Insect exoskeletons react to hypergravity

A typical feature of biological materials is their ability to adapt to mechanical load. However, it is not known whether the cuticle exoskeleton, one of the most common biological structures, also shares this trait. Here, we show direct experimental evidence that prolonged exposure to hypergravity conditions affects the morphology and biomechanics of an insect exoskeleton. Locusts were raised for several weeks in a custom-designed centrifuge at various levels of hypergravity. Biomechanical measurements and X-ray microtomography show that up to 3 g load Young's modulus of the tibiae increased by about 67%. Higher gravitational loads however decreased the survival rate, body mass and endocuticle thickness. These results directly show that cuticle exoskeletons can react to hypergravity. This ability has so far only been known for bone endoskeletons and plants. Our findings thus add important context to the discussion on general ultimate factors in the evolution of adaptive biological materials and skeletal systems.

3D escape: an alternative paradigm for spatial orientation studies in insects

Arthropods and in particular insects show a great variety of different exoskeletal sensors. For most arthropods, spatial orientation and gravity perception is not fully understood. In particular, the interaction of the different sensors is still a subject of ongoing research. A disadvantage of most of the experimental methods used to date to study the spatial orientation of arthropods in behavioral experiments is that the body or individual body parts are fixed partly in a non-natural manner. Therefore, often only the movement of individual body segments can be used to evaluate the experiments. We here present a novel experimental method to easily study 3D-escape movements in insects and analyze whole-body reaction. The animals are placed in a transparent container, filled with a lightweight substrate and rotating around two axes. To verify our setup, house crickets (Acheta domesticus) with selectively manipulated gravity-perceiving structures were analyzed. The spatial orientation behavior was quantified by measuring the time individuals took to escape toward the surface and the angular deviation toward the gravitational vector. These experiments confirm earlier results and therefore validated our experimental setup. Our new approach thus allows to investigate several comprehensive questions regarding the spatial orientation of insects and other animals.

Semi-automated differentiation of insect exo- and endocuticle in X-ray microtomography

One of the most versatile and complex biological composite materials, the insect exoskeleton shows a huge range in biomechanical properties. The cuticle exoskeleton can be differentiated into two main histologically different layers with distinct properties: the outer, more sclerotized exocuticle and inner, softer endocuticle. For most biomechanical research questions, it is of great importance to be able to selectively characterize geometrical features of these layers. However, most conventional preparation methods (cross-sections, histological staining, SEM) require complex and destructive sample preparation, which provides only two-dimensional information. Here, we present a novel, simple staining method using X-ray microtomography to distinguish between exo- and endocuticle in a 3D environment without sample destruction. We illustrate the power of our method using locust (Locusta migratoria) hindleg tibia, a well characterized biomechanical sample. Our method allows an easy and direct measurement of exo- and endocuticle and their respective geometric features. Applying our method will help to understand the biomechanical role of exo- and endocuticle within an insect exoskeleton and will allow us to understand its composition and morphological features in more detail.

Epidermal Cell Surface Structure and Chitin-Protein Co-assembly Determine Fiber Architecture in the Locust Cuticle

The geometrical similarity of helicoidal fiber arrangement in many biological fibrous extracellular matrices, such as bone, plant cell wall, or arthropod cuticle, to that of cholesteric liquid mesophases has led to the hypothesis that they may form passively through a mesophase precursor rather than by direct cellular control. In search of direct evidence to support or refute this hypothesis, here, we studied the process of cuticle formation in the tibia of the migratory locust, Locusta migratoria, where daily growth layers arise by the deposition of fiber arrangements alternating between unidirectional and helicoidal structures. Using focused ion beam/scanning electron microscopy (FIB/SEM) volume imaging and scanning X-ray scattering, we show that the epidermal cells determine an initial fiber orientation, from which the final architecture emerges by the self-organized co-assembly of chitin and proteins. Fiber orientation in the locust cuticle is therefore determined by both active and passive processes.

