activation of a molecule of T in about 1 ms. Thus, hundreds of molecules can be activated within the response time of vision in the single-photon limit, and the formation of T alpha-GTP is fast enough for it to be a key step in visual transduction.

3',5'-Cyclic-GMP Phosphodiesterases↗