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

Development and plasticity of mitochondria and electrical properties of the cell membrane in blowfly photoreceptors

Blowfly photoreceptors are highly energy demanding sensory systems. Their information processing efficiency is enabled by the high temporal resolution of the cell membrane, requiring heavy metabolic support by the mitochondria. We studied the developmental changes of the mitochondrial apparatus and electrical properties of the photoreceptor membrane in the white eyed Calliphora vicina Chalky. Using in vivo microspectrophotometry and Western blot analysis, we found an age-dependent increase in the concentration of mitochondrial pigments. The maximal change occurred during the first week. The age-related changes were smaller in dark-bred than in light-bred flies. The mitochondrial pigment content increased after the switch from dark to light rearing and decreased after the switch from light to dark rearing. The electrical parameters of the photoreceptors were investigated with intracellular recordings. The resting membrane resistance and time constant decreased significantly after eclosion. The decrease was again most significant during the first week of adult life, paralleled with changes in the Na/K pump-dependent hyperpolarizing afterpotential. We conclude that the photoreceptor mitochondria exhibit remarkable ontogenetic and phenotypic plasticity, because the quantity of mitochondrial pigments tightly follows the development of the cell membrane as well as the energy demands of the photoreceptors under different rearing conditions.

Preretinal partial pressure of oxygen gradients before and after experimental pars plana vitrectomy

PURPOSE

To evaluate preretinal partial pressure of oxygen (PO2) gradients before and after experimental pars plana vitrectomy.

METHODS

Arteriolar, venous, and intervascular preretinal PO2 gradients were recorded in 7 minipigs during slow withdrawal of oxygen-sensitive microelectrodes (10-μm tip diameter) from the vitreoretinal interface to 2 mm into the vitreous cavity. Recordings were repeated after pars plana vitrectomy and balanced salt solution (BSS) intraocular perfusion.

RESULTS

Arteriolar, venous, and intervascular preretinal PO2 at the vitreoretinal interface were 62.3 ± 13.8, 22.5 ± 3.3, and 17.0 ± 7.5 mmHg, respectively, before vitrectomy; 97.7 ± 19.9, 40.0 ± 21.9, and 56.3 ± 28.4 mmHg, respectively, immediately after vitrectomy; and 59.0 ± 27.4, 25.2 ± 3.0, and 21.5 ± 4.5 mmHg, respectively, 2½ hours after interruption of BSS perfusion. PO2 2 mm from the vitreoretinal interface was 28.4 ± 3.6 mmHg before vitrectomy; 151.8 ± 4.5 mmHg immediately after vitrectomy; and 34.8 ± 4.1 mmHg 2½ hours after interruption of BSS perfusion. PO2 gradients were still present after vitrectomy, with the same patterns as before vitrectomy.

CONCLUSION

Preretinal PO2 gradients are not eliminated after pars plana vitrectomy. During BSS perfusion, vitreous cavity PO2 is very high. Interruption of BSS perfusion evokes progressive equilibration of vitreous cavity PO2 with concomitant progressive return of preretinal PO2 gradients to their previtrectomy patterns. This indicates that preretinal diffusion of oxygen is not altered after vitrectomy. The beneficial effect of vitrectomy in ischemic retinal diseases or macular edema may be related to other mechanisms, such as increased oxygen convection currents or removal of growth factors and cytokines secreted in the vitreous.

Kinetics of oxygen consumption after a flash of light in the lateral ocellus of the barnacle

Until recently, polarographic methods for measuring the time course of transient changes in the rate of oxygen consumption (DeltaQO(2)) have been applied only to tissue preparations containing thousands of cells. Here, we describe DeltaQO(2) measurements on the lateral ocellus of the barnacle (Balanus eburneus) which contains only three photoreceptor cells. The decrement of partial pressure of oxygen (DeltaPO(2)) elicited by an 80 ms flash of light was measured near the cells with a microelectrode and the DeltaQO(2) was calculated from the DeltaPO(2) using a model of diffusion with spherical symmetry. As shown by mathematical simulation, the exact shape of the preparation is not crucial for our measurements of the time course of the DeltaQO(2). For a given DeltaQO(2), the model describes correctly the attenuation of the DeltaPO(2) measured at increased distances from the preparation. To know more about the mechanisms controlling the DeltaQO(2), we compared it with the electrical response of the photoreceptor cells: both responses have a similar spectral dependence, but only the DeltaQO(2) was abolished by a 10-min exposure to 50 muM dinitrophenol or to 3 mM amytal. We conclude that the DeltaQO(2) reflects an increase in mitochondrial respiration and that it is initiated by the phototransformation of rhodopsin, as was already found in the honeybee drone retina (Dimitracos and Tsacopoulos, 1985; Jones and Tsacopoulos, 1987).

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

A glia-neuron alanine/ammonium shuttle is central to energy metabolism in bee retina

