Effects of bicarbonate ion on chick retinal pigment epithelium: membrane potentials and light-evoked responses. (57/15260)

The purpose of this study was to determine how changes in [HCO3-] alter the electrical properties of the retinal pigment epithelium (RPE). Experiments were conducted on the isolated chick retina-RPE-choroid preparation. The chamber holding the preparation allowed independent perfusion of the retinal and the choroidal surfaces. The light-evoked trans-tissue potential (TTP), the trans-epithelial potential (TEP), the trans-retinal potentials, and the intracellularly-recorded apical and basal membrane potentials were studied. Increasing the [HCO3-]0 in the choroidal bath from 25 to 40 mEq/1 led to an increase in the TTP and TEP. The same change in the retinal bath decreased the TTP because of a biphasic change of the RPE membrane potentials. There was also an increase in the amplitudes of the TEP, the c-wave and the slow PIII. The light-evoked subretinal K+ decrease was greater which is consistent with an increase in the photoreceptor light response. These observations indicated that the decrease of TTP resulted from a basal membrane hyperpolarization followed by an apical membrane depolarization induced by an increase in retinal [HCO3-]0. The relationship of these potential changes to the human bicarbonate responses is discussed.  (+info)

Preferential transport of glutathione versus glutathione disulfide in rat liver microsomal vesicles. (58/15260)

A bi-directional, saturable transport of glutathione (GSH) was found in rat liver microsomal vesicles. GSH transport could be inhibited by the anion transport blockers flufenamic acid and 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid. A part of GSH taken up by the vesicles was metabolized to glutathione disulfide (GSSG) in the lumen. Microsomal membrane was virtually nonpermeable toward GSSG; accordingly, GSSG generated in the microsomal lumen could hardly exit. Therefore, GSH transport, contrary to previous assumptions, is preferred in the endoplasmic reticulum, and GSSG entrapped and accumulated in the lumen creates the oxidized state of its redox buffer.  (+info)

Genetic dissection of behavior: modulation of locomotion by light in the Drosophila melanogaster larva requires genetically distinct visual system functions. (59/15260)

The Drosophila larva modulates its pattern of locomotion when exposed to light. Modulation of locomotion can be measured as a reduction in the distance traveled and by a sharp change of direction when the light is turned on. When the light is turned off this change of direction, albeit significantly smaller than when the light is turned on, is still significantly larger than in the absence of light transition. Mutations that disrupt adult phototransduction disrupt a subset of these responses. In larvae carrying these mutations the magnitude of change of direction when the light is turned on is reduced to levels indistinguishable from that recorded when the light is turned off, but it is still significantly higher than in the absence of any light transition. Similar results were obtained when these responses were measured in strains where the larval photoreceptor neurons were ablated by mutations in the glass (gl) gene or by the targeted expression of the cell death gene head involution defective (hid). A mutation in the homeobox gene sine oculis (so) that ablates the larval visual system, or the targeted expression of the reaper (rpr) cell death gene, abolishes all responses to light detected as a change of direction. We propose the existence of an extraocular light perception that does not use the same phototransduction cascade as the adult photoreceptors. Our results indicate that this novel visual function depends on the blue-absorbing rhodopsin Rh1 and is specified by the so gene.  (+info)

Systemic signaling and acclimation in response to excess excitation energy in Arabidopsis. (60/15260)

Land plants are sessile and have developed sophisticated mechanisms that allow for both immediate and acclimatory responses to changing environments. Partial exposure of low light-adapted Arabidopsis plants to excess light results in a systemic acclimation to excess excitation energy and consequent photooxidative stress in unexposed leaves. Thus, plants possess a mechanism to communicate excess excitation energy systemically, allowing them to mount a defense against further episodes of such stress. Systemic redox changes in the proximity of photosystem II, hydrogen peroxide, and the induction of antioxidant defenses are key determinants of this mechanism of systemic acquired acclimation.  (+info)

Light-dependent changes in redox status of the plastidic acetyl-CoA carboxylase and its regulatory component. (61/15260)

