scholarly journals Kinetics of Turn-offs of Frog Rod Phototransduction Cascade

2008 ◽  
Vol 132 (5) ◽  
pp. 587-604 ◽  
Author(s):  
Luba A. Astakhova ◽  
Michael L. Firsov ◽  
Victor I. Govardovskii
Keyword(s):  
Time Constant ◽  
Dark Current ◽  
Main Part ◽  
Time Course ◽  
Light Adaptation ◽  
Key Factors ◽  
Time Courses ◽  
Kinetics Of ◽  
Phototransduction Cascade ◽  
Effect Of Light

The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca2+-clamping solution. A flash applied in the dark-adapted state elicits a wave of PDE activity whose rising and decaying phases have characteristic times near 0.5 and 2 seconds, respectively. Nonsaturating steady background shortens both phases roughly to the same extent. The acceleration may exceed fivefold at the backgrounds that suppress ≈70% of the dark current. The time constant of the process that controls the recovery from super-saturating flashes (so-called dominant time constant) is adaptation independent and, hence, cannot be attributed to either of the processes that shape the main part of the PDE wave. We hypothesize that the dominant time constant in frog rods characterizes arrestin binding to rhodopsin partially inactivated by phosphorylation. A mathematical model of the cascade that considers two-stage rhodopsin quenching and transducin inactivation can mimic experimental PDE activity quite well. The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding. This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

scholarly journals Kinetics of intramembrane charge movement and sodium current in frog node of Ranvier.

1982 ◽  
Vol 79 (4) ◽  
pp. 571-602 ◽  
Author(s):  
J M Dubois ◽  
M F Schneider
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Sodium Current ◽  
Node Of Ranvier ◽  
Charge Movement ◽  
Intramembrane Charge Movement ◽  
Time Courses ◽  
Charge Movements ◽  
Kinetics Of ◽  
Microsecond Pulse

Intramembrane charge movement (Q) and sodium current (INa) were monitored in isolated voltage-clamped frog nodes of Ranvier, ON charge movements (QON) for pulses from the holding potential (-100 mV) to potentials V less than or equal to 0 mV followed single exponential time courses, whereas two exponentials were found for pulses to V greater than or equal to 20 mV. The voltage dependence of both QON and its time constant tauON indicated that the two ON components resolved at V greater than or equal to 20 mV were also present, though not resolvable, for pulses to V less than or equal to 0 mV. OFF charge movements (QOFF) monitored at various potentials were well described by single exponentials. When QOFF was monitored at -30 or -40 mV after a 200-microsecond pulse to +20 mV and QON was monitored at the same potential using pulses directly from -100 mV, tauON/tauOFF = 2.5 +/- 0.3. At a set OFF potential (-90 to -70 mV), tauOFF first increased with increasing duration tON of the preceding pulse to a given potential (0 to +30 mV) and then decreased with further increases in tON. The declining phase of tauOFF followed a time course similar to that of the decline in QOFF with tON. For the same pulse protocol, the OFF time constant tauNa for INA also first increased with tON but then remained constant over the tON interval during which tauOFF and QOFF were declining. After 200- or 300-microsecond pulses to +20, +20, or +50 mV, tauOFF/tauNa at -70 to -90 mV was 1.2 +/- 0.1. Similar tauOFF/tauNa ratios were predicted by channel models having three identical charged gating particles that can rapidly and reversibly form an immobile dimer or trimer after independently crossing the membrane from their OFF to their ON locations.

scholarly journals Sodium and calcium currents in dispersed mammalian septal neurons.

1991 ◽  
Vol 97 (2) ◽  
pp. 303-320 ◽  
Author(s):  
A Castellano ◽  
J López-Barneo
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Calcium Currents ◽  
Closing Time ◽  
Patch Clamp Technique ◽  
Average Value ◽  
Time To Peak ◽  
Time Courses ◽  
Fast Activation ◽  
Septal Neurons

