Ionic Mechanisms in Excitation of Photoreceptors

Brief illumination of the isolated toad retina produces a transient increase in the extracellular free Ca concentration in the photoreceptor layer. This change in concentration arises from the release of Ca.+ by the rod outer segments.The release begins within 200 msec after the stimulus flash. The peak amplitude of the concentration change increases linearly with light intensity up to approximately 100 absorbed photons per rod and saturates at about 107 absorbed photons per rod. In the linear range, the amount of Ca released corresponds to about 2 X 104 Ca2+ per absorbed photon per rod. The high stoichiometry of this release, in relation to the low free intracellular MATERIALS AND METHODSMaterials. The organophosphate used in the electrodes (11) was the Ca salt of bis(di-n-octyl)phenyl phosphate (HDOPP) (the kind gift of S. Firstenberg, Radiometer, Copenhagen, Denmark). Di-octylphenyl phosphonate (DOPP) was obtained from Specialty Organics (Irwindale, CA). Tetrahydrofuran and poly(vinyl) chloride (PVC) were purchased from Aldrich. Li-Al hydride was added to the tetrahydrofuran to prevent accumulation of peroxides. Na tetraphenylboron was obtained from Sigma. The water used throughout was distilled and deionized.Electrode Construction. The Ca-selective membrane was made by mixing HDOPP with DOPP, 1:10 (wt/wt), and allowing the solution to stand overnight at room temperature. The solution was then incorporated into nonporous PVC (12) as follows. PVC (0.39 g) was dissolved in 13 ml of tetrahydrofuran and 0.83 g of the HDOPP/DOPP solution and 5.2 mg of Na tetraphenylboran were added. The resulting solution was poured into a clean glass petri dish (diameter, 9.5 cm), covered with several layers of filter paper, and allowed to dry for at least 24 hr. The resulting PVC membrane was 100,gm thick.For measurements from the isolated retina, a special electrode was constructed. Two holes were drilled through the top of a cylindrical PVC cap (diameter, 1.3 cm) and a disc (diameter, 0.8 cm) of the Ca membrane was glued over the holes with 0.3% PVC in tetrahydrofuran (Fig. 1). One of the holes was made into a reference electrode by cutting its overlaying PVC membrane and inserting into it polyethylene tubing containing 2% agar in Ringer's solution. The assembled PVC cap was glued onto the end of a U-shaped plexiglass tube. The tube was then filled with a Ca standard solution which contacted the lower surface of the uncut (active) membrane. The Ca electrode potential was recorded between Ag/AgCl pellets placed in the reference tubing and in the Ca standard solution with a lownoise differential amplifier (model 113, PAR Instruments, Princeton, NJ).Electrode Properties. As originally reported by Ruzicka et al. (11), the Ca membrane electrodes exhibited nearly Nernstian behavior-i.e., 29 mV per decade change in Ca concentration, down to <1 gM Ca. The electrodes exhibited a high selectivity for Ca over other biologically important ions [-log KCa/Na = 5.2; -log KCa/K = 5.7; -log Kca/Mg = 3.6 (11)]. In addition, the elect...

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“…The similarity between the isolated receptor potential that we observed and the rod photocurrent in normal Ringer's solution (19) (20,21).…”

Section: I6supporting

confidence: 67%

Light-induced calcium release by intact retinal rods.

Gold¹,

Korenbrot²

1980

Proc. Natl. Acad. Sci. U.S.A.

168

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“…If we assume that -75 mV is the K+ equilibrium potential, then the intracellular K+ concentration would be 27 mM (see also ref. 4). To preserve osmotic balance, the intracellular Na+ concentration should not exceed 90 mM, a concentration that would set the Na+ equilibrium potential to +5 mV.…”

Section: Methodsmentioning

confidence: 99%

“…The uniform organization of the vertebrate photoreceptor layer has aided in the detection of an ionic current that flows extracellularly from the inner segment toward the outer segment of the photoreceptor during darkness (1). Ion substitution experiments indicate that this current is carried by sodium ions which, in the dark, enter the outer segment and maintain the photoreceptor in a depolarized state (2)(3)(4). Light stimulation interrupts this current and hyperpolarizes the photoreceptor's membrane (1,5,6).…”

mentioning

confidence: 99%

Responses to light of solitary rod photoreceptors isolated from tiger salamander retina.

