Effects of light and darkness on oxygen distribution and consumption in the cat retina

These experiments were done to investigate the effects of light and darkness on the oxygenation of the retina in anesthetized cats. Measurements were made with double-barreled oxygen microelectrodes capable of recording both oxygen tension (PO2) and local voltages. Diffuse white illumination presented to a dark-adapted retina led to an increase in PO2 of up to 30 mmHg in the outer half of the retina. Changes were maximal at approximately 75% depth, corresponding to the outer nuclear layer. No change or decrease in PO2 was observed in the inner retina. Light-evoked increases in outer retinal PO2 were graded with the duration and strength of illumination, and were maximal in response to 60 s of illumination at rod saturation. For these stimuli, the increase at the onset of illumination was slower (average half-time, 12.2 s) than the recovery at the end of illumination (average half-time, 5.9 s), but for stimuli above rod saturation, PO2 recovered much more slowly. The profile of PO2 was measured during electrode penetration and withdrawal and during light and dark adaptation. Dark-adapted profiles were characterized by a minimum PO2 of nearly 0 mmHg at depths of 65-85%, and a steep gradient from the minimum to the choroid. During light adaptation at rod saturation, PO2 was elevated in the outer half of the retina and the minimum was eliminated. Fits of the profiles to a one-dimensional model of oxygen diffusion indicated that light reduced the oxygen consumption of the outer retina to approximately 50% of its dark-adapted value.     

Oxygen distribution and consumption in the macaque retina

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. P(O2) was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O(2) microelectrodes. Oxygen consumption (Q(O2)) of the photoreceptors was extracted by fitting a steady-state diffusion model to P(O2) measurements. In the perifovea, the P(O2) was 48 +/- 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 +/- 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The P(O2) in the inner half of the retina in darkness was 17.9 +/- 7.8 mmHg. When averaged over the outer retina, photoreceptor Q(O2) (called Q(av)) was 4.6 +/- 2.3 ml O(2).100 g(-1).min(-1) under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Q(av) to 72 +/- 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O(2) from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Q(O2) of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Q(O2) with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.     

Outer retinal anoxia during dark adaptation is not a general property of mammalian retinas

The purpose of this study was to measure the intraretinal oxygen distribution across the retina under conditions, which maximise outer retinal oxygen consumption. In particular, we looked for evidence of increased oxygen delivery from the choroid and the deep retinal capillary layer, and whether or not this was sufficient to avoid the development of intraretinal anoxia. Under dark-adapted conditions the photoreceptors need additional energy, at least part of which is derived from increased oxidative metabolism. In earlier studies in the cat retina it was revealed that dark adaptation could render some regions of the outer retina anoxic. The present study of the in vivo oxygen distribution across the rat retina in light and dark found no evidence of outer retinal anoxia in the dark. This was despite a mean increase of 52.6+/-11.4% (n=7) in outer retinal oxygen consumption in the dark. The mean value for the minimum outer retinal PO(2) in the dark was 5.2+1.2 mmHg. Oxygen delivery from both the choroid and the deep retinal capillary layer increased in the dark (P<0.01, and P<0.001, respectively). It is argued that the ability of the deep capillary layer to compensate for changes in oxygen demand in the outer retina is an important element in the maintenance of homeostasis in the retina. This is in addition to the role of the deep capillary layer in supplying oxygen to the highly consuming plexiform layers within the inner retina. These findings in the rat retina also demonstrate that intraretinal anoxia in the dark, is not, as implied by earlier work in the cat, a general feature of mammalian retinas.     

