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R Padrón

Publications and source records attributed to R Padrón.

At least 37 records · Page 2Linked to original sources
PubMed1985

Arrangement of the heads of myosin in relaxed thick filaments from tarantula muscle.

Thick filaments from leg muscle of tarantula, maintained under relaxing conditions (Mg-ATP and EGTA), were negatively stained and photographed with minimal electron dose. Particles were selected for three-dimensional image reconstruction by general visual appearance and by the strength and symmetry of their optical diffraction patterns, the best of which extend to spacings of 1/5 nm-1. The helical symmetry is such that, on a given layer-line, Bessel function contributions of different orders start to overlap at fairly low resolution and must therefore be separated computationally by combining data from different views. Independent reconstructions agree well and show more detail than previous reconstructions of thick filaments from Limulus and scallop. The strongest feature is a set of four long-pitch right-handed helical ridges (pitch 4 X 43.5 nm) formed by the elongated myosin heads. The long-pitch helices are modulated to give ridges with an axial spacing of 14.5 nm, lying in planes roughly normal to the filament axis and running circumferentially. We suggest that the latter may be formed by the stacking of a subfragment 1 (S1) head from one myosin molecule on an S1 from an axially neighbouring molecule. Internal features in the map indicate an approximate local twofold axis relating the putative heads within a molecule. The heads appear to point in opposite directions along the filament axis and are located very close to the filament backbone. Thus, for the first time, the two heads of the myosin molecule appear to have been visualized in a native thick filament under relaxing conditions.

Animals↗
PubMed1984

The effect of the ATP analogue AMPPNP on the structure of crossbridges in vertebrate skeletal muscles: X-ray diffraction and mechanical studies.

Adenylylimidodiphosphate (AMPPNP), a nonhydrolysable analogue of ATP, has been used to arrest the crossbridge cycle of muscular contraction in one of its hypothetical intermediate states. Whole frog sartorius muscles were chemically demembranated, and it was found possible to cycle such skinned muscles reversibly between the relaxed and rigor states. The effect of binding of AMPPNP on the structure and spatial arrangement of the crossbridges of such muscles was studied using low-angle X-ray diffraction, with simultaneous recording of the mechanical effects, starting from the rigor state. Saturating concentrations of MgAMPPNP produce a characteristic decrease of about 50% in the original rigor isometric tension with a concomitant increase in muscle length by 0.13%. The equatorial X-ray diffraction pattern is modified in the following way: the lattice dimensions and the intensity of the (10) equatorial reflection do not change, while the intensity of the (11) equatorial reflection increases slightly. These observations of very small equatorial changes could be explained by assuming that in these muscles (as distinct from others such as rabbit psoas) the analogue does not produce a significant degree of detachment of crossbridges; that is, there are only AMPPNP-modified attached ones. The changes in the meridional X-ray diffraction pattern are more pronounced: the meridional reflection at 14.5 nm decreases in intensity, and the meridional reflection at 7.2 nm increases considerably: the intensity of all the actin-based off-meridional layer-lines decreases. There are no signs of the characteristic relaxed layer-lines, and the changes in the layer-line intensities are probably due to there being a single population of AMPPNP-modified attached crossbridges, rather than a mixture of attached and detached crossbridges. Thus the AMPPNP X-ray pattern, both equatorially and meridionally, is somewhat similar to the rigor one, indicating that most of the crossbridges remain attached. On the other hand, the fact that there are some changes in the layer-line intensities of the AMPPNP frog pattern, without the appearance of any signs of a relaxed equatorial pattern, indicates that the attached crossbridges are in a structural state that is different from rigor, one is not seeing, apparently, simply a mixture of rigor and relaxed states. Our tentative interpretation of this result is that there may be a structural change in the crossbridge near to the junction with S2, with less significant changes occurring in the parts of the crossbridge close to actin.

Adenosine Triphosphate↗
PubMed1982

Repetitive propagation of action potentials destabilizes the structure of the myelin sheath. A dynamic x-ray diffraction study.

Time courses of myelin lattice swelling in toad sciatic nerves preexposed to different treatments were determined by x-ray diffraction using a one-dimensional position-sensitive detector. In the nerves supramaximally stimulated for 1 h at 200 Hz, the subsequent process of myelin swelling occurred 45.0 +/- 7.3 min (n = 24) sooner than in resting controls. Sciatic nerves incubated for 1 h in a Ringer's solution deprived of divalent cations (Ca++ and Mg++) exhibited a kinetics of swelling similar to that shown by the stimulated nerves, that is, 52.5 +/- 14.2 min (n = 6) sooner than controls preincubated for the same time in normal Ringer's solution (with divalent cations). The fact that both pretreatments supramaximal stimulation and removal of divalent cations from the perfusion solution produced a similar effect; namely, a decrease of the myelin lattice stability against swelling in distilled water, suggests that the repetitive propagation of action potentials could modify the ionic composition at either the intraperiod channel or the paranodal axoglial junction complexes.