It has been proposed that glial cells may supply carbon fuel to neurons and also that there are fluxes of ammonium from neurons to glia. We have investigated both these proposals in Apis retinal slices, in which virtually all the mitochondria are in the photoreceptor neurons. Normally the superfusate contained no substrate of energy metabolism; addition of glucose or alanine did not increase oxygen consumption (QO2), confirming that the neurons received adequate substrate from glycogen in the glia. 1,4-Dideoxy-1,4-imino-D-arabinitol (DAB, 100 microm), an inhibitor of glycogen phosphorylase, progressively decreased QO2. This decrease was reversed by alanine but not glucose. Ammonium-sensitive microelectrodes did not detect significant extracellular [NH(4)(+)] ([NH(4)(+)](e)) in slices superfused with normal superfusate. Removal of Cl(-), necessary for cotransport of NH(4)(+) into the glia, increased [NH(4)(+)](e) so that at the end of 2 min photostimulation mean [NH(4)(+)](e) was 0.442 mM (S.E.M. = 0.082 mM, n = 16). In 0 Cl(-), [NH(4)(+)](e) was reduced by 2-(methylamino)isobutyrate (MeAIB) an inhibitor of alanine transport. MeAIB also blocked oxidation of alanine in the presence of DAB, but did not decrease QO2 in normal superfusate. Lactate (l and d) and pyruvate (but not glucose) increased QO2 in DAB and decreased [NH(4)(+)](e) in 0 Cl(-). These results strengthen the evidence that in superfused retinal slices, glucose is metabolized exclusively in the glia, which supply alanine to the neurons, and that ammonium returns to the glia. They also show that another fuel (perhaps lactate) can be supplied by the glia to the neurons.

The role of extracellular adenosine in regulating mossy fiber synaptic plasticity

Hippocampal mossy fiber synapses show unique molecular features and dynamic range of plasticity. A recent paper proposed that the defining features of mossy fiber synaptic plasticity are caused by a local buildup of extracellular adenosine (Moore et al., 2003). In this study, we reassessed the role of ambient adenosine in regulating mossy fiber synaptic plasticity in mouse and rat hippocampal slices. Synaptic transmission was highly sensitive to activation of presynaptic adenosine A1 receptors (A1Rs), which reduced transmitter release by >75%. However, most of A1Rs were not activated by ambient adenosine. Field potentials increased only by 20-30% when A1Rs were fully blocked with the A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1 microM). Moreover, blocking A1Rs hardly altered paired-pulse facilitation, frequency facilitation, or posttetanic potentiation. Frequency facilitation was similar in A1R-/- mice and when measured with NMDA receptor-mediated EPSCs in CA3 pyramidal cells in the presence of DPCPX. Additional experiments suggested that the results obtained by Moore et al. (2003) can partially be explained by their usage of a submerged recording chamber and elevated divalent cation concentrations. In conclusion, a reduction of the basal release probability by ambient adenosine does not underlie presynaptic forms of plasticity at mossy fiber synapses.

Effect of Indomethacin on the Hypercapnia-Associated Vasodilation of the Optic Nerve Head Vessels: An Experimental Study in Miniature Pigs

PURPOSE

To evaluate whether prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels.

METHODS

We measured the PO(2) at intervascular areas of the optic disc in 9 anaesthetized miniature pigs using oxygen-sensitive microelectrodes placed at <50 mum from the optic disc. PO(2) was measured continuously under normoxia, hyperoxia (breathing of 100% O(2)), carbogen breathing (95% O(2), 5% CO(2)), and hypercapnia (40% increase in inhaled CO(2)). Similar measurements under these conditions were also done after intravenous administration of the prostaglandin inhibitor indomethacin.

RESULTS

Before the injection of indomethacin, we observed a slight increase in the optic disc PO(2) during hypercapnia (DeltaPO(2) = 2.0 +/- 1.7 mm Hg; p < 0.001; n = 18) or hyperoxia (DeltaPO(2) = 3.4 +/- 1.6 mm Hg; p < 0.001; n = 23), but a much more important increase during carbogen breathing (DeltaPO(2) = 12.0 +/- 5.1 mm Hg; p < 0.001; n = 23). After the injection of indomethacin, the increase in the optic disc PO(2) was similar during hyperoxia (DeltaPO(2) = 5.6 +/- 2.2 mm Hg; p < 0.001; n = 9) or carbogen breathing (DeltaPO(2) = 5.8 +/- 3.2 mm Hg; p < 0.001; n = 9), while in hypercapnia the variation of the optic disc PO(2) was minimal (DeltaPO(2) = 0.5 +/- 1.9 mm Hg; p > 0.1; n = 6).

CONCLUSIONS

Indomethacin inhibits the vasodilatory effect of increased systemic PaCO(2) on the optic nerve head vessels, leading to a similar moderate increase in the optic disc PO(2) during carbogen breathing as in hyperoxia. Indomethacin also inhibits the increase in the optic disc PO(2) seen during hypercapnia. Those results indicate that prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels.

Light dependence of oxygen consumption by blowfly eyes recorded with a magnetic diver balance

We measured the oxygen (O2) consumption of isolated blowfly eyes using a magnetic diver balance, a device for high-resolution volumetric O2 consumption measurements. The light-induced O2 consumption is at most three times the value of the dark consumption, which is 0.6 nl O2 s(-1) eye(-1), and is in good agreement with the estimates based on electrophysiological data. With longer stimuli the increase follows a double exponential time course. The respective time constants are approximately 2 and 20 s and show no dependence on light intensity, whereas the dependence of amplitudes can be fitted by a Hill equation. Decreasing the stimulus duration reveals that the peak in O2 consumption overshoots the time course induced by long stimuli. We suggest this may be a general feature of mitochondrial activation. The dependence of the O2 consumption peak on stimulus duration at high light intensity has a hump with stimulus durations of 10-20 ms, coinciding with the stimulus durations that start to induce the adaptation of the receptor potential.

The TRP calcium channel and retinal degeneration

The Drosophila light activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. These channels are Ca2+ permeable and have been implicated as important component of cellular Ca2+ homeostasis in neuronal and non-neuronal cells. The power of the molecular genetics of Drosophila has yielded several mutants in which constitutive activity of TRP leads to a rapid retinal degeneration in the dark. Metabolic stress activates rapidly and reversibly the TRP channels in the dark in a constitutive manner by a still unknown mechanism. The link of TRP gating to the metabolic state of the cell is shared also by mammalian homologues of TRP and makes cells expressing TRP extremely vulnerable to metabolic stress, a mechanism that may underlie retinal degeneration and neuronal cell death.

