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Biomedical subjects

M Tsacopoulos

Publications and source records attributed to M Tsacopoulos.

At least 55 records · Page 3Linked to original sources
PubMed1987

The response to monochromatic light flashes of the oxygen consumption of honeybee drone photoreceptors.

Local measurements of the fall in oxygen pressure on stimulation of slices of the retina of the honeybee drone by flashes of light were made with oxygen microelectrodes and used to calculate the kinetics of the extra oxygen consumption (delta QO2) induced by each flash. The action spectrum for delta QO2 was obtained from response-intensity curves in response to brief (40 ms) monochromatic light flashes. The action spectrum of receptor potentials was obtained with the same experimental conditions. The two action spectra match closely: they deviate slightly from the photosensitivity spectrum of the drone rhodopsin (R). The deviation is thought to be due to wavelength-dependent light scattering and absorption in the preparation. In these experiments, the visual pigment was first illuminated with orange light, which is known to convert the bistable drone photopigment predominantly to the R state from the metarhodopsin (M) state. When long (300-900 ms) light flashes were used to elicit delta QO2, the responses to different wavelengths could not be matched in time course (as for the short flashes). Flashes producing large R-to-M conversions produced a prolonged delta QO2. The prolongation did not occur after double flashes, which produced both large R-to-M and M-to-R conversions. Similar changes in the length of afterpotentials in the photoreceptor cells and in a long-lasting decrease in photoreceptor intracellular K+ activity were found after long single or double flashes. The results are interpreted to show that the initial event for stimulation by light of metabolism in the drone retina is the same as that for stimulation of electrical responses (i.e., absorption of photons by R). Absorption of photons by M can produce an inhibitory effect on this stimulation.

Animals↗
PubMed1987

The supply of metabolic substrate from glia to photoreceptors in the retina of the honeybee drone.

1. The drone retina is composed essentially of only two types of cells: a population of identical photoreceptor cells occupying 38% of the volume is embedded in a syncytium of glia (called outer pigment cells). Nearly all the mitochondria are in the photoreceptors. 2. A retinal slice consumes 18 microliter O2 (ml tissue)-1 min-1 in the dark for up to 6 h, even without exogenous substrate; in 6 h this would require the equivalent of 127 mM glucose in the photoreceptors or 8.7 mg glycogen (ml tissue)-1. 3. Freshly dissected retinas contain about 45 mg glycogen (ml tissue)-1, but this appears, from electron micrographs and from the PAS reaction, to be exclusively in the glia. After superfusion with substrate-free Ringer solution for 30 min, slices of retina contained less than 20 microM glucose. It therefore appears that to sustain respiration, carbohydrate substrate must be transferred from the glia to the photoreceptors. 4. Even after 6 h superfusion with substrate-free Ringer solution O2 consumption (QO2) was not increased by exogenous glucose, pyruvate, trehalose or lactate, nor decreased by 2-deoxy-D-glucose. QO2 was increased 2-3 fold by either light stimulation or (for at least 20 min) by 50 microM dinitrophenol. 5. QO2 was only slightly reduced when Na-dependent glucose transport was inhibited either by reduction of extracellular [Na+], or the presence of phlorizin. 6. It is suggested that drone retinal function does not require the uptake of glucose by the photoreceptors, but that the glia do take up glucose.

Animals↗
PubMed1986

Light induced sodium dependent accumulation of calcium and potassium in the extracellular space of bee retina.

Intense illumination of long duration induced a large transient increase in extracellular calcium (delta[Ca2+]o) and potassium (delta[K+]o) during and after light in bee retina when measured with ion-selective microelectrodes. Whenever a large delta[Ca2+]o appeared, it was accompanied by a transient afterdepolarization (TA). Both the increase in [Ca2+]o, [K+]o and the TA were reduced or abolished when sodium was replaced by arginine, choline or lithium (Li+) ions. At 0-Na conditions a Na independent decrease in [Ca2+]o was observed during illumination only. A pronounced transient depolarization of the photoreceptor in the dark due to transient anoxia did not result in a significant change in [Ca2+]o. In some retinae the elevated level of [K+]o after light was absent, however a small Na-dependent TA was still observed. The above findings suggest that intense long illumination induces a large Ca2+ influx into the photoreceptors which is followed by Na-dependent Ca2+ efflux due to Na-Ca exchange. The light-induced afterdepolarization arises mainly from K+ accumulation in the extracellular space but partially from the electrogenicity of Na-Ca exchange.

Animals↗
PubMed1986

Regulation of local oxygen tension and blood flow in the inner retina during hyperoxia.

