A mathematical model for the components of the receptor current in Limulus ventral nerve photoreceptors

Contzen, Klaus; Nagy, K.

doi:10.1016/1011-1344(96)07306-x

Cited by 6 publications

(11 citation statements)

References 27 publications

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“…In untreated dark-adapted normal cells, the time constant t of the decay of the C 1 component evoked by a brief flash varies between 110 and 150 ms. The time constant of the C 2 component is between 20 and 50 ms and that of the C 3 component is between 50 and 80 ms (Contzen & Nagy, 1996). In light-adapted cells t decay of C 1 is somewhat smaller, about 80 to 110 ms (unpublished results).…”

Section: Identification Of the Photocurrent In The Unusual Cellsmentioning

confidence: 82%

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Photoreceptor cells with unusual functional properties on the ventral nerve of Limulus

et al. 1999

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Normal photoreceptor cells on the ventral nerve of Limulus respond to a moderately intense flash with a large receptor potential or current. Occasionally, cells are found in which the same flash evokes only a small receptor potential or current. Our investigations reveal physiological reasons for the poor light sensitivity in these "unusual cells." In unusual cells prolonged illumination with intense light evokes a step-like inward current with an amplitude of some nanoamperes, but without a large transient peak. The current appears to be summed up of single photon responses with amplitudes smaller than about 50 pA. Their time course is similar to that of small single photon responses forming the so-called macroscopic C1 component in normal cells. The macroscopic current evoked by an intense flash has slow activation and deactivation kinetics and reaches a saturated amplitude of about 4-5 nanoamperes. The light-intensity dependence of the current evoked by flashes or by prolonged illumination has a slope of about 1 in log-log plots. The decay kinetics of the current is similar to that of the C1 component measured in normal cells after the block of the C2 component. Occasionally, the step-like current is superposed by large standard bumps. These bumps are blocked by the Ca2+-ATPase inhibitor cyclopiazonic acid, while the sustained inward current persists. We conclude that in unusual cells the light-activated current is identical to the C1 component of normal cells. The phospholipase C pathway that in normal cells presumably gives rise to the C2 component functions only with a low efficiency in unusual cells.

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Section: Identification Of the Photocurrent In The Unusual Cellsmentioning

confidence: 82%

“…When the C 2 component is inhibited, the amplitudes of C 1 and C 3 are usually affected too. It thus seems that calcium is necessary for the normal operation of the mechanisms underlying these com-ponents Contzen & Nagy, 1996;Nagy & Contzen, 1997).…”

Section: Introductionmentioning

confidence: 99%

Photoreceptor cells with unusual functional properties on the ventral nerve of Limulus

et al. 1999

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“…The C3 component has slow kinetics and is activated only at high light in tensities in the deeply dark-adapted photorecep tor. It then dominates the photoresponse and de termines the maximally possible current amplitude of the photoreceptor (Deckert et al, 1992;Contzen and Nagy, 1996). Thus, we can speculate that when the internal calcium stores were de pleted by CPA and no external calcium was avail able, this component could probably still be acti vated.…”

Section: Depleting the Internal Calcium Storesmentioning

confidence: 99%

Effects of Calcium and Cyclopiazonic Acid on the Photoresponse in the Limulus Ventral Photoreceptor

Dorlöchter

Yuan

Stieve

1999

Zeitschrift Für Naturforschung C

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Phototransduction, Current Components, Light Adaptation, Bumps, CPA 1. Single-photon responses (bumps) and small macroscopic photocurrents were studied in ventral photoreceptors of the horseshoe crab Limulus. Lowering the calcium concentration in the bath from 10 mM to 250 fiM led to increased bump size. Adaptation of the cells by a moderately bright conditioning flash was not impaired.2. Pressure-injection of 1.2 mM EG TA into the dark-adapted cells resulted in reduced bump size. EG TA weakened the effect of the conditioning light flash although it did not completely abolish light adaptation.3. The microsomal calcium-ATPase inhibitor cyclopiazonic acid strongly desensitized the cells, and bumps were suppressed below detection. When the bathing saline contained 10 m M calcium, macroscopic photoresponses after extracellular application of the agent had ampli tudes smaller than under control conditions but normal response kinetics: The response to a light step still consisted of a fast transient photocurrent and a much smaller plateau. How ever, when applied in calcium-free bathing saline, cyclopiazonic acid additionally influenced the waveform of the photoresponse. The clear distinction between transient and plateau was no longer possible, and the photocurrent appeared "square".4. Our results support the idea that a transient elevation of the cytosolic calcium concentra tion is obligatory for light adaptation in the ventral photoreceptor. It is also obligatory for the generation of the so-called C2 component of the photocurrent which is represented by "standard" bumps and the fast transient phase of a prolonged response. However, a rise in cytosolic calcium appears not necessary for the initiation of a slow electrical photoresponse.