Metabolic stress reversibly activates the Drosophila light-sensitive channels TRP and TRPL in vivo

Drosophila transient receptor potential (TRP) is a prototypical member of a novel family of channel proteins underlying phosphoinositide-mediated Ca(2+) entry. Although the initial stages of this signaling cascade are well known, downstream events leading to the opening of the TRP channels are still obscure. In the present study we applied patch-clamp whole-cell recordings and measurements of Ca(2+) concentration by ion-selective microelectrodes in eyes of normal and mutant Drosophila to isolate the TRP and TRP-like (TRPL)-dependent currents. We report that anoxia rapidly and reversibly depolarizes the photoreceptors and induces Ca(2+) influx into these cells in the dark. We further show that openings of the light-sensitive channels, which mediate these effects, can be obtained by mitochondrial uncouplers or by depletion of ATP in photoreceptor cells, whereas the effects of illumination and all forms of metabolic stress were additive. Effects similar to those found in wild-type flies were also found in mutants with strong defects in rhodopsin, Gq-protein, or phospholipase C, thus indicating that the metabolic stress operates at a late stage of the phototransduction cascade. Genetic elimination of both TRP and TRPL channels prevented the effects of anoxia, mitochondrial uncouplers, and depletion of ATP, thus demonstrating that the TRP and TRPL channels are specific targets of metabolic stress. These results shed new light on the properties of the TRP and TRPL channels by showing that a constitutive ATP-dependent process is required to keep these channels closed in the dark, a requirement that would make them sensitive to metabolic stress.

Metabolic stress reversibly activates the Drosophila light-sensitive channels TRP and TRPL in vivo

Drosophila transient receptor potential (TRP) is a prototypical member of a novel family of channel proteins underlying phosphoinositide-mediated Ca(2+) entry. Although the initial stages of this signaling cascade are well known, downstream events leading to the opening of the TRP channels are still obscure. In the present study we applied patch-clamp whole-cell recordings and measurements of Ca(2+) concentration by ion-selective microelectrodes in eyes of normal and mutant Drosophila to isolate the TRP and TRP-like (TRPL)-dependent currents. We report that anoxia rapidly and reversibly depolarizes the photoreceptors and induces Ca(2+) influx into these cells in the dark. We further show that openings of the light-sensitive channels, which mediate these effects, can be obtained by mitochondrial uncouplers or by depletion of ATP in photoreceptor cells, whereas the effects of illumination and all forms of metabolic stress were additive. Effects similar to those found in wild-type flies were also found in mutants with strong defects in rhodopsin, Gq-protein, or phospholipase C, thus indicating that the metabolic stress operates at a late stage of the phototransduction cascade. Genetic elimination of both TRP and TRPL channels prevented the effects of anoxia, mitochondrial uncouplers, and depletion of ATP, thus demonstrating that the TRP and TRPL channels are specific targets of metabolic stress. These results shed new light on the properties of the TRP and TRPL channels by showing that a constitutive ATP-dependent process is required to keep these channels closed in the dark, a requirement that would make them sensitive to metabolic stress.

Distribution and regulation of the optic nerve head tissue PO2

We investigated the distribution and regulation of the optic nerve head (ONH) tissue partial pressure of oxygen (PO2) under various stimuli and the role of the nitric oxide in the ONH circulation. Tissue PO2 was measured using double-barreled recess microelectrodes in the intact eyes of miniature pigs during normoxia, hyperoxia, hypoxia, variations of systemic blood pressure, and after inhibition of the endothelial nitric oxide synthesis by the administration of nitro-L-arginine. Measurements were performed in front of the ONH at intervascular and juxta-arteriolar areas and at a depth of 50 and 200 microm within the ONH at the center and the rim. During normoxia, PO2 was heterogeneously distributed in the ONH, higher close to the arterioles than in intervascular areas. Hyperoxia induced a significant increase of juxta-arteriolar tissue PO2, while in intervascular areas no change was noticed. Hypoxia did not modify intervascular tissue PO2 at 200 microm depth within the ONH. Variations of the systemic blood pressure did not induce any significant change in ONH tissue PO2. Similarly, no modification was noticed after the administration of nitro-L-arginine. There is a remarkable autoregulatory capacity of the ONH circulation that may compensate for parameters such as hyperoxia, hypoxia, and variations of the systemic blood pressure. Endothelially derived nitric oxide inhibition does not modify the ONH tissue PO2, probably because the tissue PO2 is stabilized by compensatory regulation.

Potassium homeostasis and glial energy metabolism

Since capillaries appear not to contribute significantly to rapid removal of K+ from brain tissue, the K+ released into extracellular clefts by neurons at the onset of electrical activity is presumably removed either by redistribution in the clefts or by uptake into cells. What appear to be the three major processes require no energy from the glial cells. These are diffusion through the extracellular clefts, spatial buffering by glial cells, and net uptake of K+ into glial cells through glial K+ channels associated with uptake of Cl- through an independent Cl- conductance. There is a relatively slow uptake by the Na+/K+-ATPase, which directly consumes ATP. In addition, some glial cells take up K+ on the Na+/K+/2Cl- cotransporter, which leads indirectly to energy consumption when the Na+ is subsequently pumped out. Currently available data suggest that the glial energy metabolism devoted to K+ homeostasis is less than a tenth of the total tissue energy metabolism, even under conditions of pathologically high extracellular [K+]. Hence, in situ, it is possible that glial cells could function with much less ATP than neurons do. All the various routes of muffling of changes in extracellular [K+] can be modulated, directly or indirectly, by transmitters liberated by neurons. A consequence of this could be regulation of the entry of Na+ into glial cells such that the Na+/K+-ATPase is activated. The degree of activation might be adjusted so that the resulting activation of the glial glycolytic pathway is appropriate to the provision of the quantity of metabolic substrates required by the neurons.