Light-induced changes in energy metabolites, guanine nucleotides, and guanylate cyclase within frog retinal layers

Freeze-dried sections were prepared from retinas of frogs which were dark-adapted or exposed to varying periods of light. Samples of the discrete layers were dissected, weighed, and analyzed for energy metabolites, guanylate compounds, and the enzyme guanylate cyclase. ATP and P-creatine were measured in both dark- and light-adapted retinas. There was a gradient in ATP and P-creatine levels in dark-adapted retinas, with the lower concentrations in the photoreceptors, and increasing concentrations in the inner retina. After light adaptation, concentrations increased, an observation which supports the concept that transmitter release occurs in the dark and ceases in the light. The sum of GTP plus GDP, GDP, and cyclic GMP were analyzed in dark-adapted retinas and after exposure to 2 min or 2 h of room light. GDP was rather uniformly distributed in the retinal layers, was increased by 2 min of light in all layers but the outer nuclear, and remained elevated at 2 h in the inner retina. GTP values showed a marked localization in the outer nuclear layer, which increased after 2 min or 2 h of illumination; in all other layers GTP was decreased by light. Cyclic GMP in the dark was highest in the photoreceptor cells, decreasing to one-third after 2 min of light; there were significant increases in the outer plexiform and inner nuclear layers at this time. Cyclic GMP remained low in the photoreceptor cells even after 2 h of light, while the inner layers returned to dark values. Guanylate cyclase, like cyclic GMP, was largely confined to the photoreceptor cells and showed a maximal increase after 2 min of light exposure.     

Site-specific phosphorylation of phosducin in intact retina. Dynamics of phosphorylation and effects on G protein beta gamma dimer binding

Phosducin (Pdc) is a G protein beta gamma dimer (G beta gamma) binding protein, highly expressed in retinal photoreceptor and pineal cells, yet whose physiological role remains elusive. Light controls the phosphorylation of Pdc in a cAMP and Ca(2+)-dependent manner, and phosphorylation in turn regulates the binding of Pdc to G(t)beta gamma or 14-3-3 proteins in vitro. To directly examine the phosphorylation of Pdc in intact retina, we prepared antibodies specific to the three principal phosphorylation sites (Ser-54, Ser-73, and Ser-106) and measured the kinetics of phosphorylation/dephosphorylation during light/dark adaptation and the subsequent effects on G(t)beta gamma binding. Ser-54 phosphorylation increased slowly (t((1/2)) approximately 90 min) during dark adaptation to approximately 70% phosphorylated and decreased rapidly (t((1/2)) approximately 2 min) during light adaptation to less than 20% phosphorylated. Ser-73 phosphorylation increased much faster during dark adaptation (t((1/2)) approximately 3 min) to approximately 50% phosphorylated and decreased more slowly during light adaptation (t((1/2)) approximately 9 min) to less than 20% phosphorylated. The Ca(2+) chelator BAPTA-AM blocked Ser-54 phosphorylation during dark adaptation but had no effect on Ser-73 phosphorylation. In contrast, Ser-106 was not phosphorylated in either the light or dark. Importantly, G beta gamma binding to Pdc was enhanced by Ca(2+) chelation and the binding kinetics closely paralleled those of Ser-54 dephosphorylation, indicating that Ser-54 phosphorylation controls G(t)beta gamma binding in vivo. These results suggest a pivotal role of Ser-54 and Ser-73 phosphorylation in determining the interactions of Pdc with its binding partners, G(t)beta gamma and 14-3-3 protein, which may regulate the light-dependent translocation of the photoreceptor G protein.     

Role of nitric oxide in capillary perfusion and oxygen delivery regulation during systemic hypoxia