Bader¹,

MacLeish²,

Schwartz³

1978

Proc. Natl. Acad. Sci. U.S.A.

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The mechanism whereby light generates an electrical signal in photoreceptors has been extensively studied in the intact retina. The uniform organization of the vertebrate photoreceptor layer has aided in the detection of an ionic current that flows extracellularly from the inner segment toward the outer segment of the photoreceptor during darkness (1). Ion substitution experiments indicate that this current is carried by sodium ions which, in the dark, enter the outer segment and maintain the photoreceptor in a depolarized state (2-4). Light stimulation interrupts this current and hyperpolarizes the photoreceptor's membrane (1, 5, 6). Intracellular recording indicates that the cessation of this current is accompanied by a conductance decrease in at least some preparations (7,8). It seems, therefore, that an effect of light on photoreceptors is to decrease a sodium conductance that is high when the photoreceptor is in the dark.The light response, however, is not shaped solely by the light-sensitive conductance changes; intracellular recordings from photoreceptors have shown that the amplitude of the response can be increased when the size of the illuminating spot is increased, even though the irradiance is kept constant (9, 10). Concomitantly, the time course of the response is altered. These changes have been attributed to interactions between photoreceptors (9-11), to voltage-dependent membrane conductances (10, 11), and, for cone photoreceptors, to feedback from second-order neurons (9). Electrical synapses have been proposed to mediate the interaction between photoreceptors (11-15). This notion is supported by experiments showing that cobalt ions, which are assumed to block chemical synaptic transmission, have no detectable effect on the interaction between rod photoreceptors (13). Also, gap junctions (15-17), but not chemical synapses, have been described between photoreceptors in several species. Thus photoreceptors in the intact vertebrate retina form a syncytium. Consequently, current can

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“…The osmolarity of the buffers was 0 048 osmol. The content of Na+ and K+ in the medium was modelled on the ionic composition found by Hagins & Yoshikami (1975) in frog rod outer segments.…”

Section: Methodsmentioning

confidence: 99%

Effect of pH on the formation and decay of the metarhodopsins of the frog.

Baumann¹,

Zeppenfeld²

1981

The Journal of Physiology

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SUMMARY1. Suspensions of membrane vesicles were prepared by sonication of dark-adapted frog rod outer segments. The pH was adjusted to one of twelve values between 5 0 and 9 8; the temperature was 15 'C. Each preparation was exposed to a single yellow flash of 120 Pts duration, and rapid and slow changes of absorbance were measured at 475 nm wavelength.2. Rapid changes consist of a transient rise followed by a diphasic decay to a new level of absorbance which is lower than that before the light exposure. The level of absorbance reached at the end of the rapid changes is lower at lower pH.3. Kinetic analysis reveals that three reactants take part in the rapid reactions, viz. metarhodopsin I and two isochromic forms of metarhodopsin II named metarhodopsin II' and metarhodopsin II". The kinetics of the reversible transition from metarhodopsin I to metarhodopsin II' are not measurably influenced by the pH of the medium. However, the reversible reaction of metarhodopsin II' with metarhodopsin II" is dependent on pH because metarhodopsin II" is encountered either as a protonated or as a deprotonated compound.4. Slow reactions are due to metarhodopsin I and to metarhodopsin III. A combined quantitative analysis of both rapid and slow reactions involves a reversible reaction between metarhodopsin II' and metarhodopsin III and a hydrolysis of metarhodopsin II' to retinal and opsin. The scheme employed accounts for the pH dependence of all equilibria although only one of the metarhodopsins reacts directly with protons.

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Ionic Mechanisms in Excitation of Photoreceptors

Cited by 94 publications

References 28 publications

Light-induced calcium release by intact retinal rods.

Light-induced calcium release by intact retinal rods.

Responses to light of solitary rod photoreceptors isolated from tiger salamander retina.

Effect of pH on the formation and decay of the metarhodopsins of the frog.

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