The osmotic link between hypoglycaemia and hypovolaemia.

OBJECTIVE: Hypoglycaemia is regularly accompanied by hypovolaemia. To suggest a mechanism for this phenomenon, we reviewed data from eight studies conducted by our group and examined the circumstances under which rebound hypoglycaemia develops after intravenous infusion of glucose solutions. MATERIAL AND METHODS: Forty healthy volunteers and 40 patients received a total of 122 infusions of glucose solutions at different rates, volumes and concentrations. Plasma glucose and the haemodilution were measured repeatedly during and for at least 2 h after the infusions ended. Glucose kinetics was calculated using a one-compartment turnover model and the plasma volume expansion was estimated from changes in Hb. RESULTS: A strong linear correlation was found between the glucose level and the plasma volume expansion in all series of experiments (p<0.001). After infusion, there was a risk of hypoglycaemia and hypovolaemia developing in healthy volunteers with a high glucose clearance and when infusing glucose solutions of higher concentrations than 2.5 %. Few and mild hypoglycaemic events occurred in patients with insulin resistance, such as in diabetics and in those undergoing surgery. The immediate linear relationship between hypoglycaemia and hypovolaemia suggests an osmotic link between the two parameters. More specifically, infused fluid accompanies glucose during uptake into the cells, while volume expansion by the same fluid has already elicited an effective diuretic response. CONCLUSION: Hypovolaemia is a consequence of hypoglycaemia after intravenous infusion of glucose solution and is caused by the osmotic translocation of fluid from the extracellular to the intracellular fluid space that occurs despite effective renal elimination.

Concepts, facts and artifacts in electron microscopy.

This communication illustrates how the electron microscope has contributed to biochemistry by revealing how multienzyme systems in mitochondria are structurally organized to secure high speed ATP synthesis and has extended physiology to the molecular level. Ribonucleoprotein complexes form a gel in the cytoplasm determining the conditions for translation... Photoreceptor stimulation involves two phases, trapping of light by a light reflecting cylinder formed by the outer segment disks and energy transduction by bleaching of photopigment molecules changing the charge of the outer segment disks driving the photoreceptor toward hyperpolarization. Revealing the synaptic connections between retinal neurons extends neurophysiology to the level of information processing by neural circuits, which are designed for high speed processing. Spatial brightness contrast enhancement is eliminated in connection with macular degeneration, which leads to partial blindness, revealing the importance of contrast enhancement for vision.

Movement perception, directionally selective ganglion cell responses and the involuntary eye movements. Neurophysiology at the level of information processing.

Two neural circuits in the retina are described, that involve neural interaction generated by image movement. One circuit increases the sensitivity in detecting movement in the visual field while the other circuit generates ganglion cell signals when an image moves in one preferred direction but not when moves in the opposite direction, explaining the observation of directionally selective ganglion cell responses described by Barlow and Hill (1963). In agreement with the recorded potentials, the circuit generates ganglion cell responses also when the light stimulus is stationary. This ambiguity in ganglion cell signals to the visual center limits the signals usefulness to images that are in constant movement. The only known such movement is that caused by the involuntary eye movements. The function of the circuit is therefore to stabilize the retinal image when transmitted to the visual center.

Volume kinetics of glucose 2.5% solution during laparoscopic cholecystectomy.

BACKGROUND: Analyses of the distribution and elimination of glucose 2.5% solutions can be used to suggest combinations of infusion rates and infusion times which yield a predetermined plasma glucose level and degree of plasma dilution during surgery. METHODS: Twelve patients aged between 27 and 51 (mean 40) underwent laparoscopic cholecystectomy. An i.v. infusion of 1.4 litres of glucose 2.5% over 60 min was started when surgery began. A volume kinetic model was fitted to measurements of the plasma glucose concentration and the degree of haemodilution. Nomograms were constructed based on the kinetic results. RESULTS: The volume of distribution for the glucose and infused fluid and the plasma insulin levels were similar to the ones recorded in previous volunteer studies, but 50-70% lower values were obtained for the clearance of glucose (mean 0.21 litres min(-1)), endogenous glucose production (1.1 mmol min(-1)) and the elimination rate constant for the infused fluid (median 37 ml min(-1)). Urinary excretion was markedly depressed and amounted to 9% of the infused fluid volume 4 h after starting surgery. To prevent hyperglycaemia, nomograms suggested that the infusion should be directed towards a "target" glucose concentration and then slowed down in a controlled way. At steady state, the infused fluid maintains a 3.5% plasma dilution for each mmol that plasma glucose remains above baseline. CONCLUSION: Metabolic changes warrant careful balancing of infusion rates of glucose 2.5% during laparoscopic cholecystectomy, which is facilitated by a nomogram. Volume expansion from the infused fluid volume should be recognized.