The role of nitric oxide (NO) and reactive oxygen species (ROS) in regulating capillary perfusion was studied in the hamster cheek pouch model during normoxia and after 20 min of exposure to 10% O2-90% N2. We measured PO2 by using phosphorescence quenching microscopy and ROS production in systemic blood. Identical experiments were performed after treatment with the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) and after the reinfusion of the NO donor 2,2'-(hydroxynitrosohydrazono)bis-etanamine (DETA/NO) after treatment with L-NMMA. Hypoxia caused a significant decrease in the systemic PO2. During normoxia, arteriolar intravascular PO2 decreased progressively from 47.0 +/- 3.5 mmHg in the larger arterioles to 28.0 +/- 2.5 mmHg in the terminal arterioles; conversely, intravascular PO2 was 7-14 mmHg and approximately uniform in all arterioles. Tissue PO2 was 85% of baseline. Hypoxia significantly dilated arterioles, reduced blood flow, and increased capillary perfusion (15%) and ROS (72%) relative to baseline. Administration of L-NMMA during hypoxia further reduced capillary perfusion to 47% of baseline and increased ROS to 34% of baseline, both changes being significant. Tissue PO2 was reduced by 33% versus the hypoxic group. Administration of DETA/NO after L-NMMA caused vasodilation, normalized ROS, and increased capillary perfusion and tissue PO2. These results indicate that during normoxia, oxygen is supplied to the tissue mostly by the arterioles, whereas in hypoxia, oxygen is supplied to tissue by capillaries by a NO concentration-dependent mechanism that controls capillary perfusion and tissue PO2, involving capillary endothelial cell responses to the decrease in lipid peroxide formation controlled by NO availability during low PO2 conditions.     

Light-dependent dynamics of gap junctions between horizontal cells in the retina of the crucian carp

Summary The dynamics of gap junctions between outer horizontal cells or their axon terminals in the retina of the crucian carp were investigated during light and dark adaptation by use of ultrathin-section and freeze-fracture electron microscopy. Light adaptation was induced by red light, while dark adaptation took place under ambient dark conditions. The two principal findings were: (1) The density of connexons within an observed gap junction is high in dark-adapted retina, and low in light-adapted retina. This, respectively, may reflect the coupled and uncoupled state of the gap junction. (2) The size of individual gap junctions is larger in light-than in dark-adapted retinae. Whereas the overall area occupied by gap junctions is reduced with dark adaptation, the percentage of small and very small gap junctions increases dramatically. A lateral shift of connexons in the gap junctional membrane is strongly suggested by these reversible processes of densification and dispersion. Two additional possibilities of gap junction modulation are discussed: (1) the de novo formation of very small gap junctions outside the large ones in the first few minutes of dark adaptation, and (2) the rearrangement of a portion of the very large gap junctions. The idea that the cytoskeleton is involved in such modulatory processes is corroborated by thin-section observations.Dedicated to Professor J. Peiffer on the occasion of his 65th birthday     

Renal oxygen delivery and consumption during progressive hypoxemia in the anesthetized dog

The relationship between renal oxygen delivery (RDO2) and function was evaluated during progressive hypoxemia. Seven anesthetized, spontaneously breathing dogs were given progressively lower oxygen concentrations to breathe while monitoring renal O2 consumption (RVO2), renal hemodynamic and excretory function. In addition, basal RVO2 was determined in three models of kidneys without filtration. RDO2 averaged 3648 mumole O2/min/100 g during normoxia. Basal RVO2 averaged 100 mumole O2/min/100 g kidney while total RVO2 was 466 mumole O2/min/100 g kidney during normoxia, leaving 366 mumole O2/min/100 g consumed by those processes involved in tubular transport. During hypoxemia, all renal parameters were well maintained until the lowest PaO2 (24.2 Torr). At this level, total RVO2 and RDO2 were significantly reduced. However, RDO2 remained well above RVO2 throughout hypoxemia. The reduction in RVO2 was a direct result of decreased O2 demand, as glomerular filtration and tubular load were also reduced. This associated decrease in O2 demand and RVO2 was indicated by the fact that the renal (a - v)O2 difference remained low and unchanged (1.9 vol%), fractional sodium excretion was unchanged, and the ratio of tubular sodium reabsorption to RVO2 also remained unchanged (30.8 meq Na/mmole O2). It was concluded that hypoxemia, while reducing both RDO2 and RVO2 at the lowest PaO2 (24.2 Torr), did not functionally impair renal excretory function by limiting RDO2 to the tubular transport processes. A reduction in RBF is far more likely to compromise the RDO2 needed to sustain basal and active transport processes than hypoxemia itself.     

