Search PubMed⌕ Search

Biomedical subjects

B Burnside

Publications and source records attributed to B Burnside.

At least 73 records · Page 4Linked to original sources
PubMed1983

Retinomotor pigment migration in the teleost retinal pigment epithelium. I. Roles for actin and microtubules in pigment granule transport and cone movement.

In lower vertebrates, retinal pigment epithelial (RPE) cells and photoreceptors undergo dramatic "retinomotor movements" in response to changes in light conditions. In the dark, RPE pigment granules aggregate to the (choroidal) base of the RPE cells, cones elongate, and rods contract. In the light, movements are reversed: pigment granules migrate out into the long apical projections of the RPE cells, cones contract, and rods elongate. In this report the time courses of dark-induced pigment aggregation and light-induced dispersion have been characterized (and compared to cone movements) in the blue stripe grunt, Haemulon sciurus. It was found that aggregation and dispersion occur at linear rates of 3.4-3.5 microns/min and that RPE movements are kinetically independent from cone movements induced by the same changes in light conditions. The roles of actin and microtubules in RPE and cone movements were also investigated by using the actin-inhibitors, cytochalasins-B and -D, and the microtubule inhibitor, colchicine. Light-induced pigment dispersion, as well as maintenance of the fully dispersed (light-adapted) position appear to require actin-dependent processes. Intraocularly injected cytochalasins-B and -D fully prevented pigment dispersion when administered to dark-adapted animals immediately prior to their exposure to light, and caused pigment aggregation to the RPE cell base when administered to fully light-adapted animals. Ultrastructural studies showed that actin filaments, which in untreated retinas were found closely associated with pigment granules and the plasma membrane, were disrupted after cytochalasin-B treatment. Both dispersive and aggregative pigment movements within the cell body appeared to require microtubule-dependent processes. Intraocularly injected colchicine disrupted microtubules and blocked pigment granule translocation in both directions in the cell body. A hypothetical model to explain pigment movements in response to changes in light conditions is proposed based on these observations as well as on data from the literature.

Actins↗
PubMed1982

Reactivation of contraction in detergent-lysed teleost retinal cones.

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 micrometers (midnight dark-adapted) to 6 micrometers in fully light-adapted state. When dark-adapted fish are exposed to light (1.4 lux), cone myoids contract with a linear rate of 1.5 +/- 0.1 micrometers/min. We report here that detergent-lysed motile models of teleost retinal cones exhibit calcium- and ATP-dependent reactivated contraction, with morphology and rate comparable to that observed in vivo. For reactivation studies isolated dark-adapted retinas were lysed with nonionic detergent Brij-58 (0.1-1.0%). In reactivation medium containing 10(-5) M free calcium and 4 mM ATP, the lysed cones contracted with normal morphology at in vivo rates (1.4 +/- 1 micrometer/min). Little contraction was observed if ATP or detergent was deleted from the medium or if free calcium levels were less than 10(-8) M. Ultrastructural examination of cone models lysed with 1% Brij-58 revealed that, in spite of extensive extraction of the cytoplasmic matrix, cytoskeletal components (thin filaments, intermediate filaments, microtubules) were still present. Thus we have produced extensively extracted motile models of teleost retinal cones which undergo calcium- and ATP-dependent reactivated contraction with normal morphology at physiological rate.

Adenosine Triphosphate↗
PubMed1982

Elevation of cyclic AMP activates an actin-dependent contraction in teleost retinal rods.