Insect wing damage: causes, consequences and compensatory mechanisms

The evolution of wings has played a key role in the success of insect species, allowing them to diversify to fill many niches. Insect wings are complex multifunctional structures, which not only have to withstand aerodynamic forces but also need to resist excessive stresses caused by accidental collisions. This Commentary provides a summary of the literature on damage-reducing morphological adaptations in wings, covering natural causes of wing collisions, their impact on the structural integrity of wings and associated consequences for both insect flight performance and life expectancy. Data from the literature and our own observations suggest that insects have evolved strategies that (i) reduce the likelihood of wing damage and (ii) allow them to cope with damage when it occurs: damage-related fractures are minimized because wings evolved to be damage tolerant and, in the case of wing damage, insects compensate for the reduced aerodynamic efficiency with dedicated changes in flight kinematics.

SAUV-A Bio-Inspired Soft-Robotic Autonomous Underwater Vehicle

Autonomous and remotely operated underwater vehicles allow us to reach places which have previously been inaccessible and perform complex repair, exploration and analysis tasks. As their navigation is not infallible, they may cause severe damage to themselves and their often fragile surroundings, such as flooded caves, coral reefs, or even accompanying divers in case of a collision. In this study, we used a shallow neural network, consisting of interlinking PID controllers, and trained by a genetic algorithm, to control a biologically inspired AUV with a soft and compliant exoskeleton. Such a compliant structure is a versatile and passive solution which reduces the accelerations induced by collisions to 56% of the original mean value acting upon the system, thus, notably reducing the stress on its components and resulting reaction forces on its surroundings. The segmented structure of this spherical exoskeleton protects the encased system without limiting the use of cameras, sensors or manipulators.

What goes up must come down: biomechanical impact analysis of falling locusts

Many insects are able to precisely control their jumping movements. Once in the air, the properties of the actual landing site, however, are almost impossible to predict. Falling insects thus have to cope with the situation at impact. In particular, for insects jumping to escape predators, a controlled landing movement appears to be a major evolutionary advantage. A quick recovery into an upright and stable body posture minimizes the time to prepare for the next escape jump. In this study, we used high-speed recordings to investigate the falling and in particular the impact behavior of Schistocerca gregaria locusts, a common model organism for studies on the biomechanics of jumping. Detailed impact analyses of free-falling locusts show that most insects typically crashed onto the substrate. Although free-falling locusts tended to spread their legs, they mostly fell onto the head and thorax first. The presence of wings did not significantly reduce impact speed; however, it did affect the orientation of the body at impact and significantly reduced the time to recover. Our results also show that alive warm locusts fell significantly faster than inactive or dead locusts. This indicates a possible tradeoff between active control versus reduced speed. Interestingly, alive insects also tended to perform a characteristic bending movement of the body at impact. This biomechanical adaptation might reduce the rebound and shorten the time to recover. The adhesive pads also play an important role in reducing the time to recover by allowing the insect to anchor itself to the substrate.

Morphology, shape variation and movement of skeletal elements in starfish (Asterias rubens)

Starfish (order: Asteroidea) possess a complex endoskeleton composed of thousands of calcareous ossicles. These ossicles are embedded in a body wall mostly consisting of a complex collagen fiber array. The combination of soft and hard tissue provides a challenge for detailed morphological and histological studies. As a consequence, very little is known about the general biomechanics of echinoderm endoskeletons and the possible role of ossicle shape in enabling or limiting skeletal movements. In this study, we used high-resolution X-ray microscopy to investigate individual ossicle shape in unprecedented detail. Our results show the variation of ossicle shape within ossicles of marginal, reticular and carinal type. Based on these results we propose an additional classification to categorize ossicles not only by shape but also by function into 'connecting' and 'node' ossicles. We also used soft tissue staining with phosphotungstic acid successfully and were able to visualize the ossicle ultrastructure at 2-μm resolution. We also identified two new joint types in the aboral skeleton (groove-on-groove joint) and between adambulacral ossicles (ball-and-socket joint). To demonstrate the possibilities of micro-computed tomographic methods in analyzing the biomechanics of echinoderm skeletons we exemplarily quantified changes in ossicle orientation for a bent ray for ambulacral ossicles. This study provides a first step for future biomechanical studies focusing on the interaction of ossicles and soft tissues during ray movements.