Contrast enhancement of the retinal image and image distortion associated with macular degeneration.

An analysis of image distortion in connection with macular degeneration revealed that seeing objects requires brightness contrast enhancement. Thus macular degeneration changes neural interaction controlling bipolar cell responses to light stimulating photoreceptors to reverse spatial brightness contrast enhancement with the consequence that objects cannot be seen. This shows that the contrast of the retinal images is too low for vision without enhancement. However images consisting of randomly arranged small spots of different brightness are seen because of cones enhancing bipolar cell responses to rod input and rods enhancing bipolar cell responses to cone input. This residual vision and the observation that image distortion disappears at low light intensities reveal that macular degeneration is a functional disorder with intact photoreceptor function. The affection may therefore be caused by a reduction of blood flow through the choriocapillaries associated with ageing. The analysis of image distortion associated with the affection led to a simple way to determine the size of the affected retinal area, making it possible to follow the progression of the affection in a direct and simple way. Basic aspects are described of synaptic interaction within the information processing unit that determines the responses of the bipolar cells to photoreceptor input. This unit is the first information processing unit that has been revealed thanks to the extension of the analysis of the nervous system to the nanometer level at which information is processed. The minute size of the information processing unit, the special conditions for synaptic transmission combined with the short distances separating the synapses create special conditions for neural interaction at the nanometer level, establishing conditions for high speed neural communication.

Color vision at low light intensity, dark adaptation, Purkinje shift, critical flicker frequency and the deterioration of vision at low illumination. Neurophysiology at the nanometer range of neural structure.

The discovery that color vision extends to low illumination reported in this communication eliminates the duplicity theory as an explanation of vision differing at high and low illumination. Instead, an explanation of the difference was found when analyzing synaptic interaction between retinal neurons, made possible by revealing the synaptic connections between the neurons through three-dimensional reconstruction of the outer plexiform layer and by applying information communicated by published recordings of the potential of retinal neurons. The synaptic connections revealed the existence of a large horizontal cell network and of cone networks. The networks contribute continuous information regarding average light intensity over an area of the retina. The opposite sign of the network input maintains bipolar cell threshold constant when illumination varies. When at low illumination the network potential approaches a minimum the consequent extensive increase of transmitter release at network synapses eliminates fine tuning of synaptic transmission at these synapses. This accounts for the deterioration of vision at low illumination by eliminating spatial brightness contrast enhancement and also accounts for the difference in critical flicker frequency at high and low illumination. Network interference accounts for the two phases of dark adaptation and for the Purkinje shift. The analysis revealed conditions for particularly fast synaptic transmission leading to cascade like transmission at sequences of synapses. The overall design of the neural circuits establishes conditions for fast processing of information. This is the consequence of the neurons responding with graded changes of the membrane potential and conducting potentials electrotonically. Such neurons are therefore particularly suitable for processing of information.

Validation of volume kinetic analysis of glucose 2.5% solution given by intravenous infusion.

BACKGROUND: The distribution and elimination of glucose solutions can be analysed by means of a volume kinetic model, but the ability of the model to predict plasma dilution ('model linearity') has not been evaluated. METHODS: Six male volunteers received four separate infusions of glucose 2.5%: 10 ml kg(-1) and 15 ml kg(-1) over 30 min, and 15 ml kg(-1) and 25 ml kg(-1 )over 60 min. The kinetic model was fitted to measurements of plasma glucose concentration and haemodilution. RESULTS: The mean volume of distribution for the glucose was 9.2 (SEM 0.4) litres while the infused fluid expanded a central body fluid space (V(1)) of 3.1 (0.3) litres. Increasing the amount of infused fluid, but not the infusion rate, resulted in a proportional increase in the area under the curve for plasma glucose and plasma dilution, the only confounder being glycosuria. The bias of computer simulation was slightly increased by rebound hypoglycaemia, which could occur with the highest infusion rates, but the accuracy was almost identical regardless of whether the kinetic parameters from all 24 experiments or from any of the subgroups were used. CONCLUSION: The volume kinetic model for glucose 2.5% is linear and can therefore be used for computer simulation as long as marked glycosuria does not occur.

