Isopotentiality and an optical determination of series resistance in Limulus ventral photoreceptors.

Brown, J. E.; Harary, HH; Waggoner, Alan S.

doi:10.1113/jphysiol.1979.sp013010

Cited by 27 publications

(24 citation statements)

References 24 publications

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“…Fernandez & Nickel (1976) made an estimate based on both methods in crayfish and Muri, Coles & Baumann (1976) (Fioravanti & Fuortes, 1972). Because of this striking contrast, and because of the scatter of our results (particularly the ratio of C measured in the dark and in the light) it was desirable that the conclusion should be confirmed by other methods: this has been done by Brown et al (1979) Spread of excitation from a photoisomerized rhodopsin molecule It has been shown that in the photoreceptor of the leech the light-induced increase in membrane conductance occurs on, or close to, the microvilli (Fioravanti & Fuortes, 1972). In squid and honey bee drone, experiments with localized illumination have shown that the light-induced increase in membrane conductance is confined to regions of the cell not more than 10-20 ,um from the site of light absorption and that there are no marked variations in light sensitivity along the length of the rhabdome (Hagins, Zonana & Adams, 1965;Carreras, 1978).…”

Section: Discussionmentioning

confidence: 96%

“…The present work addresses the question of how this saturation occurs. METHODS Photoreceptor cells of the superfused ventral rudimentary eye of Limulu8 polyphemu8 were impaled with two micro-electrodes much as described by Millecchia & Mauro (1969b) (see also Brown et al 1979). The superfusate was an artificial sea water of composition: NaCl, 423 mM; KCl, 10 mM; MgCl2, 22 mM; MgSO4, 26 mM; CaC12, 10 mm; Tris Cl buffer, 10 mM; pH 7-8.…”

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Saturation of the response to light in Limulus ventral photoreceptor.

Brown¹,

Coles²

1979

The Journal of Physiology

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SUMMARY1. Limulus ventral photoreceptor cells were voltage-clamped with two intracellular micro-electrodes. The light-induced membrane current was recorded for brief stimuli. From observation of discrete waves (quantum bumps) at low stimulus energies and the early receptor potential at high energies, the stimulus energy was related to the number of rhodopsin molecules photoisomerized.2. In the dark-adapted cell the log (peak light-induced current) reached almost its maximum value when about 103 of the 109 rhodopsin molecules in the cell were photoisomerized.3. The magnitude of the maximum light-induced current was not significantly altered after iontophoresis of EGTA into the cell. This treatment is known to counteract the Ca2+-mediated reduction in sensitivity to light.4. Current pulses were injected into the unclamped cell during the receptor potential. The form of the voltage deflexion (a step followed by a curve) suggested that the effective electrical equivalent of the cell was a membrane capacitance in parallel with a light-dependent membrane resistance, Bm, and in series with another, light-invariant, resistance, RB. R8 ranged from 7 to 24 kQ (five cells).5. During a receptor potential the ratio Rm/Rs was never observed to fall below 1*7 no matter how intense the light flash. Hence, it is concluded that the lightinduced current saturated essentially because RM fell to a minimum value.6. Charging curves gave a value for the capacitance, and hence the area, of the surface membrane. From this it was estimated that there were 105-106 microvilli on each cell.7. These results show that the light-induced increase in membrane conductance in a dark-adapted cell comes close to its maximum value when the number of photoisomerizations is about 1/1000 the total number of microvilli. We suggest that absorption of a photon by a rhodopsin molecule in a microvillus causes an increase in membrane conductance on parts of the surface membrane beyond that microvillus.8. In the presence of moderate background illumination the sensitivity to nonsaturating superimposed flashes was greatly decreased (e.g. by 103) while the saturating light-induced current was only slightly decreased (e.g. by 15 %).

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Section: Discussionmentioning

confidence: 96%

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confidence: 99%

Saturation of the response to light in Limulus ventral photoreceptor.