Investigating the efficiency of a bio-inspired insect repellent surface structure

Most insects with smooth or hairy adhesive pads have very little problems in attaching to smooth substrates. A careful selection of surface roughness, however, can effectively limit the contact area of the adhesive organs with the surface. In comparison to conventional toxin-based insect repelling methods, biologically inspired micro- and nanostructured insect repellent surface structures, thus, offer a great potential to effective and environmentally-friendly control insect pests. We here present a simple experimental approach to qualitatively and quantitatively analyse the efficiency of a micro- and nanorough surface structure. Nauphoeta cinerea and Gastrophysa viridula as model organisms for insects with smooth and hairy adhesive pads, respectively, were placed in an enclosed environment. The escape movements of freely running insects over either the repellent or a control surfaces were counted and analyzed in detail. The tested surface structures were able to significantly reduce the escape of cockroaches with smooth adhesive pads by 44.1%. Interestingly, the data indicates that N. cinerea might use mechanical cues from the antenna to discriminate between surfaces before making contact with the adhesive pads. G. viridula with hairy adhesive pads were not significantly affected by the surface structure. By carefully adjusting 'critical' surface topography parameters relevant for insect adhesion, more efficient and selective repellent surfaces might be achieved. Such nanostructure-based insect repellent surfaces could also help to utilize recruitment behavior of certain insect species and might present a novel approach to effectively control insect pests.

Effect of sample treatment on biomechanical properties of insect cuticle

Experimental limitations often prevent to perform biomechanical measurements on fresh arthropod cuticle samples. Hence, in many cases short- or long-term storage of samples is required. So far, it is not known whether any of the standard lab-techniques commonly used to fix or store insect cuticle samples in any way affects the biomechanical properties of the respective samples. In this paper we systematically address this question for the first time, with a focus on practical, easily accessible and common lab-methods including storage in water, ethanol, glutaraldehyde, freezing and desiccation. We performed a comprehensive and sensitive non-destructive Dynamic Mechanical Analysis (DMA) on locust hind leg tibiae using a three-point-bending setup. Our results show that from all tested treatments, freezing samples at -20 °C was the best option to maintain the original values for Young's modulus and damping properties of insect cuticle. In addition, our results indicate that the damping properties of locust hind legs might be mechanically optimized in respect to the jumping and kicking direction.

Damage, repair and regeneration in insect cuticle: The story so far, and possibilities for the future

The exoskeleton of an insect can contain countless specializations across an individual, across developmental stages, and across the class Insecta. Hence, the exoskeleton's building material cuticle must perform a vast variety of functions. Cuticle displays a wide range of material properties which are determined by several known factors: the amount and orientation of the chitin fibres, the constituents and degree of cross-linking and hydration of the protein matrix, the relative amounts of exo- and endocuticle, and the shape of the structures themselves. In comparison to other natural materials such as wood and mammal bone, relatively few investigations into the mechanical properties of insect cuticle have been carried out. Of these, very few have focussed on the need for repair and its effectiveness at restoring mechanical stability to the cuticle. Insect body parts are often subject to prolonged repeated cyclic loads when running and flying, as well as more extreme "emergency" behaviours necessary for survival such as jumping, wedging (squeezing through small holes) and righting (when overturned). What effects have these actions on the cuticle itself? How close to the limits of failure does an insect push its body parts? Can an insect recover from minor or major damage to its exoskeleton "bones"? No current research has answered these questions conclusively.

Bridging the gap: wound healing in insects restores mechanical strength by targeted cuticle deposition

If an insect is injured, can it repair its skeleton in a manner which is mechanically strong and viable? Previous work has described the biological processes that occur during repair of insect cuticle, but until now, there has been no biomechanical assessment of the repaired area. We analysed the biomechanics of the injury repair process in the desert locust (Schistocerca gregaria). We show that after an incision, a healing process occurred which almost doubled the mechanical strength of locust tibial cuticle, restoring it to 66% of the original, intact strength. This repair process occurred by targeted cuticle deposition, stimulated by the presence of the injury. The cut surfaces remained unrepaired, but a patch of endocuticle was deposited, reinforcing the area and thus increasing the effective fracture toughness. The deposition rate of endocuticle inside the tibia increased fourfold compared with uninjured controls, but only on the dorsal side, where the incision was placed. The limb is highly loaded during jumping, so this partial restoration of strength will have a profound effect on the fitness of the insect. A finite-element model provided insights into the mechanics of the repair, predicting that the patch material reaches its ultimate strength before the fracture toughness of the existing cuticle is exceeded.

