Actions of EGTA and high calcium on the cones in the turtle retina.

Bertrand, Daniel; Fuortes, M. G. F.; Pochobradsky, J.

doi:10.1113/jphysiol.1978.sp012198

Cited by 41 publications

(21 citation statements)

References 16 publications

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“…5). Second, evidence from our experiments and from other work (Yoshikami & Hagins, 1973;Hagins & Yoshikami, 1974Bertrand et al 1978;Wormington & Cone, 1978;Bastian & Fain, 1979 Fig. 10) all cross the curve measured under normal conditions.…”

Section: Discussionsupporting

confidence: 82%

The effects of low calcium and background light on the sensitivity of toad rods

Bastian¹,

Fain²

1982

The Journal of Physiology

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1. We have examined the effects of decreases in extracellular Ca2+ concentration on the intracellularly recorded light responses of rods from the toad, Bufo marinus. In agreement with previous results (Brown & Pinto, 1974; Lipton, Ostroy & Dowling, 1977), Ca2+ concentrations below 10−6 M produced a depolarization of rod resting membrane potential of approximately 30‐40 mV and a corresponding increase in the maximum amplitude of the rod's light responses, so that saturating flashes in normal and low Ca2+ Ringer produced hyperpolarizations to approximately the same membrane potential. 2. The rod's sensitivity was reduced in low Ca2+ Ringer by an amount dependent upon the extracellular Ca2+ concentration. At 10−6 M‐Ca2+, sensitivity was approximately 0·6 log units below normal. Thereafter, it dropped nearly linearly with [Ca2+]o to a value approximately 4·0 log units below normal at 10−9 M‐Ca2+. Most of the decline occurred within 1‐2 min after the solution change as the membrane potential depolarized, but sensitivity continued to fall slowly with time at the lowest Ca2+ concentrations. Exposure to low Ca2+ solutions altered the kinetics of the receptor response to brief flashes, delaying response onset and time‐to‐peak but affecting the time course of decay very little. 3. The sensitivity of the rod to maintained steps of light was also reduced in low Ca2+. Furthermore, the changes in sensitivity produced by background illumination were very much smaller in low Ca2+ than in normal Ringer. In some cases backgrounds actually increased sensitivity. 4. In 10−8 M‐Ca2+, backgrounds which themselves produced no response in the rod and no changes in rod sensitivity produced large decreases in response latency for responses of all amplitudes, and pronounced changes in time‐to‐peak and time‐to‐decay for moderate and large amplitude responses. 5. Since the effects of background light and low Ca2+ on the wave form of the rod are distinct and in some cases antagonistic, and since the changes in receptor sensitivity produced by backgrounds and low Ca2+ are not additive, the decreases in sensitivity produced by exposure to low Ca2+ appear to be caused by a mechanism distinct from normal light adaptation. We suggest that they are caused by an increase in the buffering capacity of the receptor cytosol for Ca2+ and that Ca2+ is the excitatory messenger or ‘internal transmitter’, as originally suggested by Yoshikami & Hagins (1971).

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Section: Discussionsupporting

confidence: 82%

The effects of low calcium and background light on the sensitivity of toad rods

Bastian¹,

Fain²

1982

The Journal of Physiology

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“…Another possibility is that the effects depended critically on low-Ca2A-induced modifications of photoreceptor responses. It is well known that the amplitude of the response induced by bright flashes in photoreceptors increases in low-Ca 2 media as a consequence of modifications of the intracellular level of cyclic GMP, the second messenger of phototransduction in vertebrate photoreceptors (24,28). Even if modifications of photoreceptor light responses and membrane potential induced by low Ca2+ influence some of the results of our experiments, they cannot be the main cause of the response recovery after Zn2+, Ni2+, or Co>2 block.…”

Section: Discussioncontrasting

confidence: 49%

Low-calcium-induced enhancement of chemical synaptic transmission from photoreceptors to horizontal cells in the vertebrate retina.