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“…Therefore it seems unlikely that this calcium-carrying C l current causes the supralinearity of the slow rising phase of the arsenazo signal. A receptor current attrib uted to the C2 component showed supralinearity in response-versus-stimulus intensity curves (Deckert et al, 1992;Contzen and Nagy, 1996). Since the C2 component is the only component were supralinearity of the receptor current was ob served we assume that C2 is partially carried by calcium, and causes supralinearity of the slow ris ing phase in arsenazo signals.…”

Section: Stimulus Intensity and Photoresponsementioning

confidence: 99%

“…the re sponse increases disproportionate to the stimulus intensity (Brown and Coles, 1979;Stieve et al, 1986;Stieve and Schlösser, 1989). More recently it has been reported that this supralinearity can be found only in one of three different receptor cur rent components (Deckert, Nagy, Helrich and Stieve, 1992;Contzen and Nagy, 1996). In the pre sent study we measured the light-stimulated electrical response simultaneously with the tran sient change of intracellular free Ca2+ or Sr2+ con centration.…”

Section: Introductionmentioning

confidence: 99%

Cytosolic Calcium Transient and Light Response in Limulus Ventral Nerve Photoreceptors: Supralinearity in Response-Intensity Curves

1997

Zeitschrift Für Naturforschung C

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Light-evoked intracellular electrical responses were measured simultaneously with the changes in intracellular free Ca2+ or Sr2+ concentration by means of arsenazo III. The response-versus-stimulus intensity characteristics was measured in two states of relative light adaptation in strontium-containing saline (10 mM Sr2+; 0.25 mM Ca2+). Supralinear parts of the curves with slopes > 1 were found for the receptor current amplitude and the current time integral as well as for the amplitude of the arsenazo signal within at least 0.3 log units of identical stimulus intensity range. Maximum slopes of response-versus-intensity curves are on average approximately 1.7 for the receptor current and the arsenazo signal in both adaptational states. The rise in time of the arsenazo signal evoked by a light flash shows two phases. In strontium saline we observe a fast increase at the onset of the arsenazo signal which is detectable within the first 20-100 ms of the signal rise and a slow further increase to the maximum, with a duration of 0.7-6 s. In the detailed analysis of both phases we observed that the amplitude of each phase increases supralinear, i.e. disproportionate to the stimulus intensity. The fast rising phase of the arsenazo signal is not visibly voltage-dependent. This fast rising phase might be a monitor of an IP3-sensitive calcium or strontium release. Since the amplitude of the fast rising phase increases supralinear with the stimulus intensity, we propose the interpretation that more than one IP3-molecule must bind to the IP3-receptor to open this channel of the intracellular calcium store. Only the slow rising phase is voltage-dependent. The kinetic of this slow rising phase was accelerated with more negative membrane potentials. This predominant slow phase correlates with the receptor current-time integral. Therefore the slow rising phase of the arsenazo signal might be a monitor for influx of calcium or strontium through light activated ion channels in the plasma membrane. The supralinearity of the amplitude of the slow rising phase might indicate the supralinear increase in calcium or strontium, which is carried by the receptor current. A ddi tionally, we calculated the fraction of strontium and calcium ions of the total receptor current. When calcium was replaced by strontium in the superfusate about 3-30% of the receptor current are carried by strontium and calcium ions. In physiological saline calcium contributes about 1-1.5% to the receptor current.

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A mathematical model for the components of the receptor current in Limulus ventral nerve photoreceptors

Cited by 6 publications

References 27 publications

Photoreceptor cells with unusual functional properties on the ventral nerve of Limulus

Photoreceptor cells with unusual functional properties on the ventral nerve of Limulus

Effects of Calcium and Cyclopiazonic Acid on the Photoresponse in the Limulus Ventral Photoreceptor

Cytosolic Calcium Transient and Light Response in Limulus Ventral Nerve Photoreceptors: Supralinearity in Response-Intensity Curves

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