Physical principles of fluid-mediated insect attachment - Shouldn't insects slip?

Insects use either hairy or smooth adhesive pads to safely adhere to various kinds of surfaces. Although the two types of adhesive pads are morphologically different, they both form contact with the substrate via a thin layer of adhesive fluid. To model adhesion and friction forces generated by insect footpads often a simple "wet adhesion" model is used, in which two flat undeformable substrates are separated by a continuous layer of fluid. This review summarizes the key physical and tribological principles that determine the adhesion and friction in such a model. Interestingly, such a simple wet-adhesion model falls short in explaining several features of insect adhesion. For example, it cannot predict the observed high static friction forces of the insects, which enable them to cling to vertical smooth substrates without sliding. When taking a closer look at the "classic" attachment model, one can see that it is based on several simplifications, such as rigid surfaces or continuous layers of Newtonian fluids. Recent experiments show that these assumptions are not valid in many cases of insect adhesion. Future tribological models for insect adhesion thus need to incorporate deformable adhesive pads, non-Newtonian properties of the adhesive fluid and/or partially "dry" or solid-like contact between the pad and the substrate.

Fatigue of insect cuticle

Many parts of the insect exoskeleton experience repeated cyclic loading. Although the cuticle of insects and other arthropods is the second most common natural composite material in the world, so far nothing is known about its fatigue properties, despite the fact that fatigue undoubtedly limits the durability of body parts in vivo. For the first time, we here present experimental fatigue data of insect cuticle. Using force-controlled cyclic loading, we determined the number of cycles to failure for hind legs (tibiae) and hind wings of the locust Schistocerca gregaria, as a function of the applied cyclic stress. Our results show that, although both made from cuticle, these two body parts behaved very differently. Wing samples failed after 100,000 cycles when we applied 46% of the stress needed for instantaneous failure (the UTS). Legs, in contrast, were able to sustain a stress of 76% of UTS for the same number of cycles to failure. This can be explained by the difference in the composition and structure of the material and related to the well-known behaviour of engineering composites. Final failure of the tibiae occurred via one of two different failure modes - crack propagation in tension or buckling in compression - indicating that the tibia is evolutionary optimized to resist both failure modes equally. These results are further discussed in relation to the evolution and normal use of these two body parts.

Shape optimization in exoskeletons and endoskeletons: a biomechanics analysis

This paper addresses the question of strength and mechanical failure in exoskeletons and endoskeletons. We developed a new, more sophisticated model to predict failure in bones and other limb segments, modelled as hollow tubes of radius r and thickness t. Five failure modes were considered: transverse fracture; buckling (of three different kinds) and longitudinal splitting. We also considered interactions between failure modes. We tested the hypothesis that evolutionary adaptation tends towards an optimum value of r/t, this being the value which gives the highest strength (i.e. load-carrying capacity) for a given weight. We analysed two examples of arthropod exoskeletons: the crab merus and the locust tibia, using data from the literature and estimating the stresses during typical activities. In both cases, the optimum r/t value for bending was found to be different from that for axial compression. We found that the crab merus experiences similar levels of bending and compression in vivo and that its r/t value represents an ideal compromise to resist these two types of loading. The locust tibia, however, is loaded almost exclusively in bending and was found to be optimized for this loading mode. Vertebrate long bones were found to be far from optimal, having much lower r/t values than predicted, and in this respect our conclusions differ from those of previous workers. We conclude that our theoretical model, though it has some limitations, is useful for investigating evolutionary development of skeletal form in exoskeletons and endoskeletons.