Effects of photoreceptor metabolism on interstitial and glial cell pH in bee retina: evidence of a role for NH4+

1. Measurements were made with pH microelectrodes in superfused slices of the retina of the honey-bee drone. In the dark, the mean +/- S.E.M. pH values in the three compartments of the tissue were: neurones (photoreceptors), 6.99 +/- 0.04; glial cells (outer pigment cells), 7.31 +/- 0.03; extracellular space, 6.60 +/- 0.03. 2. Stimulation of the photoreceptors with light caused transient pH changes: a decrease in the photoreceptors (pHn) and in the glial cells (pHg), and an increase in the interstitial clefts (pHo). 3. The effects of inhibition and activation of aerobic metabolism showed that part, perhaps all, of the light-induced delta pHo resulted from the increased aerobic metabolism in the photoreceptors. 4. Addition of 2 mM NH4+ to the superfusate produced changes in pHo and pHg of the same sign as and similar amplitude to those caused by light stimulation. Manipulation of transmembrane pH gradients had similar effects on changes in pHo induced by light or by exogenous NH4+. 5. Measurements with NH(4+)-sensitive microelectrodes showed that stimulation of aerobic metabolism in the photoreceptors increased [NH4+]o and also that exogenous NH4+/NH3 was taken up by cells, presumably the glial cells. 6. We conclude that within seconds of an increase in the aerobic metabolism in the photoreceptors, they release an increased amount of NH4+/NH3 which affects pHo and enters glial cells. Other evidence suggests that in drone retina the glial cells supply the neurones with amino acids as substrates of energy metabolism; the present results suggest that fixed nitrogen is returned to the glial cells as NH4+/NH3.

Kinetics of oxygen consumption and light-induced changes of nucleotides in solitary rod photoreceptors

We made simultaneous measurements of light-induced changes in the rate of oxygen consumption (QO2) and transmembrane current of single salamander rod photoreceptors. Since the change of PO2 was suppressed by 2 mM Amytal, an inhibitor of mitochondrial respiration, we conclude that it is mitochondrial in origin. To identify the cause of the change of QO2, we measured, in batches of rods, the concentrations of ATP and phosphocreatine (PCr). After 3 min of illumination, when the QO2 had decreased approximately 25%, ATP levels did not change significantly; in contrast, the amount of PCr had decreased approximately 40%. We conclude that either the light-induced decrease of QO2 is not caused by an increase in [ATP] or [PCr], or that the light-induced change of [PCr] is highly heterogeneous in the rod cell.

Cellular and subcellular localization of hexokinase, glutamate dehydrogenase, and alanine aminotransferase in the honeybee drone retina

Subcellular localization of hexokinase in the honeybee drone retina was examined following fractionation of cell homogenate using differential centrifugation. Nearly all hexokinase activity was found in the cytosolic fraction, following a similar distribution as the cytosolic enzymatic marker, phosphoglycerate kinase. The distribution of enzymatic markers of mitochondria (succinate dehydrogenase, rotenone-insensitive cytochrome c reductase, and adenylate kinase) indicated that the outer mitochondrial membrane was partly damaged, but their distributions were different from that of hexokinase. The activity of hexokinase in purified suspensions of cells was fivefold higher in glial cells than in photoreceptors. This result is consistent with the hypothesis based on quantitative 2-deoxy[3H]glucose autoradiography that only glial cells phosphorylate significant amounts of glucose to glucose-6-phosphate. The activities of alanine aminotransferase and to a lesser extent of glutamate dehydrogenase were higher in the cytosolic than in the mitochondrial fraction. This important cytosolic activity of glutamate dehydrogenase was consistent with the higher activity found in mitochondria-poor glial cells. In conclusion, this distribution of enzymes is consistent with the model of metabolic interactions between glial and photoreceptor cells in the intact bee retina.

Membrane conductances involved in amplification of small signals by sodium channels in photoreceptors of drone honey bee

1. Voltage signals of about 1 mV evoked in photoreceptors of the drone honey bee by shallow modulation of a background illumination of an intensity useful for behaviour are thought to be amplified by voltage-dependent Na+ channels. To elucidate the roles of the various membrane conductances in this amplification we have studied the effects of the Na+ channel blocker tetrodotoxin (TTX) and various putative K+ channel blockers on the membrane potential, Vm. 2. Superfusion of a slice of retina with 0.5-10 mM-4-aminopyridine (4-AP) depolarized the membrane and, in fifty of sixty-three cells induced repetitive action potentials. Ionophoretic injection of tetraethylammonium produced similar effects. 3. In order to measure the depolarization caused by 4-AP, action potentials were prevented by application of TTX: 4-AP was applied when the membrane was depolarized to different levels by light. 4-AP induced an additional depolarization at all membrane potentials tested (-64 to -27 mV). We conclude that there are 4-AP-sensitive K+ channels that are open at constant voltage over this range. 4. 4-AP slowed down the recovery phase of the action potential induced by a light flash by a factor that ranged from 0.51 to 0.16. This reduction could be accounted for by the reduction in a voltage-independent K+ conductance estimated from the steady-state depolarization. 5. After the voltage-gated Na+ channels had been blocked by TTX, exposure to 4-AP further changed the amplitude of the response to a small (approximately 10%) decremental light stimulus. The change was an increase when the background illumination brought Vm to potentials more negative than about -40 mV; it was a decrease when Vm > -40 mV. The data could be fitted by a circuit representation of the membrane with a light-activated conductance and a K+ conductance (EK = -66 mV) that was partly blocked by 4-AP. The voltage range studied was from -52 to -27 mV; neither conductance in the model was voltage dependent. 6. The responses to small changes in light intensity in the absence of TTX were mimicked by a model. We conclude that a voltage-dependent Na+ conductance described by the Hodgkin-Huxley equations can amplify small voltage changes in a cell membrane that is also capable of generating action potentials; the magnitude of the K+ conductance is critical for optimization of signals while avoiding membrane instability.

