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W A Hagins

Publications and source records attributed to W A Hagins.

At least 19 recordsLinked to original sources
PubMed1989

Transduction heats in retinal rods: tests of the role of cGMP by pyroelectric calorimetry.

The sensory dark current of vertebrate retinal rods is believed to be controlled by light activation of a chain of coupled biochemical cycles that finally regulate the cationic conductance of the plasma membrane by hydrolytically reducing the level of cGMP in rod outer segment cytoplasm. The scheme has been tested by measuring heat production by live frog retinas when stimulated with sequences of light flashes of progressively increasing energy. Using pyroelectric poly(vinylidene 1,1-difluoride) detectors that simultaneously measure transretinal voltage and retinal temperature change, four heat effects assignable to known biochemical cycles in rods have been found. As the dark current shuts down after a flash causing 180-1800 rhodopsin photoisomerizations per rod, a heat burst, q1, raises the retinal temperature 1-2 microK. q1 is closely regulated in size and slightly precedes dark current shutdown. Isobutylmethylxanthine slows and enlarges q1, delaying the dark-current response. Increasing cytoplasmic Ca2+ stops the dark current without affecting q1. Although rod heat production is consistent with splitting of 1-3 microM of free cytoplasmic cGMP during transduction, the kinetics of the two processes do not match the predictions of current cGMP control models.

Animals↗
PubMed1983

Control of Ca2+ in rod outer segment disks by light and cyclic GMP.

Photons absorbed in vertebrate rods and cones probably cause electrochemical changes at the photoreceptor plasma membrane by changing the cytoplasmic concentration of a diffusible transmitter substance, reducing the Na+ current flowing into the outer segment of the cell in the dark, to produce the observed membrane hyperpolarization that is the initial excitatory response. Cyclic GMP has been proposed as the transmitter because a light-activated cyclic GMP phosphodiesterase (PDE) has been found in rod disk membranes and because intracellularly injected cyclic GMP reduces rod membrane potentials. Free Ca2+ has also been proposed because increasing external [Ca2+] quickly and reversibly reduces the dark current and divalent cationophores increase the Ca2+ sensitivity. Ca2+ efflux from rod outer segments (ROS) of intact retinas occurs simultaneously with light responses. Vesicles prepared from ROS disk membranes become more permeable on illumination, releasing trapped ions or molecules, but intact outer segment disks have not previously been found to store sufficient Ca2+ in darkness and to release enough in light to meet the theoretical requirements for control of the dark current by varying cytoplasmic Ca2+ (refs 14-18). We now report experiments that show the required Ca2+ storage and release from rod disk membranes suspended in media containing high-energy phosphate esters and electrolytes approximating the cytoplasmic composition of live rod cells. Cyclic GMP stimulates Ca2+ uptake by ROS disks in such media.

Animals↗
PubMed1977

Liposome-cell interaction: transfer and intracellular release of a trapped fluorescent marker.

When small, unilamellar lipid vesicles containing a high concentration of the fluorescent dye 6-carboxyfluorescein are incubated with either frog retinas or human lymphocytes, fluroescence distributes widely throughout each cell. Since "self-quenching" largely prevents the dye from fluorescing as long as it remains sequestered in vesicles, it is clear that a considerable amount of dye is released from the vesicles and diluted into the much larger volume of the cell.

Animals↗
PubMed1975

Ionic mechanisms in excitation of photoreceptors.

Both measurements and rough theoretical calculations suggest that both sites in the plasma membrane where the dark current enters and the mechanism that produces the intracellular excitatory transmitter of rod vision operate via ionophoric mechanisms with very low specific conductivities and high ionic specificities properties similar to those of carriers in artificial and natural membranes.

Animals↗
PubMed1975

Ionic aspects of excitation in rod outer segments.

The current status of the problem of ionic mechanisms underlying excitation of vertebrate photoreceptors is reviewed. Evidence is presented that the ionic dark current of retinal rods is sustained by the action of a ouabain-sensitive Na-K exchange pump driven by oxidative metabolism. The photoreceptors are depleted of K when the pump is stopped by ouabain. Considerations of cell membrane topology, kinetics of the light response, and signal amplification indicate that the light-induced suppression of the ionic dark current is mediated by a diffusible internal chemical transmitter substance. The desensitizing effect of low CA2+ Ringer's on the light responses of vertebrate rods, along with other indirect evidence, suggests that the transmitter substance may be calcium ions released from the internal membranous disks of the outer segments.