Voltage-gated Na+ and Ca2+ conductances of freshly dissociated septal neurons were studied in the whole-cell configuration of the patch-clamp technique. All cells exhibited a large Na+ current with characteristic fast activation and inactivation time courses. Half-time to peak current at -20 mV was 0.44 +/- 0.18 ms and maximal activation of Na+ conductance occurred at 0 mV or more positive membrane potentials. The average value was 91 +/- 32 nS (approximately 11 mS cm-2). At all membrane voltages inactivation was well fitted by a single exponential that had a time constant of 0.44 +/- 0.09 ms at 0 mV. Recovery from inactivation was complete in approximately 900 ms at -80 mV but in only 50 ms at -120 mV. The decay of Na+ tail currents had a single time constant that at -80 mV was faster than 100 microseconds. Depolarization of septal neurons also elicited a Ca2+ current that peaked in approximately 6-8 ms. Maximal peak Ca2+ current was obtained at 20 mV, and with 10 mM external Ca2+ the amplitude was 0.35 +/- 0.22 nA. During a maintained depolarization this current partially inactivated in the course of 200-300 ms. The Ca2+ current was due to the activity of two types of conductances with different deactivation kinetics. At -80 mV the closing time constants of slow (SD) and fast (FD) deactivating channels were, respectively, 1.99 +/- 0.2 and 0.11 +/- 0.03 ms (25 degrees C). The two kinds of channels also differed in their activation voltage, inactivation time course, slope of the conductance-voltage curve, and resistance to intracellular dialysis. The proportion of SD and FD channels varied from cell to cell, which may explain the differential electrophysiological responses of intracellularly recorded septal neurons.

scholarly journals Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors

2010 ◽  
Vol 135 (4) ◽  
pp. 355-366 ◽  
Author(s):  
Hugh R. Matthews ◽  
Alapakkam P. Sampath
Keyword(s):  
Time Constant ◽  
Light Adaptation ◽  
Light Response ◽  
Response Duration ◽  
Photoreceptor Cells ◽  
Quenching Rate ◽  
Vertebrate Photoreceptor ◽  
Cone Response ◽  
Rate Limiting ◽  
Phototransduction Cascade

The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment's effector, the transducin-GTP–phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca2+] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca2+-sensitive step involved the pigment itself was provided by the observation that removal of Cl− from the pigment's anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca2+], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.

scholarly journals Delays in inactivation development and activation kinetics in myxicola giant axons.

1982 ◽  
Vol 80 (1) ◽  
pp. 83-102 ◽  
Author(s):  
L Goldman ◽  
J L Kenyon
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Series Resistance ◽  
Activation Kinetics ◽  
Time To Peak ◽  
Exponential Decline ◽  
Na Inactivation ◽  
The Difference ◽  
True Property ◽  
Kinetics Of

Na inactivation was studied in Myxicola (two-pulse procedure, 6-ms gap between conditioning and test pulses). Inactivation developed with an initial delay (range 130-817 microseconds) followed by a simple exponential decline (time constant tau c). Delays (deviations from a simple exponential) are seen only for brief conditioning pulses were gNa is slightly activated. Hodgkin-Huxley kinetics with series resistance, Rs, predict deviations from a simple exponential only for conditioning pulses that substantially activate gNa. Reducing INa fivefold (Tris substitution) had no effect on either tau c or delay. Delay in not generated by Rs or by contamination from activation development. The slowest time constant in Na tails is approximately 1 ms (Goldman and Hahin, 1978) and the gap was 6 ms. Shortening the gap to 2 ms had no effect on either tau c or delay. Delay is a true property of the channel. Delay decreased with more positive conditioning potentials, and also decreased approximately proportionally with time to peak gNa during the conditioning pulse, as expected for sequentially coupled activation and inactivation. In a few cases the difference between Na current values for brief conditioning pulses and the tau c exponential could be measured. Difference values decayed exponentially with time constant tau m. The inactivation time course is described by a model that assumes a process with the kinetics of gNa activation as a precursor to inactivation.

scholarly journals The kinetics of inactivation of the rod phototransduction cascade with constant Ca2+i.

1996 ◽  
Vol 107 (1) ◽  
pp. 19-34 ◽  
Author(s):  
A Lyubarsky ◽  
S Nikonov ◽  
E N Pugh
Keyword(s):  
G Protein ◽  
Time Constant ◽  
Inactivation Kinetics ◽  
First Order ◽  
Successful Model ◽  
Ringer’S Solution ◽  
Rhodopsin Kinase ◽  
Solution Bathing ◽  
Kinetics Of ◽  
Phototransduction Cascade