Metabolic signaling between photoreceptors and glial cells in the retina of the drone (Apis mellifera)

Experimental evidence showing metabolic interaction and signaling between photoreceptors-neurons and glial cells of the honeybee drone retina is presented. In this tissue [3H]2-deoxyglucose ([3H]2DG) in the dark and during repetitive light stimulation is phosphorylated to [3H]2-deoxyglucose-6P ([3H]2DG-6P) almost exclusively in the glial cells. Hence, stimulus-induced changes in the rate of formation of [3H]2DG-6P occurs predominantly in the glial cells. Repetitive stimulation of the photoreceptors with light flashes induced about a 47% rise in the rate of formation of [3H]2DG-6P in the glial cells and this effect is probably due to the activation of hexokinase. The potent inhibitor of glycolysis iodoacetic acid (IAA), inhibited this phosphorylation by about 75%. Probably this was largely due to an about 70% decrease of adenosine triphosphate (ATP). Exposure of the retina to IAA suppressed the transient rise in oxygen consumption (delta QO2) in the photoreceptors and subsequently the light-induced receptor potential. This indicates that the supply of a glycolytic substrate by glial cells to the photoreceptors is greatly reduced by IAA. Anoxia, by rapidly suppressing QO2, abolished the receptor potential of the photoreceptors and caused a rapid drop of about 50% in the ATP content of the retina. At the same time the formation of [3H]2DG-6P was inhibited by about 30%. This indicates that respiring photoreceptors send a metabolic signal to glial cells which is suppressed by anoxia.

Single K+ channels in membrane patches of arterial chemoreceptor cells are modulated by O2 tension

Type I cells of the carotid body are known to participate in the detection of O2 tension in arterial blood but the primary chemotransduction mechanisms are not well understood. Here we report the existence in excised membrane patches of type I cells of a single K+ channel type modulated by changes in PO2. Open probability of the O2-sensitive K+ channel reversibly decreased by at least 50% on exposure to hypoxia but single-channel conductance (approximately 20 pS) was unaltered. In the range between 70 and 150 mmHg (1 mmHg = 133 Pa) the decrease of single-channel open probability was proportional to the PO2 measured in the vicinity of the membrane patch. The inhibition of K+ channel activity by low PO2 was independent of the presence of non-hydrolyzable guanine triphosphate analogues at the internal face of the membrane. The results indicate that the O2 sensor of type I cells is in the plasma membrane and suggest that environmental O2 interacts directly with the K+ channels.

Experimental retinal branch vein occlusion in miniature pigs induces local tissue hypoxia and vasoproliferative microangiopathy

In miniature pigs, retinal veins were experimentally occluded using argon laser coagulation. Microvascular modifications leading to retinal hemorrhages and retinal edema were observed some hours after the occlusion. These lesions resolved progressively within 3 weeks after the occlusion, but in most cases ischemic retinal territories persisted. Preretinal partial pressure of oxygen (PO2) measurements, using double barrelled O2-sensitive microelectrodes, showed that all the ischemic areas were indeed hypoxic. In half of the experiments, preretinal and intravitreal new vessels grew on the ischemic territories. Tissue hypoxia appears to be a key step in triggering neovascularization. However, the critical level of hypoxia was not determined.

Scatter photocoagulation restores tissue hypoxia in experimental vasoproliferative microangiopathy in miniature pigs

Experimental retinal branch vein occlusion using argon laser photocoagulation in miniature pigs induced the development of ischemic retinal territories associated with preretinal neovascularization. Preretinal partial pressure of oxygen (PO2) measurements on the ischemic territories, using O2-sensitive microelectrodes, established that the ischemic retinal areas were hypoxic. Scatter photocoagulation of these ischemic hypoxic territories restores the local PO2 to the normal values within 2 weeks. Hence, the reported inhibitory effect of photocoagulation on the development of retinal neovascularization could be due to a reversal effect on tissue hypoxia.

Diffusion of O2 in normal and ischemic retinas of anesthetized miniature pigs in normoxia and hyperoxia

Transretinal PO2 profiles were recorded with O2-sensitive microelectrodes in the normal retina and in ischemic retinal foci induced by the occlusion of a retinal branch vein with argon laser photocoagulation in anesthetized miniature pigs. In the normal retina there are two PO2 gradients: one from the inner retina and the other from the choroid, both directed toward the middle of the retina. Both PO2 gradients persisted during hyperoxia. Thus, even in hyperoxia, the choroid does not supply the whole thickness of the normal retina with O2. Preretinal and transretinal PO2 measurements in ischemic inner retinal foci showed the existence of two PO2 gradients in steady-state systemic normoxia, as did those in the normal retina. This finding indicates that even in ischemia the choroid does not supply O2 to the inner retina; as a result, tissue hypoxia is maintained. During systemic hyperoxia, the intraretinal PO2 measurements in the ischemic foci showed only one gradient going from the choroid toward the inner retina. This gradient indicates that under these conditions, the choroid can supply O2 to the entire thickness of the ischemic retina. Extending a previously formulated hypothesis, we propose that in the ischemic retina as opposed to the normal retina, hyperoxia does not induce an increase in the O2 consumption of the outer retina. This suggestion could explain the rise in PO2 in the inner ischemic retina during hyperoxia.