Brown¹,

Coles²

1979

The Journal of Physiology

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“…1 W cm-2 corresponded to approximately 2 x 1010 photons 8-1 absorbed by the cell. Micropipettes filled with 2 M-KCI were used to record membrane voltage and to pass voltage-clamp current as described previously (Millecchia & Mauro, 1969a,b;Brown, Harary & 382 CALCIUM AND PHOTORECEPTOR EXCITATION Waggoner, 1979). Ionophoretic injections were made into a voltage-clamped cell from a third intracellular micropipette.…”

Section: Electrophysiologymentioning

confidence: 99%

“…The factor by which the peak of the receptor potential initially increased was in the range 12-9. Some cells were impaled by two or three micropipettes and the sensitivity (defined as the peak light-induced current per unit irradiance) was measured by a voltage clamp (Brown et al 1979 (Millecchia & Mauro, 1969a). Therefore we do not ascribe all the increased responsiveness to an artifact of impalement.…”

Section: Cells Bathed In Egta-aswmentioning

confidence: 99%

Calcium ion, an intracellular messenger of light adaptation, also participates in excitation of Limulus photoreceptors.

Bolsover¹,

Brown²

1985

The Journal of Physiology

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SUMMARY1. Photoreceptor cells of Limulus ventral eyes were bathed in artificial sea water (ASW) that contained 10 mM-EGTA and no added Ca2+ (EGTA-ASW). Test flashes elicited responses that increased to a maximum size within 10 min in EGTA-ASW but did not change further when dark-adapted cells were bathed for an additional 35 min in this solution.2. Light responses progressively declined from this maximum size if the cells were repetitively illuminated in EGTA-ASW. In this state of reduced responsiveness, response amplitudes were further reduced by intracellular ionophoretic injection of EGTA; response amplitudes were increased by intracellular ionophoretic injection of Ca2+. Both of these findings are opposite to what is normally observed for cells bathed in ASW. Also, after repetitive illumination in EGTA-ASW, both the slope of the response versus intensity relationship became steeper and light responses often had a delayed increase in amplitude.3. The light responses and the response versus intensity relation returned to normal when the bathing medium was changed back to ASW containing 10 mM-Ca2+.4. The light-induced rise in luminescence recorded from photoreceptors injected with the photoprotein aequorin (the 'aequorin response') declined by at most 50 % after dark-adapted photoreceptors were bathed in EGTA-ASW for 45 min. However, the aequorin response progressively declined by 98 % if cells were repetitively illuminated while bathed in EGTA-ASW. 5. The total intracellular Ca content of whole end-organs was measured by atomic absorption spectroscopy. Total intracellular Ca content did not change significantly while photoreceptors were bathed in EGTA-ASW even after repetitive illumination.6. We suggest that cytosolic Ca2+ is required by one or more steps in the mechanisms that link rhodopsin isomerization to both (i) an increase in the conductance of the cell membrane to Na+ and (ii) a release of Ca2+ from a light-labile store.

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“…It is now possible to optically measure changes in the membrane potential (for reviews see COHEN and SALZBERG,1978;WAGGONER,1979).Merocyaninerhodanine dyes have been used as rapid molecular probes to monitor action potentials in various nervous systems (WOOLUM andSTRUMWASSER,1978 COHEN et al,1978;GRINVALD et al,1981;Ross and REICHARDT,1979;BROWN et al, 1979).In addition,the merocyanine-rhodanine dyes were also applied to contractile tissues. BAYLOR and CHANDLER(1978)and NAKAJIMA and GILAI(1980) measured optical action potential in frog skeletal muscle stained with a merocyaninerhodanine dye(Dye XVII),and spontaneous action potentials were monitored in early embryonic chick hearts in normal solution FuJII et al, 1980FuJII et al, ,1981a.…”

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Optical recording of conducted action potential in heart muscle using a voltage-sensitive dye.

et al. 1981

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Isopotentiality and an optical determination of series resistance in Limulus ventral photoreceptors.

Abstract: 3. From these measurements, as well as from calculations for a model spherical cell, we conclude that membrane potential can be controlled to within 2 mV using our micropipette 'point clamp' methods, for all but the brightest stimuli.

Cited by 27 publications

References 24 publications

Saturation of the response to light in Limulus ventral photoreceptor.

Saturation of the response to light in Limulus ventral photoreceptor.

Calcium ion, an intracellular messenger of light adaptation, also participates in excitation of Limulus photoreceptors.

Optical recording of conducted action potential in heart muscle using a voltage-sensitive dye.

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