A rich variety of mechanisms govern the inactivation of the rod phototransduction cascade. These include rhodopsin phosphorylation and subsequent binding of arrestin; modulation of rhodopsin kinase by S-modulin (recoverin); regulation of G-protein and phosphodiesterase inactivation by GTPase-activating factors; and modulation of guanylyl cyclase by a high-affinity Ca(2+)-binding protein. The dependence of several of the inactivation mechanisms on Ca2+i makes it difficult to assess the contributions of these mechanisms to the recovery kinetics in situ, where Ca2+i is dynamically modulated during the photoresponse. We recorded the circulating currents of salamander rods, the inner segments of which are held in suction electrodes in Ringer's solution. We characterized the response kinetics to flashes under two conditions: when the outer segments are in Ringer's solution, and when they are in low-Ca2+ choline solutions, which we show clamp Ca2+i very near its resting level. At T = 20-22 degrees C, the recovery phases of responses to saturating flashes producing 10(2.5)-10(4.5) photoisomerizations under both conditions are characterized by a dominant time constant, tau c = 2.4 +/- 0.4 s, the value of which is not dependent on the solution bathing the outer segment and therefore not dependent on Ca2+i. We extended a successful model of activation by incorporating into it a first-order inactivation of R*, and a first-order, simultaneous inactivation of G-protein (G*) and phosphodiesterase (PDE*). We demonstrated that the inactivation kinetics of families of responses obtained with Ca2+i clamped to rest are well characterized by this model, having one of the two inactivation time constants (tau r* or tau PDE*) equal to tau c, and the other time constant equal to 0.4 +/- 0.06 s.

scholarly journals Kinetics of oxygen consumption after a single flash of light in photoreceptors of the drone (Apis mellifera).

1982 ◽  
Vol 80 (1) ◽  
pp. 19-55 ◽  
Author(s):  
M Tsacopoulos ◽  
S Poitry
Keyword(s):  
Oxygen Consumption ◽  
Time Constant ◽  
Time Course ◽  
Sodium Pump ◽  
Fourier Transforms ◽  
Recovery Phase ◽  
Peak Amplitude ◽  
Single Flash ◽  
High Concentrations ◽  
Kinetics Of

The time course of the rate of oxygen consumption (QO2) after a single flash of light has been measured in 300-micrometers slices of drone retina at 22 degrees C. To measure delta QO2(t), the change in QO2 from its level in darkness, the transients of the partial pressure of O2 (PO2) were recorded with O2 microelectrodes simultaneously in two sites in the slice and delta QO2 was calculated by a computer using Fourier transforms. After a 40-ms flash of intense light, delta QO2, reached a peak of 40 microliters O2/g.min and then declined exponentially to the baseline with a time constant tau 1 = 4.96 +/- 0.49 s (SD, n = 10). The rising phase was characterized by a time constant tau 2 = 1.90 +/- 0.35 s (SD, n = 10). The peak amplitude of delta QO2 increased linearly with the log of the light intensity. Replacement of Na+ by choline, known to decrease greatly the light-induced transmembrane current, caused a 63% decrease of delta QO2. With these changes, however, the kinetics of delta QO2 (t) were unchanged. This suggest that the recovery phase is rate-limited by a single reaction with apparent first-order kinetics. Evidence is provided that suggests that this reaction may be the working of the sodium pump. Exposure of the retina to high concentrations of ouabain or strophanthidin (inhibitors of the sodium pump) reduced the peak amplitude of delta QO2 by approximately 80% and increased tau 1. The increase of tau 1 was an exponential function of the time of exposure to the cardioactive steroids. Hence, it seems likely that the greatest part of delta QO2 is used for the working of the pump, whose activity is the mechanism underlying the rate constant of the descending limb of delta QO2 (t).

scholarly journals Adaptation in the Ventral Eye of Limulus is Functionally Independent of the Photochemical Cycle, Membrane Potential, and Membrane Resistance

1973 ◽  
Vol 61 (3) ◽  
pp. 273-289 ◽  
Author(s):  
A. Fein ◽  
R. D. DeVoe
Keyword(s):  
Membrane Potential ◽  
Time Constant ◽  
Dark Adaptation ◽  
Light Adaptation ◽  
Receptor Potential ◽  
Membrane Resistance ◽  
Initial Time ◽  
Early Receptor Potential ◽  
Time Courses ◽  
Sensitivity Changes

The early receptor potential (ERP), membrane potential, membrane resistance, and sensitivity were measured during light and/or dark adaptation in the ventral eye of Limulus. After a bright flash, the ERP amplitude recovered with a time constant of 100 ms, whereas the sensitivity recovered with an initial time constant of 20 s. When a strong adapting light was turned off, the recovery of membrane potential and of membrane resistance had time-courses similar to each other, and both recovered more rapidly than the sensitivity. The receptor depolarization was compared during dark adaptation after strong illumination and during light adaptation with weaker illumination; at equal sensitivities the cell was more depolarized during light adaptation than during dark adaptation. Finally, the waveforms of responses to flashes were compared during dark adaptation after strong illumination and during light adaptation with weaker illumination. At equal sensitivities (equal amplitude responses for identical flashes), the responses during light adaptation had faster time-courses than the responses during dark adaptation. Thus neither the photochemical cycle nor the membrane potential nor the membrane resistance is related to sensitivity changes during dark adaptation in the photoreceptors of the ventral eye. By elimination, these results imply that there are (unknown) intermediate process(es) responsible for adaptation interposed between the photochemical cycle and the electrical properties of the photoreceptor.