Effects of hypoxia on potassium homeostasis and pigment epithelial cells in the cat retina

Intracellular recordings show that light-evoked hyperpolarizations of the apical and basal membranes of the cat retinal pigment epithelium (RPE) are altered by mild hypoxia. RPE cells, like glia, have a high K+ conductance, and measurements with K+-sensitive microelectrodes show that the hypoxic changes in the RPE cell are largely the result of changes in extracellular [K+] in the subretinal space [( K+]o) rather than direct effects on RPE cells. During hypoxia, light-evoked [K+]o responses and membrane responses have longer times to peak, slower and less complete recovery during illumination, and larger amplitudes. In addition to the effects on light-evoked responses, hypoxia causes a depolarization of first the apical and then the basal membranes of RPE cells under dark-adapted conditions. The basal depolarization is accompanied by a decrease in basal membrane resistance. These depolarizations appear to be caused by a rapid increase in [K+]o at the onset of hypoxia, which is maximal in dark adaptation, and smaller if the retina is subjected to maintained illumination. All of the effects are graded with the severity of hypoxia and can be observed at arterial oxygen tensions as high as 65 mmHg, although the threshold may be even higher. We argue that the origin of hypoxic [K+]o changes is probably an inhibition of the photoreceptors' Na+/K+ pump. This work then suggests that photoreceptors are more sensitive to hypoxia than previously believed, and that the high oxygen tension normally provided by the choroidal circulation is necessary for normal photoreceptor function.     

Low oxygen consumption in the inner retina of the visual streak of the rabbit

The oxygen requirements of different retinal layers are of interest in understanding the vulnerability of the retina to hypoxic damage in retinal diseases with an ischemic component. Here, we report the first measurements of retinal oxygen consumption in the visual streak of the rabbit retina, the region with the highest density of retinal neurons, and compare it with that in the less-specialized region of the retina underlying the vascularized portion of the rabbit retina. Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of retinal depth in anesthetized animals. Measurements were performed in the region of the retina containing overlying retinal vessels and in the center of the visual streak. Established mathematical analyses of the intraretinal oxygen distribution were used to quantify the rate of oxygen consumption in the inner and outer retina and the relative oxygen contributions from the choroidal and vitreal sides. Outer retinal oxygen consumption was higher in the visual streak than in the vascularized area (means +/- SE, 284 +/- 20 vs. 210 +/- 23 nl O2.min(-1) x cm(-2), P = 0.026, n = 10). However, inner retinal oxygen consumption in the visual streak was significantly lower than in the vascular area (57 +/- 4.3 vs. 146 +/- 12 nl O2 x min(-1) x cm(-2), P < 0.001). We conclude that despite the higher processing requirements of the inner retina in the visual streak, it has a significantly lower oxygen consumption rate than the inner retina underlying the retinal vasculature. This suggests that the oxygen uptake of the inner retina is regulated to a large degree by the available oxygen supply rather than the processing requirements of the inner retina alone.     

Gill chloride cell proliferation and respiratory responses to hypoxia of the neotropical erythrinid fish Hoplias malabaricus

The effect of chloride cell proliferation on the respiratory function was evaluated by measuring oxygen consumption (VO2) and ventilatory parameters during normoxia and gradual hypoxia in the tropical fish Hoplias malabaricus. Chloride cell proliferation was induced by keeping fish in deionized water, and the effect on the respiratory function was measured on the 1st, 2nd, and 7th day in this water using a flow-through respirometry system. Plasma osmolarity and the partial pressure of arterial oxygen (PaO2) and carbon dioxide (PaCO2) were measured under conditions of normoxia and severe hypoxia. Chloride cell proliferation on the lamellae significantly increased the water-blood diffusion distance on the 2nd and 7th day in deionized water. VO2 was kept constant until the critical oxygen pressure (PcO2) of 21.6+/-0.9 mmHg in both the control and deionized water fish was reached. The ventilatory parameters were higher in deionized water fish in normoxia, and increased during hypoxia, matching decreases in the water's partial O2 pressure. Impairment of the respiratory function was evidenced by the decrease of PaO2 of deionized water fish in normoxic condition. However, despite the changes in the epithelial morphology of gills in fish kept in deionized water, H. malabaricus proved be a hypoxic-tolerant tropical species.     