Agents which elevate cyclic AMP (cAMP) cause teleost retinal rods to contract. We have characterized this cAMP effect and have evaluated the role of the cytoskeleton in cyclic nucleotide-induced contraction, using actin and microtubule inhibitors. The necklike myoid region of the rod contracts in the dark and elongates in the light. If long, light-adapted rods are cultured with cAMP analogs and IBMX, rods contract to their short dark-adapted position. Cyclic nucleotide-induced rod contraction occurs in constant light, requires a phosphodiesterase inhibitor, and is specific to cAMP (db cyclic GMP, 8-bromocyclic GMP, 5'AMP, and adenosine have no effect on rod myoid length). Cyclic AMP effects on rod length are consistent with observations from several species that cAMP levels are higher in dark-adapted than in light-adapted retinas. Since rod myoids contain paraxially aligned actin filaments and microtubules, we have used the motility inhibitors cytochalasin D and cold and nocodazole to investigate the roles of these cytoskeletal elements in rod contraction. Cyclic nucleotide-induced contraction is not inhibited when myoid microtubules are disrupted with cold and nocodazole treatments, but contraction is blocked if myoid actin filaments are disrupted with cytochalasin D. Thus, we conclude that actin filaments, but not microtubules, are required for rod contraction. We propose that rod contraction in vivo is triggered by a rise of cytoplasmic cAMP at onset of darkness and that this contraction is mediated by an actin-dependent mechanism.

1-Methyl-3-isobutylxanthine↗
PubMed1982

Induction of dark-adaptive retinomotor movement (cell elongation) in teleost retinal cones by cyclic adenosine 3','5-monophosphate.

In the teleost retina, the photoreceptors and retinal pigment epithelium (RPE) undergo extensive movements (called retinomotor movements) in response to changes in light conditions and to an endogenous circadian rhythm. Photoreceptor movements serve to reposition the light-receptive outer segments and are effected by changes in inner segment length. Melanin granule movements within the RPE cells provide a movable melanin screen for rod outer segments. In the dark (night), cones elongate, rods contract, and pigment granules aggregate to the base of the RPE cell; in the light (day), these movements are reversed. We report here that treatments that elevate cytoplasmic cyclic adenosine 3',5'-monophosphate (cAMP) provoke retinomotor movements characteristic of nighttime dark adaptation, even in bright light at midday. To illustrate this response, we present a quantitative description of the effects of cyclic nucleotides on cone length in the green sunfish, Lepomis cyanellus. Cone elongation is induced when light-adapted retinas are exposed to exogenous cAMP analogues accompanied by phosphodiesterase (PDE) inhibitors (either by intraocular injection or in retinal organ culture). Cone movements is not affected by cyclic GMP analogies. Dose-response studies indicate that the extent, but not the rate, of cone elongation is proportional to the concentration of exogenous cAMP and analogue presented. As has been reported for other species, we find that levels of cAMP are significantly higher in dark- than in light-adapted green sunfish retinas. On the basis of these observations, we suggest that cAMP plays a role in the light and circadian regulation of teleost cone length.

Animals↗
PubMed1982

Effects of cyclic adenosine 3',5'-monophosphate on photoreceptor disc shedding and retinomotor movement. Inhibition of rod shedding and stimulation of cone elongation.

As a test of the hypothesis that cyclic nucleotides play a role in the regulation of retinomotor movements and disc shedding in the photoreceptor-pigment epithelial complex, we have used an in vitro eyecup preparation that sustains both disc shedding and cone retinomotor movements, Eyecups were prepared in white light from animals in which both shedding and cone movement had been blocked by 4 d of constant-light treatment. In eyecups incubated for 3 h in light, disc shedding was negligible and cones remained in the light-adapted (contracted) position. In eyecups incubated in darkness, however, a massive shedding response (dominated by rod photoreceptors) was induced, and at the same time cone photoreceptors elongated to their dark-adapted position. In eyecups incubated in light dbcAMP promoted cone elongation and thus mimicked darkness; the dbcAMP effect was potentiated by the phosphodiesterase inhibitors papaverine and 3-isobutylmethylxanthine. In eyecups incubated in darkness, on the other hand, both phosphodiesterase inhibitors and dbcAMP reduced the phagosome content of the pigment epithelium. The effects of dbcAMP on the cone elongation and rod shedding appear to be specific in that dbcGMP, adenosine, and adenosine 5'-monophosphate had no significant effect. Our results suggest that cAMP plays a role in the regulation of both retinomotor movements and disc shedding.

Animals↗
PubMed1981

Actin-dependent cell elongation in teleost retinal rods: requirement for actin filament assembly.