Fracture toughness of locust cuticle

Insect cuticle is one of the most common biological materials, yet very little is known about its mechanical properties. Many parts of the insect exoskeleton, such as the jumping legs of locusts, have to withstand high and repeated loading without failure. This paper presents the first measurements of fracture toughness for insect cuticle using a standard engineering approach. Our results show that the fracture toughness of cuticle in locust hind legs is 4.12 MPa m(1/2) and decreases with desiccation of the cuticle. Stiffness and strength of the tibia cuticle were measured using buckling and cantilever bending and increased with desiccation. A combination of the cuticle's high toughness with a relatively low stiffness of 3.05 GPa results in a work of fracture of 5.56 kJ m(-2), which is amongst the highest of any biological material, giving the insect leg an exceptional ability to tolerate defects such as cracks and damage. Interestingly, insect cuticle achieves these unique properties without using reinforcement by a mineral phase, which is often found in other biological composite materials. These findings thus might inspire the development of new biomimetic composite materials.

In vivo dynamics of the internal fibrous structure in smooth adhesive pads of insects

Many insects with smooth adhesive pads can rapidly enlarge their contact area by centripetal pulls on the legs, allowing them to cope with sudden mechanical perturbations such as gusts of wind or raindrops. The short time scale of this reaction excludes any neuromuscular control; it is thus more likely to be caused by mechanical properties of the pad's specialized cuticle. This soft cuticle contains numerous branched fibrils oriented almost perpendicularly to the surface. Assuming a fixed volume of the water-filled cuticle, we hypothesized that pulls could decrease the fibril angle, thereby helping the contact area to expand laterally and longitudinally. Three-dimensional fluorescence microscopy on the cuticle of smooth stick insect pads confirmed that pulls significantly reduced the fibril angle. However, the fibril angle variation appeared insufficient to explain the observed increase in contact area. Direct strain measurements in the contact zone demonstrated that pulls not only expand the cuticle laterally, but also add new contact area at the pad's outer edge.

Biomechanics of the stick insect antenna: damping properties and structural correlates of the cuticle

The antenna of the Indian stick insect Carausius morosus is a highly specialized near-range sensory probe used to actively sample tactile cues about location, distance or shape of external objects in real time. The length of the antenna's flagellum is 100 times the diameter at the base, making it a very delicate and slender structure. Like the rest of the insect body, it is covered by a protective exoskeletal cuticle, making it stiff enough to allow controlled, active, exploratory movements and hard enough to resist damage and wear. At the same time, it is highly flexible in response to contact forces, and returns rapidly to its straight posture without oscillations upon release of contact force. Which mechanical adaptations allow stick insects to unfold the remarkable combination of maintaining a sufficiently invariant shape between contacts and being sufficiently compliant during contact? What role does the cuticle play? Our results show that, based on morphological differences, the flagellum can be divided into three zones, consisting of a tapered cone of stiff exocuticle lined by an inner wedge of compliant endocuticle. This inner wedge is thick at the antenna's base and thin at its distal half. The decay time constant after deflection, a measure that indicates strength of damping, is much longer at the base (τ>25 ms) than in the distal half (τ<18 ms) of the flagellum. Upon experimental desiccation, reducing mass and compliance of the endocuticle, the flagellum becomes under-damped. Analysing the frequency components indicates that the flagellum can be abstracted with the model of a double pendulum with springs and dampers in both joints. We conclude that in the stick-insect antenna the cuticle properties described are structural correlates of damping, allowing for a straight posture in the instant of a new contact event, combined with a maximum of flexibility.

Arachnids secrete a fluid over their adhesive pads

Background

Many arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey. Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion.

Methodology and Principal Findings

We investigated the adhesive contact zone of living spiders, solifugids and mites using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48–1.50; contact angle: 3.7–11.2°). Fluid was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest (26%±2.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%±1.7 SE). This may be explained by the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.

Significance

This finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance, improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic adhesives should consider whether model species with fluid secretions are appropriate to their design goals.