Oxygen transport in the rat brain cortex at normobaric hyperoxia

The distribution of oxygen tension (PO(2)) in microvessels and in the tissues of the rat brain cortex on inhaling air (normoxia) and pure oxygen at atmospheric pressure (normobaric hyperoxia) was studied with the aid of oxygen microelectrodes (diameter = 3-6 microm), under visual control using a contact optic system. At normoxia, the PO(2) of arterial blood was shown to decrease from [mean (SE)] 84.1 (1.3) mmHg in the aorta to about 60.9 (3.3) mmHg in the smallest arterioles, due to the permeability of the arteriole walls to oxygen. At normobaric hyperoxia, the PO(2) of the arterial blood decreased from 345 (6) mmHg in the aorta to 154 (11) mmHg in the smallest arterioles. In the blood of the smallest venules at normoxia and at normobaric hyperoxia, the differences between PO(2) values were smoothed out. Considerable differences between PO(2) values at normoxia and at normobaric hyperoxia were found in tissues at a distance of 10-50 microm from the arteriole walls (diameter = 10-30 microm). At hyperbaric hyperoxia these values were greater than at normoxia, by 100-150 mmHg. In the long-run, thorough measurements of PO(2) in the blood of the brain microvessels and in the tissues near to the microvessels allowed the elucidation of quantitative changes in the process of oxygen transport from the blood to the tissues after changing over from the inhalation of air to inhaling oxygen. The physiological, and possibly pathological significance of these changes requires further analysis.     

Visual pigment and photoreceptor sensitivity in the isolated skate retina

Photoreceptor potentials were recorded extracellularly from the aspartate-treated, isolated retina of the skate (Raja oscellata and R. erinacea), and the effects of externally applied retinal were studied both electrophysiologically and spectrophotometrically. In the absence of applied retinal, strong light adaptation leads to an irreversible depletion of rhodopsin and a sustained elevation of receptor threshold. For example, after the bleaching of 60% of the rhodopsin initially present in dark-adapted receptors, the threshold of the receptor response stabilizes at a level about 3 log units above the dark-adapted value. The application of 11-cis retinal to strongly light-adapted photoreceptors induces both a rapid, substantial lowering of receptor threshold and a shift of the entire intensity-response curve toward greater sensitivity. Exogenous 11-cis retinal also promotes the formation of rhodopsin in bleached photoreceptors with a time-course similar to that of the sensitization measured electrophysiologically. All-trans and 13-cis retinal, when applied to strongly light-adapted receptors, fail to promote either an increase in receptor sensitivity or the formation of significant amounts of light-sensitive pigment within the receptors. However, 9-cis retinal isin. These findings provide strong evidence that the regeneration of visual pigment in the photoreceptors directly regulates the process of photochemical dark adaptation.     

Relation between fetal arterial PO2 and oxytocin-induced uterine contractions in pregnant sheep

The relation between oxytocin-induced type A uterine contractions and fetal arterial PO2, measured continuously with an intravascular oxygen electrode, was studied in nine chronically catheterized sheep during late pregnancy. Oxytocin provoked dose-related increases in intrauterine pressure (IUP) and decreases in fetal PaO2. There was a significant positive relationship between changes in IUP and the maximum decrease in fetal PaO2 (average r = 0.696, df = 92; P less than 0.001). We conclude that changes in uterine activity contribute to transient fetal hypoxemia, and that administration of exogenous oxytocin provides an experimental paradigm to examine the consequences of this relationship.     