Teleost retinal rods elongate when exposed to light. Elongation is mediated by a narrow necklike region called the myoid. In the cichlid Sarotherodon mossambicus, the rod inner segment (composed of the myoid with adjacent ellipsoid) increases in length from 12 micrometers in the dark to 41 micrometers in the light. Long light-adapted myoids contain longitudinally oriented microtubules and bundles of parallel 60-A filaments that we have identified as actin by their ability to bind myosin subfragment 1. In short dark-adapted myoids, only microtubules are recognizable. Colchicine experiments reveal that light-induced rod elongation can occur in the absence of myoid microtubules. Intraocular injections of colchicine at concentrations that disrupt virtually all rod myoid microtubules do not block rod elongation. However, rod elongation is blocked by intraocular injections of cytochalasin B or cytochalasin D. The hierarchy of effectiveness of these drugs is consistent with their effectiveness in inhibiting actin assembly and in disrupting other actin-dependent motile processes. On the basis of ultrastructural observations and the results of these inhibitor studies, we propose that the forces responsible for rod elongation are dependent not on microtubules but on actin filament assembly.

Actins↗
PubMed1981

Circadian rhythms in teleost retinomotor movement. A comparison of the effects of circadian rhythm and light condition on cone length.

The long, slender cones of the teleost retina elongate at night and contract during the day. In the Midas cichlid, Cichlasoma citrinellum, this cone excursion is elicited both by changes in light conditions and by strong endogenous circadian rhythms. In a normal day/night cycle. C. citrinellum cones change length by 69 micron. We have found that in this species an endogenous circadian rhythm induces substantial cone excursion in contrast light as well as in constant darkness. Total excursion in constant light is 34% of that seen in a normal cycle. Total excursion in constant darkness is 58% of that seen in a normal cycle. Similar excursions are observed on the second and third days of constant darkness. A change from light to darkness at a time in the cycle other than dusk induces elongation averaging 49% of the total excursion observed in the normal cycle. A change from darkness to light at a time other than dawn induces cone contraction averaging 30% of the total excursion observed in the normal cycle. The response of retinal cones to either of the above changes in light conditions is a relatively constant magnitude at all sampling times over a 24 hr period. We conclude that both the endogenous circadian rhythm and the responses to changes in light conditions of the cones and required to produces the full excursion observed in the normal day/night cycle. Full elongation of cones can occur only at night in darkness, and full contraction of the cones can occur only in the day in the light. Changing light conditions at inappropriate times produces intermediate cone lengths.

Adaptation, Physiological↗
PubMed1980

Actin filaments in retinal pericytes and endothelial cells.

The contractile protein actin was identified within retinal capillary pericytes and endothelial cells. In capillary cross-sections, circumferential pericyte processes showed numerous parallel bundles of actin filaments forming a cap over the adjacent endothelial cells. The latter contained few actin filaments but a considerable number of intermediate filaments. Pericytes and endothelial cells communicated via gaps in the adluminal basal lamina. These special anatomic relationships are interpreted as evidence for a contractile role of retinal capillary pericytes and against a contractile role of retinal capillary endothelial cells

Actins↗
PubMed1978

Thin (actin) and thick (myosinlike) filaments in cone contraction in the teleost retina.

The long slender retinal cones of fishes shorten in the light and elongate in the dark. Light-induced cone shortening provides a useful model for stuying nonmuscle contraction because it is linear, slow, and repetitive. Cone cells contain both thin (actin) and thick (myosinlike) filaments oriented parallel to the axis of contraction. This study examines the polarities of the cone's thin filaments and the changes in filament distribution which accompany light-induced contraction, in an attempt to elucidate the structural basis for the cone's contractile process. The proximal half of the cone is fixed to its cellular neighbors in the outer nuclear layer while the distal half is free. Thus, all shortening takes place in a necklike region (the myoid) in the distal half of the cone which extends into the space between the neural retina and the pigmented retinal epithelium. Thin filaments are found throughout the length of the cone, whereas thick filaments occur predominantly in the proximal (axon) regions of both light- and dark-adapted cones. Thus, thick filaments are primarily localized outside the region where shortening takes place. Observations from myosin subfragment-1 binding studies suggest that the cone's thin filaments are organized into two opposing sets. In the distal half of the cone (including the myoid), virtually all filaments have proximally directed arrowheads. In the more proximal regions of the axon, many thin filaments have opposite polarity, their arrowheads being distally directed. Near the synaptic proximal end of the light-adapted (contracted) cone, filaments of opposite polarities occur in approximately equal numbers. Thus, in the cone axon there appear to be two overlapping sets of actin filaments whose opposite polarities correspond to the two actin halves of a muscle sarcomere. In elongated, dark-adapted cones, thick filaments are localized throughout the axon region of the cone. In light, thick filaments accumulate towards the proximal end of the cone. These observations are consistent with a "sliding hypothesis" for cone contraction, in which thick myosinlike filaments produce sliding interdigitation of the two sets of oppositely directed actin filaments in the proximal axon region. Thus, the myoid thin filaments would be essentially reeled into the axon region to produce shortening. The mechanism of re-elongation depends on microtubules, as discussed in the companion paper.