A method for measuring the oxygen consumption of photoreceptor cells in the steady state and after a brief stimulation by light

The rate of oxygen consumption (QO2) in living tissue cannot be directly measured but may be estimated by mathematically modelling the diffusion of oxygen in the tissue and measuring the local partial pressure of oxygen (PO2). The retina of arthropods contains only two types of cells, photoreceptor and glial cells, which are regularly distributed. Because of this simple structure, simple models of diffusion can be used to estimate the QO2 of the tissue, both in steady state and after a brief stimulation by light. We used a model of diffusion in a plane sheet to calculate the QO2 in a slice of honeybee drone retina, which contains a few thousand cells. We then modified the method slightly and used a model with spherical symmetry to calculate the QO2 in the cluster of three photoreceptor cells of the barnacle and in the single ventral photoreceptor cells of Limulus.

Aerobic glycolysis in the retina of the crab Ocypode ryderi

Our experiments on the isolated and superfused crab retina reveal that pronounced gradients for PO2 and pH exist in this tissue. The PgO2 profiles and a delayed recovery of the PgO2 after hypoxia seem to be a consequence of the oxygen consumption inside the tissue, as much as both characteristics can be abolished by impairment of electron transport in the respiratory chain after application of antimycin A. The pH profile is obviously created by a production and steady release of lactate, which could be measured in the superfusate. As this lactate release is occurring inspite of a sufficient oxygen supply and consumption, it can be concluded that this tissue performs aerobic glycolysis.

Diffusion of O2 in the retina of anesthetized miniature pigs in normoxia and hyperoxia

Intraretinal oxygen tension (pO2) and local electroretinogram (ERG) were simultaneously measured in miniature pigs using double-barreled recess type microelectrodes. Transretinal pO2 profiles were recorded during normoxia and hyperoxia in areas close to (juxta-arteriolar) or far from (intervascular) retinal arterioles. In normoxia, in both areas, the pO2 decreased from the inner retina and the choroid towards the middle of the retina. In the inner retina the gradient of the juxta-arteriolar pO2 profile was steeper than that of the intervascular profile. This characteristic persisted during the breathing of 100% O2. Analysis of the pO2 profiles shows that, even in hyperoxia, the choroid cannot supply O2 to the whole retina. The results also support the conclusions of previous work (Riva, Pournaras and Tsacopoulos, 1986) indicating that in the normal retina it is not the O2 diffusing from the choroid to the retinal arterioles the induces vasoconstriction of these vessels. In the miniature pig this constriction appears to maintain inner retina tissue pO2 at a constant level during hyperoxia. From the pO2 transretinal profiles and previously published choroidal O2 fluxes and pO2 values near retinal vessels an explanatory working hypothesis is formulated according to which O2 consumption (qO2) of the outer retina increases during hyperoxia in the miniature pig.

Honeybee retinal glial cells transform glucose and supply the neurons with metabolic substrate

The retina of the honeybee drone is a nervous tissue in which glial cells and photoreceptor cells (sensory neurons) constitute two distinct metabolic compartments. Retinal slices incubated with 2-deoxy[3H]glucose convert this glucose analogue to 2-deoxy[3H]glucose 6-phosphate, but this conversion is made only in the glial cells. Hence, glycolysis occurs only in glial cells. In contrast, the neurons consume O2 and this consumption is sustained by the hydrolysis of glycogen, which is contained in large amounts in the glia. During photostimulation the increased oxidative metabolism of the neurons is sustained by a higher supply of carbohydrates from the glia. This clear case of metabolic interaction between neurons and glial cells supports Golgi's original hypothesis, proposed nearly 100 years ago, about the nutritive function of glial cells in the nervous system.

Light-induced oxygen consumption in Limulus ventral photoreceptors does not result from a rise in the intracellular sodium concentration

Illumination of Limulus ventral photoreceptors leads to an increase in the intracellular concentration of sodium, [Na+]i, and to an increase in the consumption of O2 (delta QO2). After a 1-s light flash, it takes approximately 480 s for [Na+]i to return to within 10% of its preillumination level, whereas delta QO2 takes approximately 90 s. Thus, the delta QO2 is complete long before [Na+]i has returned to its resting level. Pressure injection of Na+ into the cell in order to elevate [Na+]i to the same levels as attained by illumination causes a rise in [Na+]i that returns to baseline with the same time course as the light-induced rise in [Na+]i. However, the injection of Na+ does not lead to an increase of the consumption of O2. We conclude that activation of the Na pump by a rise in [Na+]i is not a factor involved in the light-induced activation of O2 consumption in these cells.

Activation of mitochondrial oxidative metabolism by calcium ions in Limulus ventral photoreceptor

Cells regulate their metabolic energy production to meet the requirements of their energy consuming activities. For most animal cells the prime site of energy production, in the form of ATP, is the mitochondrion. Extensive in vitro studies of isolated mitochondria have provided detailed information about the specific biochemical reactions involved in energy production. At present there is a debate about whether respiration in excitable cells is controlled by the availability of ADP to the mitochondrion and/or by calcium ions. Using the large ventral photoreceptor of the horseshoe crab (Limulus polyphemus) we describe a method for measuring the transient increase in the mitochondrial O2 consumption (delta QO2) following a flash of light of a single photoreceptor. We then show that this delta QO2 results in part from a rise in the intracellular concentration of calcium (Cai).