Kuffler's inhibitory surround, the function of the inner plexiform layer and an information processing unit in the retina. Neural interaction at the nanometer level.

Comparing Kuffler's recordings of ganglion cell discharges and bipolar cell responses to the same stimuli, deduced on the basis of a knowledge of synaptic connections between the neurons, revealed that the bipolar cell signals had not been modified by synaptic interaction in the inner plexiform layer. This layer therefore receives bipolar cell signals generated by groups of bipolar cells within the center of ganglion cell receptive fields, sorts and distributes the signals to a (compared to the number of photoreceptors and bipolar cells), small number of ganglion cells in such a way that the retinal image can be reconstructed in the visual center by reversing the fusion. Transmission between photoreceptor and bipolar cell is controlled by an information processing circuit receiving information from one photoreceptor, from the large horizontal cell network, formed by synaptic connections between the large horizontal cell processes, from cone networks formed by the cone processes connecting cones and from one small horizontal cell. Interaction between input neurons shapes the input to the bipolar cell. The interaction establishes a gate like control of transmission at the bipolar cell synapse and maintains bipolar cell threshold at a constant level, two features that prevent noise in the output signal. The output is generated by simultaneous input from all input neurons at the bipolar cell synapse, a multiinput synapse. Bipolar cell response is therefore based on perfect timing of fusion of information and of the neural interaction preceding fusion. Proper timing is secured by the dimensions of the components of the circuit measuring in the nanometer range. The volume of the information processing circuit is only 0.3 cubic micrometer, which is less than one two hundredth the volume of the soma of a bipolar cell. Extension of the study of the nervous system to the nanometer level opens a new field of research by making it possible to analyze how information contributed by the sense organs is processed in the nervous system to regulate body functions.

Sequential pictorial presentation of neural interaction in the retina. 2. The depolarizing and hyperpolarizing bipolar cells at rod terminals.

Each rod is connected to one depolarizing and one hyperpolarizing bipolar cell. The synaptic connections of cone processes to each bipolar cell and presynaptically to the two rod-bipolar cell synapses establishes conditions for lateral interaction at this level. Thus, the cones raise the threshold for bipolar cell depolarization which is the basis for spatial brightness contrast enhancement and consequently for high visual acuity (Sjöstrand, 2001a). The cones facilitate ganglion cell depolarization by the bipolar cells and cone input prevents horizontal cell blocking of depolarization of the depolarizing bipolar cell, extending rod vision to low illumination. The combination of reduced cone input and transient hyperpolarization of the hyperpolarizing bipolar cell at onset of a light stimulus facilitates ganglion cell depolarization extensively at onset of the stimulus while no corresponding enhancement applies to the ganglion cell response at cessation of the stimulus, possibly establishing conditions for discrimination between on- vs. off-signals in the visual centre. Reduced cone input and hyperpolarization of the hyperpolarizing bipolar cell at onset of a light stimulus accounts for Granit's (1941) 'preexcitatory inhibition'. Presynaptic inhibition maintains transmitter concentration low in the synaptic gap at rod-bipolar cell and bipolar cell-ganglion cell synapses, securing proportional and amplified postsynaptic responses at these synapses. Perfect timing of variations in facilitatory and inhibitory input to the ganglion cell confines the duration of ganglion cell depolarization at onset and at cessation of a light stimulus to that of a single synaptic transmission.

Volume kinetics of intravenous fluid therapy in the prehospital setting.

INTRODUCTION: To study the volume effect of isotonic and hypertonic crystalloid fluid during ambulance transports after mild trauma, a prospective case-control study was initiated, using the ambulance and helicopter transport system in Stockholm. METHODS: The hemodilution resulting from intravenous infusion of 1.0 L of Ringer's acetate solution (n = 7) or 250 ml of 7.5% sodium chloride (n = 3) over 30 minutes (min) was measured every 10 min during 1 hour when fluid therapy was instituted at the scene of an accident, or on arrival at the hospital. The dilution was studied by volume kinetic analysis and compared to that of matched, healthy controls who received the same fluid in hospital. RESULT: The hemodilution at the end of the infusions averaged 7.7% in the trauma patients and 9.1% in the controls, but the dilution was better maintained after trauma. The kinetic analysis showed that the size of the body fluid space expanded by Ringer's solution was 4.6 L and 3.8 L for the trauma and the control patients, respectively, while hypertonic saline expanded a slightly larger space. For both fluids, trauma reduced the elimination rate constant by approximately 30%. CONCLUSION: Mild trauma prolonged the intravascular persistence of isotonic and hypertonic crystalloid fluid as compared to a control group.