Influence of hypoxia and of hypoxemia on the development of cardiac activity in zebrafish larvae

Cardiac activity and anaerobic metabolism were analyzed in zebrafish larvae raised under normoxia (PO(2) = 20 kPa) and under chronic hypoxia (PO(2) = 10 kPa) at three different temperatures (25, 28, and 31 degrees C). Heart rate increased with development and with temperature. Under normoxia, cardiac output increased significantly at high temperature (31 degrees C), but not at 28 or at 25 degrees C. Under chronic hypoxia, however, heart rate as well as cardiac output increased at all temperatures in larvae at about hatching time or shortly thereafter. Cardiac activity of larvae raised for 2 wk after fertilization with a reduced hemoglobin oxygen-carrying capacity in their blood (hypoxemia; due to the presence of CO or of phenylhydrazine in the incubation water) was not different from control animals. Whole body lactate content of these animals did not increase. Thus there was no indication of a stimulated anaerobic energy metabolism. The increase in cardiac activity observed during hypoxia suggests that at about hatching time receptors are present that sense hypoxic conditions, and this information can be used to induce a stimulation of convective oxygen transport to compensate for a reduction in bulk oxygen diffusion in the face of a reduced oxygen gradient between environmental water and tissues. Under normoxia, however, the PO(2) gradient between environmental water and tissues and diffusional oxygen transport assure sufficient oxygen supply even if hemoglobin oxygen transport in the blood is severely impaired. Thus, under normoxic conditions and with a normal metabolic rate of the tissues, convective oxygen transport is not required until approximately 2 wk after fertilization.     

Active and resting states of the O2-evolving complex of photosystem II

During dark adaptation, a change in the O2-evolving complex (OEC) of spinach photosystem II (PSII) occurs that affects both the structure of the Mn site and the chemical properties of the OEC, as determined from low-temperature electron paramagnetic resonance (EPR) spectroscopy and O2 measurements. The S2-state multiline EPR signal, arising from a Mn-containing species in the OEC, exhibits different properties in long-term (4 h at 0 degrees C) and short-term (6 min at 0 degree C) dark-adapted PSII membranes or thylakoids. The optimal temperature for producing this EPR signal in long-term dark-adapted samples is 200 K compared to 170 K for short-term dark-adapted samples. However, in short-term dark-adapted samples, illumination at 170 K produces an EPR signal with a different hyperfine structure and a wider field range than does illumination at 160 K or below. In contrast, the line shape of the S2-state EPR signal produced in long-term dark-adapted samples is independent of the illumination temperature. The EPR-detected change in the Mn site of the OEC that occurs during dark adaptation is correlated with a change in O2 consumption activity of PSII or thylakoid membranes. PSII membranes and thylakoid membranes slowly consume O2 following illumination, but only when a functional OEC and excess reductant are present. We assign this slow consumption of O2 to a catalytic reduction of O2 by the OEC in the dark. The rate of O2 consumption decreases during dark adaptation; long-term dark-adapted PSII or thylakoid membranes do not consume O2 despite the presence of excess reductant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Light-initiated changes of cyclic guanosine monophosphate levels in the frog retina measured with quick-freezing techniques

Although there is good agreement that light reduces the amount of cyclic GMP (cGMP) in the retina, the exact time-course of this decrease is not well established. Bullfrog retinal sections were isolated under infrared light and quick-frozen with liquid nitrogen-cooled, metal hammers after exposure to various intensities of continuous illumination. This quick-freezing should stop the degradation of cGMP within 50-100 ms. The frozen retinal sections were then slowly warmed up in the presence of perchloric acid to denature enzymes involved in cGmp metabolism. cGMP was determined by radioimmunoassay and comparison was made between light- and dark-adapted retinal sections from the same animal. The average cGMP concentration was 44.3 +/- 0.7 pmol cGMP/mg protein or 170.9 +/- 3.2 pmol cGMP/retina. After 1 s of illumination no significant change in cGMP concentration was found even with the brightest light used (approximately 7 x 10(7) rhodopsins bleached/second per rod. At this intensity the first significant decrease in cGMP from dark-adapted levels was detected 3-5 s after the initiation of illumination; cGMP decayed to 70-75% of the dark-adapted value after approximately 30 s. With lower intensity illumination the cGMP levels recovered to dark-adapted levels after the initial decrease even though the bleaching light remained on.     