A study has been undertaken to determine whether local changes in PO2 could be a factor in the regulation of retinal blood flow during 100% O2 breathing. For this purpose we have measured simultaneously in eyes of anesthetized and artificially ventilated miniature pigs the change in local preretinal PO2 and retinal blood flow using O2-sensitive microelectrodes and laser Doppler velocimetry. Although preretinal intervascular PO2 changed little, periarteriolar PO2 increased markedly during 100% O2 breathing. When measured less than 50 microns from the arteriolar wall, the time course of this increase preceded that of the decrease in retinal blood flow. The results indicate that O2 diffusing through the wall of the large retinal arterioles represents the most important component of periarteriolar PO2. This diffusion coupled with the decrease in retinal blood flow could play a major role in the regulation of inner retinal PO2. The data also suggest that diffusional shunting of O2 between retinal arterioles and veins could explain the unexpected increase in venous O2 saturation during 100% O2 breathing that has been previously reported by others. The finding that retinal periarteriolar PO2 is always higher than the PO2 in the outer retina does not support the previously formulated hypothesis that O2 from the choroid is responsible for the constriction of the arterioles during hyperoxia.

Animals↗
PubMed1985

[Various aspects of the existing relationship between retinal neuronal function, oxidative metabolism and the constant O2 supply by the microcirculation].

Retinal excitation implies transmembrane ion movement in the retinal neurones. In order to maintain excitability, the neurones utilize energy derived from the hydrolysis of ATP. ATP is produced in the mitochondria, which consume O2 and carbohydrates for this purpose. Thus, O2 consumption is essential for maintaining ATP in the retinal neurons and, therefore, for maintaining excitability. O2 is delivered to the retinal mitochondria by the microcirculation, where variations of the blood flow cause dramatic fluctuations in the local PO2 in the tissue. The retinal blood flow can be impaired by experimental changes in the pH of the glial cells surrounding the arterioles. According to a hypothesis, intraglial pH modulates the release of a mediator (prostaglandins) which in turn acts on the smooth musculature of the arteriole wall and thus controls vascular motility.

Adenosine Triphosphate↗
PubMed1985

[Experimental venous thrombosis: preretinal PO2 before and after photocoagulation].

Experimental occlusion of a retinal vein leads to retinopathy in the affected drainage area. In some cases the formation of ischemic regions, where the retina is completely destroyed, is typical for the development of this retinopathy. Laser photocoagulation restores the physiological preretinal oxygen pressure in these hypoxic regions. It is possible that the rise in O2 pressure which has been observed is caused by destruction of pigment epithelium and the photoreceptor layer, as oxygen diffuses freely from the choroid to the inner retina without being consumed by the outer layers.

Animals↗
PubMed1985

Effect of laser photocoagulation on oxygenation of the retina in miniature pigs.

A large area of the posterior pole of the retina of the miniature pig was photocoagulated, and 2-3 wk later the PO2 in the preretinal vitreous was mapped with O2-sensitive microelectrodes. In the control retina, the PO2 was highly heterogeneous being much higher close to an artery than opposite an intervascular zone. After photocoagulation, PO2 opposite an intervascular zone was found to be significantly increased. A quantitative histologic analysis showed that in the photocoagulated areas more than 28% of the outer retina was destroyed. The authors conclude from these results that photocoagulation, by partially destroying the pigment epithelium-photoreceptor complex, causes an increase of the oxygenation of the retina.

Animals↗
PubMed1982

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

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).

Animals↗
PubMed1981

Clearance of extracellular potassium: evidence for spatial buffering by glial cells in the retina of the drone.

Work with ion-selective microelectrodes on the retina of the honeybee drone has shown that potassium is released from photoreceptors during activity and enters glial cells. Measurements of the extracellular voltage gradients indicate that, in this preparation, currents flowing through the glial cells in the 'spatial buffer' pattern account for a large fraction of the glial K+ entry in the active region of the tissue.

Animals↗
PubMed1981

Diffusion and consumption of oxygen in the superfused retina of the drone (Apis mellifera) in darkness.

Double-barreled O2 microelectrodes were used to study O2 diffusion and consumption in the superfused drone (Apis mellifera) retina in darkness at 22 degrees C. Po2 was measured at different sites in the bath and retinas. It was found that diffusion was essentially in one dimension and that the rate of O2 consumption (Q) was practically constant (on the macroscale) down to Po2 s less than 20 mm Hg, a situation that greatly simplified the analysis. The value obtained for Q was 18 +/- 0.7 (SEM) microliter O2/cm3 tissue . min (n = 10), and Krogh's permeation coefficient (alpha D) was 3.24 +/- 0.18 (SEM) X 10(-5) ml O1/min . atm . cm (n = 10). Calculations indicate that only a small fraction of this Q in darkness is necessary for the energy requirements of the sodium pump. the diffusion coefficient (D) in the retina was measured by abruptly cutting off diffusion from the bath and analyzing the time-course of the fall in Po2 at the surface of the tissue. The mean value of D was 1.03 +/- 0.08 (SEM) X 10(-5) cm2/s (n = 10). From alpha D and D, the solubility coefficient alpha was calculated to be 54 +/- 4.0 (SEM) microliter O2 STP/cm3 . atm (n = 10), approximately 1.8 times that for water.