Volume kinetics of glucose solutions given by intravenous infusion.

Glucose solutions given by intravenous (i.v.) infusion exert volume effects that are governed by the amount of fluid administered and also by the metabolism of the glucose. To understand better how the body handles glucose solutions, two volume kinetic models were developed in which consideration was given to the osmotic fluid shifts that accompany the metabolism of glucose. These models were fitted to data obtained when 21 volunteers who were given approximately 1 litre of glucose 2.5 or 5% or Ringer's solution (control) over 45 min. The maximum haemodilution was similar for all three fluids, but it decreased more rapidly when glucose had been infused. The volume of distribution for the infused glucose molecules was larger (approximately 12 litres) than for the infused fluid, which amounted to (mean (SEM)) 3.7 (0.3) (glucose 2.5%), 2.8 (0.2) (glucose 5%), and 2.5 (0.2) litres (Ringer). Fluid accumulated in a remote (cellular) body fluid space when glucose had been administered (approximately 0.2 and 0.4 litres, respectively), while expansion of an intermediate fluid space (7.1 (1.3) litres) could be demonstrated in 33% of the Ringer experiments. In conclusion, kinetic models were developed which consider the relationship between the glucose metabolism and the disposition of intravenous fluid. One of them, in which infused fluid expands two instead of three body fluid spaces, was successfully fitted to data on blood glucose and blood haemoglobin obtained during infusions of 2.5 and 5% glucose.

Sequential pictorial presentation of neural interaction in the retina. 1. The synaptic ribbon complex.

The analysis of circuits in the retina revealed by three-dimensional reconstructions (Sjöstrand, 1958, 1974, 1976,1978, 1990) is presented in a simplified way by series of schematic drawings illustrating each individual step in the sequence of synaptic transmission between the neurones. In addition, the analysis has been extended further compared to earlier presentation (Sjöstrand, 1998a). The circuit of the synaptic ribbon complex controls transmission between cones and bipolar cells by the inhibitory input from horizontal cells being controlled by the cone. At low light intensity, horizontal cell inhibition blocks transmission between cones and bipolar cells because of high concentration of transmitter at the critical synapse, explaining fading of cone vision. The requirement for maintaining a low transmitter concentration at the synapses to secure proportionality between input and output signals is established by horizontal cell inhibitory input both at the cone-bipolar cell and the cone-horizontal cell synapses. How one bipolar cell can transmit information regarding average light intensity over a large area of the retina is discussed in detail.

Molecular pathology of Luft disease and structure and function of mitochondria.

Luft disease, studied in detail by Luft et al. (1962) is characterized clinically by hypermetabolism and consequent abnormal transpiration. In their study, Luft and coworkers revealed that the hypermetabolism is caused by extensive uncoupling of mitochondrial respiration in skeletal muscle tissue. They also discovered that the muscle mitochondria had been structurally modified with the cristae assuming a zig-zag conformation. The mitochondrial enzymes functioned normally, the abnormality being confined to the extensive uncoupling of respiration. In an earlier study (Sjöstrand et al., 1988) it was revealed that the zig-zag conformation is caused by removal of some tricarboxylic acid cycle enzymes from the cristae, exposing the interior of the cristae to the matrix fluid. Applying a coupling theory proposed by Sjöstrand (1990, 1991), leakage of protons from the cristae should impair coupling of respiration and ATP synthesis. The structural damage is conceived of to be caused by a genetic defect preventing proper aggregation of the enzyme molecules in the cristae. Luft disease may therefore be the first known disease caused primarily by a structural disorder at the molecular level. The symptoms of the disease reveal the importance of the compact and ordered aggregation of the enzyme molecules in the cristae for coupling of respiration.

Structure determines function of the retina, a neural center. 4. The 'duplex' nature of vision.

Part 4 reveals how the differences in the quality of vision at high and low illumination are explained by synaptic interaction between the neurons in the outer plexiform layer changing with illumination. Analysis of data available in the literature revealed that rod vision is active over the entire range of light intensities generating visual responses. It is revealed that the input from the retina to brain centers involves information regarding a black and white image based on rod vision, which is improved with respect to spatial brightness contrast and darkness contrast by cone interference. Cones add color to the image, further increasing image contrast. The duplicity theory is shown to be based on erratic assumptions regarding how rods and cones function. The theory lacks an experimental basis and is not required to explain differences in the quality of the perceived images at high and low illumination.