Animals↗
PubMed1974

Topology of the outer segment membranes of retinal rods and cones revealed by a fluorescent probe.

When N,N'-didansyl cystine binds to the cell membranes of vertebrate rods and cones its fluorescence efficiency increases about 20-fold. The entire outer segments of living cones become brilliantly fluorescent. Stained live rods, as well as most freshly detached outer segments, are only weakly fluorescent, but they become brightly fluorescent within a few seconds if their plasma membranes are osmotically ruptured. The difference in staining of rod and cones suggests that disk membranes of rods are not continuous with the plasma membranes are osmotically ruptured. The difference in staining of rod and cones plasma membrane on outer segments of photoreceptors in electrophysiological and biochemical experiments, and to study the infolding pattern of rod and cone disks.

Animals↗
PubMed1972

Kinetics of the photocurrent of retinal rods.

The shapes of the photocurrent responses of rat rods, recorded with microelectrodes from the receptor layer of small pieces of isolated retinas, have been investigated as a function of temperature and of stimulus energy. Between 27 and 37 degrees C the responses to short flashes can be described formally as the output of a chain of at least four linear low-pass filters with time constants in the range 50-100 msec. The output of the filter chain is then distorted by a nonlinear amplitude-limiting process with a hyperbolic saturation characteristic. Flashes producing approximately 30 photons absorbed per rod yield responses of half-maximal size independently of temperature. The maximum response amplitude is that just sufficient to cancel the dark current. The rate of rise of a response is proportional to flash energy up to the level of 10(5) photons absorbed per rod, where hyperbolic rate saturation ensues. The responses continue to increase in duration with even more intense flashes until, at the level of 10(7) photons absorbed per rod, they last longer than 50 min. The time-courses of the photocurrent and of the excitatory disturbance in the rod system are very similar. The stimulus intensity at which amplitude saturation of the photocurrent responses begins is near that where psychophysical "rod saturation" is seen. An analysis of these properties leads to the following conclusions about the mechanism of rod excitation. (a) The kinetics of the photocurrent bear no simple relation to the formation or decay of any of the spectroscopic intermediates so far detected during the photolysis of rhodopsin. (b) The forms of both the amplitude- and rate-limiting processes are not compatible with organization of rhodopsin into "photoreceptive units" containing more than 300 chromophores. Even at high stimulus intensities most rhodopsin chromophores remain connected to the excitatory apparatus of rods. (c) The maximum rate of rise of the photocurrent is too fast to be consistent with the infolded disks of a rod outer segment being attached to the overlying plasma membrane. Most of the disks behave electrically as if isolated within the cell. (d) Control of the photocurrent at the outer segment membrane is not achieved by segregation of the charge carriers of the current within the rod disks. Instead, it is likely to depend on control of the plasma membrane permeability by an agent released from the disks.

Animals↗
PubMed1970

Dark current and photocurrent in retinal rods.

The interstitial voltages, currents, and resistances of the receptor layer of the isolated rat retina have been investigated with arrays of micropipette electrodes inserted under direct visual observation by infrared microscopy. In darkness a steady current flows inward through the plasma membrane of the rod outer segments. It is balanced by equal outward current distributed along the remainder of each rod. Flashes of light produce a photocurrent which transiently reduces the dark current with a waveform resembling the PII and a-wave components of the electroretinogram. The photocurrent is produced by a local action of light within 12 mum of its point of absorption in the outer segments. The quantum current gain of the photocurrent is greater than 10(6). The electrical space constant of rat rods is greater than 25 mum, so that the electrical effects of the photocurrent are large enough at the rod synapses to permit single absorbed photons to be detected by the visual system. The photocurrent is apparently the primary sensory consequence of light absorption by rhodopsin.

Animals↗
PubMed1968

Membrane origin of the fast photovoltage of squid retina.

When a bright light flash is absorbed by a small region in the outer segments of squid photoreceptors fixed in glutaraldehyde, a brief pulse of membrane current flows locally. The passive spreading of this current along the outer segments produces the photochemical component of the "early receptor potential." The source of the current lies electrically in parallel with the cell membranes and perhaps is located within them. Fixation with glutaraldehyde apparently does not reduce the resistance of the cell membrane to less than 5 percent of its value in live cells.

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