Animals↗
PubMed1981

Ionic and possible metabolic interactions between sensory neurones and glial cells in the retina of the honeybee drone.

This is a review paper that includes original calculations and figures. The drone retina is composed of two essentially uniform populations of cells, the photoreceptors and the glial cells. The photoreceptors contain many mitochondria but no glycogen has been detected; the glial cells contain much glycogen and very few mitochondria. The oxygen consumption of the photoreceptors in the dark is 20 microliters min-1 per g of retinal tissue and in response to a single flash of light there is an extra consumption that reaches a maximum of 40 microliters min-1 per g. In addition, light stimulation of the photoreceptors leads to changes in the glycogen metabolism of the glial cells, and to movements of K+. Measurements with intracellular K+-sensitive micro-electrodes showed that during light stimulation with a series of flashes the K+ activity (alpha K) in the photoreceptors fell by an average of 27% while in the glial cells alpha K rose by an amount that is estimated to correspond to most of the quantity of K+ lost by the photoreceptors. The relative contributions to the clearance of extracellular K+ of extracellular diffusion, spatial buffering and possible net K+ uptake by glial cells are discussed.

Animals↗
PubMed1980

[The effect of light on glycogen turnover in the retina of the honeybee drone (author's transl)].

The retina of the compound eye of the drone (Apis mellifera) comprises two distinct classes of cells: the photoreceptors and the glial cells. The photoreceptors contain mitochondriae and the photopigment (rhodopsin). The glial cells do not contain mitochondriae, but large amounts of glycogen granules. Light stimulation causes intraglial glycogen metabolism to accelerate. Since glial cells are not directly excitable by light, it is deduced that the absorption of light by rhodopsin must somehow cause a signal to pass the glial cells to activate the glycogen metabolism.

Animals↗
PubMed1980

[Studies on potassium transport through glial cell membranes (author's transl)].

The retina of the honeybee drone is used as a model for the study of ion movements across the membranes of the glial cells caused by changes in the extracellular potassium concentration. The values found for changes in extracellular potential suggest that at least some of the potassium that enters glial cells in an active region of tissue is associated with an efflux of potassium from parts of the glial syncytium not affected by an increase in extracellular potassium concentration. In addition, it appears that ions other than K+ cross the glial membrane.

Animals↗
PubMed1979

Potassium activity in photoreceptors, glial cells and extracellular space in the drone retina: changes during photostimulation.

1. A double-barrelled potassium-sensitive micro-electrode was developed that was fine enough to record intracellular electrical potentials and potassium activities (aK) in the drone retina. 2. aK was measured in the photoreceptor cells, in the pigment (glial) cells, and in the extracellular space, in the superfused, cut, retina. The effect of photostimulation was studied: 20 msec light flashes, intense enough to evoke receptor potentials of maximum amplitude were presented, 1/sec, in a train lasting about 2 min. 3. In photoreceptors with membrane potentials greater than or equal to 50 mV aK in the dark was 79 mM, S.D. = 27 mM, n = 11. During photostimulation aK fell by 21.5 +/- 9.5 mM with a half-time of 30 +/- 22 sec. (A tentative conversion from activities to free concentrations can be made by taking the activity coefficient as 0.70 its value in the Ringer solution). 4. In pigment cells with membrane potentials greater than or equal to 50 mV, aK in the dark was 52 mM, S.D. = 13 mM, n = 11. During photostimulation aK increased by 14 +/- 5 mM. 5. In the extracellular space aK increased during photostimulation with a mean half-time of less than 1.3 sec to a maximum (mean value 14 mM, S.D. = 8.4 mM, n = 22), and then fell to a plateau. 6. It is estimated from the anatomy that the photoreceptors occupy approximately 38% of the total volume of the retina, the pigment cells 57%, and extracellular space 5%. Hence, it seems possible that during photostimulation nearly all the net loss of potassium from the photoreceptors is temporarily stored in the pigment cells. 7. Recordings were made in the extracellular space of the intact animal by passing the electrode through a hole in the cornea. The mean aK in the dark was 7.7 mM, S.E. = 0.4 mM, n = 22. In the superfused retina, aK in the dark was 6.3 mM, S.E. = 0.7 mM, n = 22, even though aK in the Ringer solution was 2.2 mM. Increasing the aK of the Ringer solution to 7.0 mM had no apparent effect on aK in the extracellular space at depths greater than 20 micron. 8. In the intact animal the amplitude and time course of the change in extracellular aK evoked by the standard pattern of photostimulation were within the range observed in the superfused preparation.

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