Diffusion in slice preparations bathed in unstirred solutions

A diffusion model is described here, which allows for the estimations of drug concentration changes in porous media, such as in slice tissues of the central nervous system (CNS) bathed in unstirred solutions following abrupt changes of drug concentration. This model may be used for the interpretation of data obtained in neuropharmacological studies if (i) the diffusion coefficient of the molecules under investigation is constant within the excised tissue, (ii) drug molecules are diffusing only in the extracellular space (ECS) and are not bound by the tissue, (iii) drug molecules diffuse mainly within one dimension, (iv) the drug concentration in the bath is changed within 5 s, and (v) the bathing solutions at the surfaces of the slices are stagnant during the period of diffusion. To test this model, estimated tetramethylammonium (TMA) ion concentrations within a tissue slice were compared to actual TMA concentration changes measured at the same depth in the tissue of hippocampal slices by means of TMA-sensitive microelectrodes. A statistically significant correlation (P less than 0.05) was observed between the estimated and measured TMA concentrations which indicates that the model is valid under the defined conditions.

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.

Oxygen diffusion in the trout retina

Oxygen tensions were measured in vivo within the different layers of the rainbow trout retina. Oxygen microelectrodes were advanced in 10 micron increments through the retinas and the PO2 measured at each location. Mean retinal PO2 ranged from 124 mmHg at the retinal-vitreal interface to 381 mmHg at the choriocapillaris. These data reaffirm that the cellular layers of the normal trout retina are continually exposed to supra-arterial oxygen tensions that are known to cause toxicity in other species. The intraretinal PO2 gradient was mathematically characterized and results indicate that the in vivo oxygen profile can be described by a two-component exponential function. In order to gain a better understanding of oxygen delivery to the retina, the data were also subjected to an analysis based upon the classical equation for planar diffusion. The trout retina is an excellent model for studying oxygen diffusion in vivo since this tissue is supplied with oxygen from a single source, thereby simplifying the mathematical analysis. Calculations yielded values of 1.86 X 10(-5) and 0.58 X 10(-5) ml O2 min-1 cm-1 atm-1 (at 9 degrees C) for the Krogh permeation coefficient (DS) for the photoreceptor region and the remaining neural retina, respectively.

Reappraisal of diffusion, solubility, and consumption of oxygen in frog skeletal muscle, with applications to muscle energy balance

Previously we tested the validity of the one-dimensional diffusion equation for O2 in the excised frog sartorius muscle and used it to measure the diffusion coefficient (D) for O2 in this muscle and the time course of its rate of O2 consumption (Qo2) after a tetanus (Mahler, 1978, 1979, J. Gen. Physiol., 71:533-557, 559-580, 73:159-174). A transverse section of the frog sartorius is in fact well fit by a hemi-ellipse with width divided by maximum thickness averaging 5.1 +/- 0.2. Using the previous techniques with the two-dimensional diffusion equation and this hemi-elliptical boundary yields a value for D that is 30% smaller than reported previously; the revised values at 0, 10, and 22.8 degrees C are 6.2, 7.9, and 10.8 X 10(-6) cm2/s, respectively. After a tetanus at 20 degrees C, Qo2 rose quickly to a peak and then declined exponentially, with a time constant (tau) approximately 15% faster than that reported previously; tau averaged 2.1 min in Rana temporaria and 2.6 min in Rana pipiens. A technique was devised to measure the solubility (alpha) of O2 in intact, respiring muscles, and yielded alpha (muscle)/alpha (H2O) = 1.26 +/- 0.04. With these modifications, the values for O2 consumption obtained with the diffusion method were in agreement with those measured by the direct method of Kushmerick and Paul (1976, J. Physiol. [Lond.]., 254:693-709). Using results from both methods, at 20 degrees C the ratio of phosphorylcreatine split during a tetanus to O2 consumption during recovery ranged from 5.2 to 6.2 mumol/mumol, and postcontractile ATP hydrolysis was estimated to be 13.6 +/- 4.1 (n = 3) nmol/mumol total creatine.

Interaction between a normoxic and a hypoxic region of guinea pig and ferret papillary muscles

Papillary muscles were mounted in a three-compartment bath. The tip of the muscle was exposed to hypoxic and glucose-free solution. The other parts of the preparation were superfused with Tyrode's solution, building a free-flow border between hypoxic and normoxic superfusates. The normoxic part of the bath was subdivided by a rubber membrane so that current pulses could be applied between segments of the preparation. Signs of electrotonic interaction between normoxic and hypoxic parts were observed a few minutes after the onset of hypoxia. Transmembrane action potentials in the normoxic part retained their plateau, but progressively shortened. Those in the hypoxic tip showed an early phase of rapid repolarization followed by a plateau phase near the resting potential. Terminal repolarization in the two parts coincided for many minutes. After 35 minutes, fast propagated activity ceased in the tip and was replaced first by conducted slow responses, then by decremental conduction. At 50 minutes, cells near the borderline had resting potentials of either -76 +/- 7 mV (SD, n = 9) in normoxic tissue or -16 +/- 3 mV (SD, n = 9) in hypoxic tissue. Concurrently, subthreshold potentials no longer appeared to spread into the tip. Unipolar electrograms remained diphasic over the normoxic part but lost their negative deflection near the borderline, implying the absence of axial current flow into the hypoxic part. Furthermore, electrotonic potentials generated by current flow across the rubber membrane did not spread beyond a line of demarcation. Reduced nicotinamide adenine dinucleotide fluorescence increased in the hypoxic part, and appeared to correlate with the development of electrical decoupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of potassium movement through the retina of the drone (Apis mellifera male) by glial uptake