Plastidic acetyl-CoA carboxylase (ACCase; EC 6.4.1.2), which catalyses the synthesis of malonyl-CoA and is the regulatory enzyme of fatty acid synthesis, is activated by light, presumably under redox regulation. To obtain evidence of redox regulation in vivo, the activity of ACCase was examined in pea chloroplasts isolated from plants kept in darkness (dark-ACCase) or after exposure to light for 1 h (light-ACCase) in the presence or absence of a thiol-reducing agent, dithiothreitol (DTT). The protein level was similar for light-ACCase and dark-ACCase, but the activity of light-ACCase in the absence of DTT was approx. 3-fold that of dark-ACCase. The light-ACCase and dark-ACCase were activated approx. 2-fold and 6-fold by DTT respectively, indicating that light-ACCase was in a much more reduced, active form than the dark-ACCase. This is the first demonstration of the light-dependent reduction of ACCase in vivo. Measurement of the activities of ACCase, carboxyltransferase and biotin carboxylase in the presence and absence of DTT, and the thiol-oxidizing agent, 5, 5'-dithiobis-(2-nitrobenzoic) acid, revealed that the carboxyltransferase reaction, but not the biotin carboxylase reaction, was redox-regulated. The cysteine residue(s) responsible for redox regulation probably reside on the carboxyltransferase component. Measurement of the pH dependence of biotin carboxylase and carboxyltransferase activities in the ACCase suggested that both components affect the activity of ACCase in vivo at a physiological pH range. These results suggest that the activation of ACCase by light is caused partly by the pH-dependent activation of two components and by the reductive activation of carboxyltransferase.  (+info)

Kinetics of photoacclimation in cultures of Chromatium vinosum DSM 185 during shifts in light irradiance. (62/15260)

Continuous cultures of Chromatium vinosum DSM 185 were shifted from a high to a low irradiance (67 to 4 microE m(-2) s(-1)) and vice versa (4 to 67 microE m(-2) s(-1)). The kinetics of photoacclimation of the cultures were analysed during these transitions until steady state was reached. When irradiance was shifted from 4 to 67 microE m(-2) s(-1), bacteriochlorophyll synthesis halted for 4 h. During this period, pigments were progressively diluted in the newly formed biomass, resulting in a lower specific pigment content. The specific growth rate of the organisms did not change immediately after the shift, but rather underwent a gradual increase during the following 10 h. This transition was accompanied by a transient increase in the levels of glycogen, indicating that CO2 fixation rates increased immediately after the shift, and that unused photosynthate was stored as glycogen. The shift from a high to a low irradiance was characterized by an immediate drop in the specific growth rate to virtually zero, and by comparatively sharp decreases in the specific rates of sulfur and sulfide oxidation and in the specific rate of glycogen accumulation. The specific content of bacteriochlorophyll a increased during the first 10 h. During the same period the specific content of glycogen decreased.  (+info)

An empirical basis for Mach bands. (63/15260)

Mach bands, the illusory brightness maxima and minima perceived at the initiation and termination of luminance gradients, respectively, are generally considered a direct perceptual manifestation of lateral inhibitory interactions among retinal or other lower order visual neurons. Here we examine an alternative explanation, namely that Mach bands arise as a consequence of real-world luminance gradients. In this first of two companion papers, we analyze the natural sources of luminance gradients, demonstrating that real-world gradients arising from curved surfaces are ordinarily adorned by photometric highlights and lowlights in the position of the illusory bands. The prevalence of such gradients provides an empirical basis for the generation of this perceptual phenomenon.  (+info)

Mach bands as empirically derived associations. (64/15260)

If Mach bands arise as an empirical consequence of real-world luminance profiles, several predictions follow. First, the appearance of Mach bands should accord with the appearance of naturally occurring highlights and lowlights. Second, altering the slope of an ambiguous luminance gradient so that it corresponds more closely to gradients that are typically adorned with luminance maxima and minima in the position of Mach bands should enhance the illusion. Third, altering a luminance gradient so that it corresponds more closely to gradients that normally lack luminance maxima and minima in the position of Mach bands should diminish the salience of the illusion. Fourth, the perception of Mach bands elicited by the same luminance gradient should be changed by contextual cues that indicate whether the gradient is more or less likely to signify a curved or a flat surface. Because each of these predictions is met, we conclude that Mach bands arise because the association elicited by the stimulus (the percept) incorporates these features as a result of past experience.  (+info)