Actins↗
PubMed1976

Microtubules and actin filaments in teleost visual cone elongation and contraction.

Teleost retinal cones contract in light and elongate in darkness. This paper describes the disposition of microtubules and cytoplasmic filaments in cone cells of 2 species of fish (Haemulon sciurus and Lutjanus griseus). In Haemulon, the neck-like "myoid" region of the cone changes in length from 5 mu to 75 mu. Maximal observed rates of elongation and contraction are comparable to that of chromosome movement in mitosis (2-3 mu/min). Microtubules presumably participate in cone elongation, since numerous longitudinal microtubules are present in the myoid region, and colchicine blocks dark-induced elongation. Myoid shortening, on the other hand, appears to be an active contractile process. Disruption of microtubules in dark-adapted cones does not produce myoid shortening in the absence of light, and light-induced myoid shortening is blocked by cytochalasin-B. Cone cells possess longitudinally-oriented thin filaments which bind myosin subfragment-1 to form arrowhead complexes typical of muscle actin. Myoid thin filaments are clearly observed in negatively stained preparations of isolated cones which have been disrupted with detergent after attachment to grids. These myoid filaments are not, however, generally preserved by conventional fixation, though bundles of thin filaments are preserved in other regions of the cell. Thus, actin filaments are poorly retained by fixation in precisely the region of the cone cell where contraction occurs. Cone cells also possess longitudinally-oriented thick filaments 130-160 A in diameter. That these thick filaments may be myosin is suggested by the presence of side-arms with approximately 150 A periodicity. The linear organization of the contractile apparatus of the retinal cone cell makes this cell a promising model for morphological characterization of the disposition of actin and myosin filaments during contraction in a nonmuscle cell.

Actins↗
PubMed1976

Actin filaments in apical projections of the primate pigmented epithelial cell.

A highly-ordered array of filaments is found within the apical processes of retinal pigmented epithelial cells in monkeys and humans. These filaments, approximately 100 A in diameter and 250 A apart, line the cytoplasmic face of the plasma membrane, in parallel with the long axis of the apical processes. Since these filaments bind rabbit myosin subfrafment-1 to form arrowhead complexes, we conclude that they contain actin. Such membrane-bound actin filaments could have any of several different functions: they could stabilize the apical projections and by so doing play a cytoskeletal role, and/or they could take part in the phagocytosis of shed outer segment discs.

Actins↗
PubMed1972

The sensitivity of developing cardiac myofibrils to cytochalasin-B (electron microscopy-polarized light-Z-bands-heartbeat).

Developing cardiac muscle cells of 11- to 13-somite chick embryos are sensitive to cytochalasin-B. In cultured chick embryos, ranging in development from 11 to 13 somites, hearts stop beating in the presence of this agent. Both polarized light and electron microscopic examination show that cytochalasin-B disrupts existing myofibrils and inhibits the formation of new ones. Discrete Z-bands are not present in treated heart cells and thick, presumably myosin, filaments are found in disarray. These effects are reversible; after cytochalasin-B is removed from the medium, heartbeat recovers and myofibrils with discrete Z-bands reappear. Fibrillar sensitivity appears to be a function of age since fibrils in hearts of embryos having from 22 to 28 pairs of somites are more resistant.

Animals↗