Piccolino

Byzov

Kurennyi

et al. 1996

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

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According to the classical calcium hypothesis of synaptic transmission, the release of neurotransmitter from presynaptic terminals occurs through an exocytotic process triggered by depolarization-induced presynaptic calcium influx. However, evidence has been accumulating in the last two decades indicating that, in many preparations, synaptic transmitter release can persist or even increase when calcium is omitted from the perfusing saline, leading to the notion of a "calcium-independent release" mechanism. Co2+, Ni2+, and Zn2+ is to a large extent accounted for by a depolarizing shift of the Ca2+ current activation curve, by means of an interaction with the negative fixed charges on the photoreceptor presynaptic membrane. Presynaptic depolarization with extrinsic currents, or reduction of extracellular Ca2+, relieves the transmission block because these conditions bring presynaptic transmembrane potential to levels where Ca>2 current can still be activated. Our results may give new insights into the problem of so-called "calcium-independent synaptic transmission" (6, 7). MATERIALS AND METHODSLight Responses in Retinal Eyecups. Eyecups were prepared from the retina of turtles (Pseudemys scripta elegans) or salamanders (Ambystoma tigrinum) and continuously superfused with a modified Ringer saline bubbled continuously with a mixture of 95% 02 and 5% Co2. The composition (in mM) of the saline used in turtle experiments was the following: NaCl, 110; KCI, 2.6; NaHCO3, 22; MgCl2, 2; CaCl2, 2; Dglucose, 10 (pH 7.4). Salamander saline had the followingThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.composition: NaCl, 95; KCI, 2.5; NaHCO3, 30; MgCl2, 2; CaCl2, 2; D-glucose, 10 (pH 7.6). Intracellular recording of the light responses induced by 5-mm-diameter white light spots of variable intensity and duration were made in HCs (and in other retinal neurons) by using fine tip microelectrodes prepared with a Brown-Flaming puller (Sutter Instruments, Novato, CA). The concentration of Ca2> in the nominally zero Ca2> Ringer solution, without EGTA added, was less than 10 ,uM. The pH of the solutions containing EGTA or EDTA was readjusted with NaOH. The concentration of Ca2> in media containing CaCl2 in variable proportions and EGTA (2-10 ,uM) or EDTA (2-5 ,.tM) ranged from 0.45 to 2.4 nM and was estimated by using the computer program SOL i.d. written by Eric Ertel (Hoffmann-La Roche). To keep a normal concentration of Mg2+ in media containing EGTA the proportion of MgCl2 in the perfusing saline was adjusted accordingly. Chemicals were obtained from Sigma. Bicuculline was used as the soluble methobromide salt and added to the perfusing medium just before application to the retina. Picrotoxin was first dissolved in slightly basic, hot distilled water and then added to the Ringer solution. Neutral density filters were used to attenuate light intensity. The flux de...

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“…Although the crossing-over of intensity-response curves was not observed by Lipton et al (1977) in their experiments on toad rods (see their Fig. 3), a similar phenomenon has recently been demonstrated for cones in the turtle eyecup superfused with Ringer containing 5 mM-EGTA (Bertrand et al 1978).…”

Section: Discussionmentioning

confidence: 89%

Light adaptation in toad rods: requirement for an internal messenger which is not calcium.

Bastian¹,

Fain²

1979

The Journal of Physiology

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SUMMARY1. The mechanism of light adaptation was investigated by recording intracellularly from single rods in the isolated, superfused retina of the toad, Bufo marines. Steady background lights produce decreases in rod sensitivity and changes in response wave form similar to those previously observed in the toad eyecup.2. The sensitivity of a dark-adapted rod is halved by a background light which bleaches about 4 rhodopsins per rod per second. Since a toad rod contains over 2000 disks, a rhodopsin bleached in one disk must alter the effectiveness of rhodopsins bleached in others. This could occur if the state of adaptation in the rod were regulated by the concentration of some diffusable substance.3. This diffusable substance cannot be Ca2+. Increases in intracellular Ca2+, produced experimentally either by increasing extracellular Ca2+ or by facilitating Ca2+ permeability into the rod with the ionophore X537A, cause a hyperpolarization of membrane potential and a decrease in response amplitude; but they do not produce changes in sensitivity and response wave form like those produced by background light.4. Either Ca2+ is not the internal transmitter released from the disks during excitation, or the disks release or otherwise alter the concentration of a second diffusable substance, in addition to Ca2 , which regulates the state of adaptation.

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Actions of EGTA and high calcium on the cones in the turtle retina.

Cited by 41 publications

References 16 publications

The effects of low calcium and background light on the sensitivity of toad rods

The effects of low calcium and background light on the sensitivity of toad rods

Low-calcium-induced enhancement of chemical synaptic transmission from photoreceptors to horizontal cells in the vertebrate retina.

Light adaptation in toad rods: requirement for an internal messenger which is not calcium.

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