Structure determines function of the retina, a neural center. 3. Contrast enhancement, directional selectivity, some incomplete circuits.

In Part 3 a circuit for spatial brightness contrast enhancement is analyzed as well as a circuit for generation of directionally selective bipolar cell responses. Several types of bipolar cells with a special function contribute to the complexity of the circuitry of the outer plexiform layer, indicating that the analysis presented in this review only represents the tip of the iceberg.

Structure determines function of the retina, a neural center. 2. The second, third and fourth circuits.

Three retinal circuits are analyzed on the bases of established circuit diagrams. The synaptic interaction at one circuit involves depolarizing bipolar cells connected to rods and shows the important contribution to function of the connections between cones and rods including connections of cones to bipolar cells connected to rods. Another circuit explains that one type of bipolar cell is depolarized at cessation of the stimulus. Because each photoreceptor is connected to both on- and off- bipolar cells, on- and off- responses are generated from each photoreceptor. The cone connections to rods and to bipolar cells connected to rods extend rod vision to low ambient illumination while the absence of corresponding connections at cone terminals leads to cone vision being blocked at low ambient illumination. The cone connections raise the threshold for stimulation of bipolar cells connected to rods and lower temporal resolution of rod vision relative to cone vision. The timing of interaction between bipolar cells, amacrine cells and ganglion cells in the inner plexiform layer makes inhibitory and facilitatory modulation of ganglion cell potential establish optimal conditions for ganglion cell activation. The agreement between bipolar cell responses determined on the basis of the analysis of the circuit diagrams and the responses recorded from bipolar cells shows that the function of a neural center such as the retina can be analyzed on the basis of circuit diagrams.

Structure determines function of the retina, a neural center. 1. The synaptic ribbon complex.

A new approach to study the function of the retina is described. It involves a precise revealing of synaptic connections between the neurons by means of three-dimensional volume reconstructions. Identification of several hundreds of synapses revealed the existence of groups of synaptically interconnected neurons constituting discrete neural circuits accounting for processing of information contributed by the photoreceptors. Supplementary information contributed by electrophysiological recordings of membrane potentials of neurons in these circuits made it possible to determine how information is processed by the circuits. Two versions of one of the circuits are analyzed in this communication.

The physical chemical basis for preserving cell structure for electron microscopy at the molecular level and available preparatory methods.

A method to prepare tissues for electron microscopy based on the kinetic theory of physical chemistry makes it possible to study the structure of cells at the molecular level because extensive denaturation of proteins is avoided in contrast to using conventional preparatory methods. The highly ordered arrangement of mitochondrial enzymes revealed by applying this method is discussed. Low temperature embedding, freeze-substitution, freeze-drying combined with low temperature embedding and freeze-fracturing are evaluated with respect to conditions for protein denaturation.

Information and misinformation regarding ischemia of heart muscle tissue. The cause of cell death during blood reperfusion and reactivation of heart muscle tissue after prolonged ischemia.

An electron microscopic study of heart muscle tissue exposed to six hours ischemia and prepared according to the low denaturation embedding technique revealed a structural modification confined to the mitochondrial cristae. The modification consisted of a removal of Krebs cycle enzymes from the cristae. Reperfusion of the ischemic tissue after four hours ischemia led to extensive breakdown of the mitochondrial structure and contractility could not be restored. However, when after six hours ischemia the ischemic tissue was reperfused with blood, the composition of which had been modified to stimulate mitochondrial function, no additional structural changes were observed and contractility was restored. The structural damage caused by reperfusion with non modified blood is explained by a loss of control of plasma membrane permeability caused by impaired ATP production which makes the ionic composition of the cytosol approach that of blood plasma, stopping oxidative phosphorylation. A treatment to restore heart muscle function after long periods of ischemia and after heart transplantation is proposed. The structural damage revealed that the Krebs cycle and the respiratory chain enzymes are associated according to a regular periodic pattern and that the enzyme molecules are closely aggregated three-dimensionally. Earlier electron microscopic studies revealing massive structural deterioration of heart muscle cells already after 45 to 60 minutes ischemia leading to the conclusion that the cells are irreversibly damaged, is based on fixation artifacts caused by osmium fixation. This study has been carried out in collaboration with the research team of Gerald D. Buckberg at the Thoracic Surgery Division at University of California at Los Angeles.