Time course and kinetics of proximal tubular processing of insulin

1992 ◽  
Vol 262 (5) ◽  
pp. F813-F822 ◽  
Author(s):  
S. Nielsen
Keyword(s):  
Steady State ◽  
Time Course ◽  
Accumulation Rate ◽  
Compartmental Analysis ◽  
Proximal Tubules ◽  
Hydrolysis Rate ◽  
Time Courses ◽  
High Concentrations ◽  
Kinetics Of

The present study was undertaken to determine the time courses and kinetics of the subcellular processing of 125I-insulin in isolated and in vitro perfused proximal tubules. Morphometric analysis demonstrated well-preserved ultrastructure after 90 min of perfusion. After luminal perfusion for 90 min the absorption was constant with time and reached steady state within 5 min (177 +/- 7 fg.min-1.mm-1). Also the hydrolysis rate and tubular accumulation rate were constant and averaged 84 +/- 8 and 93 +/- 10 fg.min-1.mm-1, respectively. Free 125I appeared already within 5 min of perfusion and reached steady state within 10 min. From proximal tubules perfused with 125I-insulin for 30 min and chased for 60 min, a compartmental analysis revealed two compartments; half time (t1/2) for delivery of insulin to the lysosomes was determined to be 8.5 min, and t1/2 for lysosomal degradation was 72 min. The results demonstrated that internalization by endocytic invaginations, incorporation in endocytic vacuoles, fusion with lysosomes, and hydrolysis were rapid processes and reached maximum rates within few minutes. A significant transtubular transport of insulin to the peritubular compartment was determined to be a constant rate of 11.2 +/- 0.7 fg.min-1.mm-1. Perfusion of tubules with insulin at high concentrations in the perfusate revealed that the transport was dependent on the absorbed amount and not on the perfused load, compatible with transport through the cells and not via a paracellular mechanism. The intactness of the tight junctions was supported by the following: 1) [14C]inulin leak did not increase with time and 2) enzyme-free intercellular spaces were evident after perfusion for only 5 min with microperoxidase (mol wt of 1,700). The transported 125I-insulin was trichloroacetic acid precipitable and immunoprecipitable.

scholarly journals Light Adaptation of Discrete Waves in the Limulus Photoreceptor

1972 ◽  
Vol 60 (1) ◽  
pp. 86-101 ◽  
Author(s):  
Richard Srebro ◽  
Mahmood Behbehani
Keyword(s):  
Time Course ◽  
Light Adaptation ◽  
Limulus Photoreceptor ◽  
Average Size ◽  
Effect Of Light

Light adaptation affects discrete waves in two ways. It reduces their average size and decreases the probability that a photon incident at the cornea causes a discrete wave. There is no effect of light adaptation on the latency of discrete waves, or on their time-course.

Kinetics of activation of the potassium conductance in the squid giant axon

1988 ◽  
Vol 232 (1269) ◽  
pp. 375-394 ◽  
Keyword(s):  
Time Constant ◽  
Potassium Current ◽  
Time Course ◽  
Potassium Conductance ◽  
Giant Axon ◽  
Squid Giant Axon ◽  
A Value ◽  
Initial Rise ◽  
Principal Effect ◽  
Kinetics Of

A quantitative re-investigation of the time course of the initial rise of the potassium current in voltage-clamped squid giant axons is described. The n 4 law of the Hodgkin–Huxley equations was found to be well obeyed only for the smallest test pulses, and for larger ones a good fit of the inflected rise required use of the expression (1 – exp {– t / ז n 1 }) X –1 (1 – exp { – t / ז n 2 }), where both of the time constants and the power X varied with the size of the test pulse. Application of a negative prepulse produced a delay in the rise resulting mainly from an increase of X from a value of about 3 at –70 mV to 8 at –250 mV, while ז n 1 remained constant and ז n 2 was nearly doubled. The process responsible for generating this delay was switched on with a time constant of 8 ms at 4°C, which fell to about 1 ms at 15°C. Analysis of the inward tail currents at the end of a voltage-clamp pulse showed that there was a substantial external accumulation of potassium owing to the restriction of its diffusion out of the Schwann cell space, which, when duly allowed for, roughly doubled the calculated value of the potassium conductance. Computations suggested that the principal effect of such a build-up of [K] o would be to reduce the fitted values of ז n 1 and ז n 2 to two-thirds or even half their true sizes, while the power X would generally be little changed; but it would not affect the necessity to introduce a second time constant, nor would it invalidate our findings on the effect of negative prepulses.

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