Action Potentials↗
PubMed1981

In vivo structure of frog sciatic nerve myelin membranes: an x-ray diffraction study at 13A resolution.

The x-ray diffraction pattern of myelin membranes from a frog sciatic nerve was recorded in vivo at 13 A resolution using a new experimental technique recently developed in our laboratory. The electron density profile was determined for the myelin membranes first in vivo and then, after elimination of the intraneural blood microcirculation, by transection of the nerve. The small structural differences observed between these conditions were statistically nonsignificant (at 95% confidence level). In addition, our results show that the stacking disorder that is present in the myelin membranes of freshly dissected frog sciatics is also characteristic of the myelin sheath in vivo.

Animals↗
PubMed1980

A dynamic X-ray diffraction study of anaesthesia action. Changes in myelin structure and electrical activity recorded simultaneously from frog sciatic nerves treated with n-alkanes.

Changes induced in the structure and electrical activity of myelin were recorded simultaneously from frog sciatic nerves treated with n-alkanes. The results suggest that the effect of n-alkanes seems to be two-fold: (a) there is an initial reversible phase, in which a significant modification of the X-ray diffraction patterns, concomitant with the continuous fall of the action potential, is observed; (b) there is a final phase which is irreversible. This occurs some time after the complete abolition of the electrical activity. At this stage, further changes of the X-ray diffraction patterns are detected, the most significant of them being in the n-pentane-treated myelin, and consist of an increase in the membrane bilayer thickness.

Action Potentials↗
PubMed1980

The effect of the repetitive propagation of action potentials on the structure of toad sciatic nerve myelin membranes: an x-ray diffraction study at 11 A resolution.

Equatorial sections of the x-ray diffraction patterns of myelin from toad sciatic nerves recorded at 11 A resolution during long supramaximal repetitive stimulation (240 Hz for two hours) were used to calculate the relative electron density profile of the myelin membranes in this physiological state. The profile was compared to the one determined for the same nerve at rest. The difference between the stimulated profile and the control profile was found to be statistically non-significant. Therefore, when the sciatic nerve fibers are excited the structure of the internodal myelin membranes does not significantly change at 11 A resolution.

Animals↗
PubMed1979

X-ray diffraction study of the kinetics of myelin lattice swelling. Effect of divalent cations.

The time-course of myelin lattice swelling and its reversal in dissected peripheral nerves was determined by small-angle x-ray diffraction using a position-sensitive proportional detector. The process of swelling can take place either in several hours or in less than 1 h depending on pretreatment of the nerves. The reversal of swelling was always completed within 1 h. The rapid structural transitions involved the disordering of membrane pairs as indicated by the transient appearance of a continuous intensity distribution similar to the membrane pair transform for myelin. The slow transitions involved the gradual replacement of the discrete reflections from the native structure by the reflections from the swollen lattice. Myelin membrane arrays reformed in normal Ringer's solution were much more stable to subsequent swelling than arrays reformed in Ca+2 and Mg+2-free Ringer's. These results suggest that these ions participate in stabilizing the interactions between the external surfaces of adjacent membrane pairs.

Animals↗
PubMed1978

Familial Cushing's syndrome.

Cushing's syndrome has been demonstrated in four of seven siblings with clinical manifestations appearing around puberty in three of the four siblings. The only other associated findings in these cases were short stature and disturbed carbohydrate metabolism. Adenomatous hyperplasia of the adrenal glands was demonstrated in 3 of the patients, and a virilizing adrenal carcinoma in the fourth sibling. The pathogenesis of the adrenocortical disorders in these siblings is discussed.

Adolescent↗
PubMed1977

Small-angle x-ray scattering study of human serum low-density lipoproteins with differential reactivity for an arterial proteoglycan.

The structure and thermal behavior of human serum low-density lipoproteins showing either a high or a low reactivity against a proteoglycan isolated from human arteries have been found to be different from each other. It is suggested that modifications in the lipoprotein surface structure induced by the physical state of the neutral lipids could modulate the affinity of the macromolecule for the arterial component.

Adult↗