Intracellular recordings were made in photoreceptors and glial cells (outer pigment cells) of the superfused cut head of the honey-bee drone (Apis mellifera male). When the [K+] in the superfusate was abruptly increased from 3.2 mM to 17.9 mM both photoreceptors and glial cells depolarized. The time course of the depolarization of the photoreceptors was slower with increasing depth from the surface. Half time of depolarization was plotted against depth: this graph was compatible with the arrival of K+ being exclusively by diffusion through the extracellular clefts. However, as we then showed, this interpretation is inadequate. The time course of depolarization of the glial cells was almost the same at all depths. This indicates that they are electrically coupled. Consequently, current-mediated K+ flux (spatial buffering) through glial cells will contribute to the transport of K+ through the tissue: K+ ions enter the glial syncytium in the region of high external potassium concentration, [K+]0, and an equivalent quantity of K+ ions leave in regions of low [K+]0. Intracellular K+ activity (aiK) was measured with double-barrelled K+-sensitive micro-electrodes in slices of retina superfused on both faces. When [K+] in the superfusate was increased from 7.5 mM to 17.9 mM an increase in aiK was observed in glial cells at all depths in the slice (initial rate 1.7 mM min-1, S.E. of the mean = 0.2 mM min-1), but there was little increase in the photoreceptors (0.3 +/- 0.2 mM min-1). The increase in aiK in glial cells near the centre of the slice could not have been caused by spatial buffering; it presumably resulted from net uptake. We conclude that when [K+] is increased at the surface of this tissue, the build up of K+ in the extracellular clefts depends on extracellular diffusion, spatial buffering and net uptake. The latter two processes, which have opposing effects, involve about 10 times as much K+ as the first. This is in rough agreement with less direct experiments on mammalian brain (Gardner-Medwin, 1977, 1983b).

Resting membrane potential, extracellular potassium activity, and intracellular sodium activity during acute global ischemia in isolated perfused guinea pig hearts

Transmembrane potentials, extracellular potassium activity, and intracellular sodium activity were determined during acute global ischemia in Langendorff perfused guinea pig ventricles by microelectrode techniques. Resting membrane potential decreased with a sigmoidal time course from -82 mV to -49.5 +/- 2.7 mV (SD, n = 6) and extracellular potassium activity increased from 4 to 5 mM to 14.7 +/- 1.3 mM (n = 8) during 15 minutes of ischemia. The estimated potassium equilibrium potential was 7 mV more negative than resting membrane potential prior to occlusion, but approached resting membrane potential during ischemia. An increase in extracellular potassium accumulation occurred when heart rate was increased abruptly from 60 to 170 beats/min. After rapid stimulation, a transient decrease of extracellular potassium activity occurred which was abolished in the presence of 10(-6) M strophanthidin. If the preparations were paced before and after aortic occlusion at a constant rate, potassium accumulation was independent of heart rate within a range of 50-170 beats/min. Intracellular sodium activity was 8.8 +/- 2.8 mM (n = 8) prior to occlusion and decreased slightly to values between 4.7 and 7.6 mM after 10-15 minutes of ischemia. The results suggest that relative potassium permeability largely predominates over relative sodium permeability during the decrease of resting membrane potential after interruption of aortic flow. Furthermore, active sodium-potassium exchange compensates for the rate-dependent fraction of potassium efflux and maintains a low intracellular sodium activity. For reasons of electroneutrality, the potassium efflux underlying extracellular potassium accumulation must be balanced by an equivalent charge movement which is not carried by sodium. The most probable hypothesis regarding the charge carriers is that net potassium efflux occurs secondary to efflux of phosphate and lactate generated during ischemia.

Oxygen uptake occurs faster than sodium pumping in bee retina after a light flash

When neurones are active there is an entry of Na+, which must subsequently be pumped out, and an increase in their oxygen consumption rate (Qo2). The Na+ pump derives its energy from ATP, splitting it into ADP and Pi, and it has reasonably been proposed that the changes in concentrations of ATP, ADP and Pi lead to a stimulation of the O2 consumption by the mitochondria and hence to a restoration of the stock of ATP. Here we present evidence suggesting that Qo2 must be controlled differently in the retinal photoreceptor cells of the honeybee drone. Stimulation of drone photoreceptors with a flash of light causes an entry of Na+ (ref. 4) and a transient increase in Qo2 that indicates respiration of the right order of magnitude to provide ATP to pump the Na+ out. We report intracellular recordings of changes in intracellular sodium (Nai+) and potassium (Ki+) in response to single light flashes and have compared the time course of extra oxygen consumption (delta Qo2) with these ion changes and other indices of Na+ pumping. We found that the time course of pumping seems to lag behind the time course of delta Qo2. It follows that the mitochondrial respiration must be stimulated by some signal which is generated earlier than the rise in ADP produced by the Na+ pump.

PO2-profiles in hippocampal slices of the guinea pig

For a detailed analysis of the oxygen supply of hippocampal slices, tissue PO2 (Pt,O2) was recorded polarographically in the neural layers of thick and thin slice preparations from the guinea pig. The experiments showed that the Pt,O2-gradients were extremely steep in the outer zones of vital slices. In an air equilibrated salt solution the surface PO2 was reduced to less than 50% within ca. 25 micron. Minimum values were measured at a depth of ca. 150 micron. A rise of temperature lowered the oxygen supply in the deeper layers of the excised tissue. An elevation of the surface PO2 hardly improved Pt,O2 in the deep structures, because the O2-consumption of the hippocampal slices increased with rising PO2.

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).