Outer retinal fine structure of the garfish Belone belone (L.) (Belonidae, Teleostei) during light and dark adaptation – photoreceptors, cone patterns and densities

In the present EM study, we investigate the retina of Belone belone , a visually-orientated marine predator living close to the water surface. In the duplex retina, four morphologically different cone types are observed: unequal and equal double cones, long single cones and triple cones. In the light-adapted state, five different cone patterns occur: row, twisted row, square, pentagonal and hexagonal patterns. High double cone densities are found ventro-nasally, ventro-temporally and dorso-temporally. Throughout the retina the double cone/single cone ratio is 2 : 1, in the ventral part, however, a 1 : 1 ratio occurs. In the vitreous body we found a curtain-like intraocular septum dividing the retina into two morphologically different regions. In most areas of the dark-adapted retina the cone patterns are absent at the ellipsoid level, with long single cones standing more vitreally in the light path than double cones. The mosaics are retained, however, in the outer nuclear layer. Typical dark adaptation, i.e. the retinomotor movements of the retinal pigment epithelium and photoreceptors in response to the dark adaptation (light change) is not present in the peripheral ventral and parts of the central ventral area. In both regions we found a twisted row pattern of cones having a vitreal position. The findings are discussed with respect to the photic habitat and feeding habits of this species.     

Studies on the Relation between the Pigment Migration and the Sensitivity Changes during Dark Adaptation in Diurnal and Nocturnal Lepidoptera

The functional significance of the pigment migration in the compound insect eye during dark adaptation has been studied in diurnal and nocturnal Lepidoptera. Measurements of the photomechanical changes were made on sections of eyes which had been dark-adapted for varying periods of time. In some experiments the sensitivity changes during dark adaptation were first determined before the eye was placed in the fixation solution. No change in the position of the retinal pigment occurred in Cerapteryx graminis until the eye had been dark-adapted for about 5 minutes. The start of the migration was accompanied by the appearance of a break in the dark adaptation curve. During longer periods of dark adaptation the outward movement of the pigment proceeded in parallel with the change in sensitivity, the migration as well as the adaptive process being completed within about 30 minutes. In the diurnal insects chosen for the present study (Erebia, Argynnis) the positional changes of the retinal pigment were insignificant in comparison with the movement of the distal pigment in Cerapteryx graminis. On the basis of these observations the tentative hypothesis is put forward that the second phase of adaptive change in nocturnal Lepidoptera is mediated by the migration of the retinal pigment while the first phase is assumed to be produced by the resynthesis of some photochemical substance. In diurnal insects which have no appreciable pigment migration the biochemical events alone appear to be responsible for the increase in sensitivity during dark adaptation.     

Ventilatory effects and interactions with change in PaO2 in awake goats.

We utilized selective carotid body (CB) perfusion while changing inspired O2 fraction in arterial isocapnia to characterize the non-CB chemoreceptor ventilatory response to changes in arterial PO2 (PaO2) in awake goats and to define the effect of varying levels of CB PO2 on this response. Systemic hyperoxia (PaO2 greater than 400 Torr) significantly increased inspired ventilation (VI) and tidal volume (VT) in goats during CB normoxia, and systemic hypoxia (PaO2 = 29 Torr) significantly increased VI and respiratory frequency in these goats. CB hypoxia (CB PO2 = 34 Torr) in systemic normoxia significantly increased VI, VT, and VT/TI; the ventilatory effects of CB hypoxia were not significantly altered by varying systemic PaO2. We conclude that ventilation is stimulated by systemic hypoxia and hyperoxia in CB normoxia and that this ventilatory response to changes in systemic O2 affects the CB O2 response in an additive manner.