Mechanisms of fluid production in smooth adhesive pads of insects

Insect adhesion is mediated by thin fluid films secreted into the contact zone. As the amount of fluid affects adhesive forces, a control of secretion appears probable. Here, we quantify for the first time the rate of fluid secretion in adhesive pads of cockroaches and stick insects. The volume of footprints deposited during consecutive press-downs decreased exponentially and approached a non-zero steady state, demonstrating the presence of a storage volume. We estimated its size and the influx rate into it from a simple compartmental model. Influx was independent of step frequency. Fluid-depleted pads recovered maximal footprint volumes within 15 min. Pads in stationary contact accumulated fluid along the perimeter of the contact zone. The initial fluid build-up slowed down, suggesting that flow is driven by negative Laplace pressure. Freely climbing stick insects left hardly any traceable footprints, suggesting that they save secretion by minimizing contact area or by recovering fluid during detachment. However, even the highest fluid production rates observed incur only small biosynthesis costs, representing less than 1 per cent of the resting metabolic rate. Our results show that fluid secretion in insect wet adhesive systems relies on simple physical principles, allowing for passive control of fluid volume within the contact zone.

Insect tricks: two-phasic foot pad secretion prevents slipping

Many insects cling to vertical and inverted surfaces with pads that adhere by nanometre-thin films of liquid secretion. This fluid is an emulsion, consisting of watery droplets in an oily continuous phase. The detailed function of its two-phasic nature has remained unclear. Here we show that the pad emulsion provides a mechanism that prevents insects from slipping on smooth substrates. We discovered that it is possible to manipulate the adhesive secretion in vivo using smooth polyimide substrates that selectively absorb its watery component. While thick layers of polyimide spin-coated onto glass removed all visible hydrophilic droplets, thin coatings left the emulsion in its typical form. Force measurements of stick insect pads sliding on these substrates demonstrated that the reduction of the watery phase resulted in a significant decrease in friction forces. Artificial control pads made of polydimethylsiloxane showed no difference when tested on the same substrates, confirming that the effect is caused by the insects' fluid-based adhesive system. Our findings suggest that insect adhesive pads use emulsions with non-Newtonian properties, which may have been optimized by natural selection. Emulsions as adhesive secretions combine the benefits of 'wet' adhesion and resistance against shear forces.

Friction ridges in cockroach climbing pads: anisotropy of shear stress measured on transparent, microstructured substrates

The contact of adhesive structures to rough surfaces has been difficult to investigate as rough surfaces are usually irregular and opaque. Here we use transparent, microstructured surfaces to investigate the performance of tarsal euplantulae in cockroaches (Nauphoeta cinerea). These pads are mainly used for generating pushing forces away from the body. Despite this biological function, shear stress (force per unit area) measurements in immobilized pads showed no significant difference between pushing and pulling on smooth surfaces and on 1-microm high microstructured substrates, where pads made full contact. In contrast, on 4-mum high microstructured substrates, where pads made contact only to the top of the microstructures, shear stress was maximal during a push. This specific direction dependence is explained by the interlocking of the microstructures with nanometre-sized "friction ridges" on the euplantulae. Scanning electron microscopy and atomic force microscopy revealed that these ridges are anisotropic, with steep slopes facing distally and shallow slopes proximally. The absence of a significant direction dependence on smooth and 1-microm high microstructured surfaces suggests the effect of interlocking is masked by the stronger influence of adhesion on friction, which acts equally in both directions. Our findings show that cockroach euplantulae generate friction using both interlocking and adhesion.

Meeting report on the Bellagio Conference ‘prevention of vascular diseases in the emerging world: An approach to global health equity’

Representatives from five international organizations (International Society of Nephrology, World Heart Federation, International Diabetes Federation, International Atherosclerosis Federation, and International Society of Hypertension) participated in a strategic planning workshop in December 2005 in Bellagio, Italy sponsored by the Rockefeller Foundation. There were equal representatives from developed and developing countries. Global perspectives on diabetes and cardiovascular and renal diseases were presented, with special emphasis on China, India, Latin America, and Africa. The rationale and effectiveness of preventive measures were discussed. It was apparent that measures for primary prevention and early intervention for all the chronic vascular diseases are similar. The five organizations agreed that an integrated global approach to chronic vascular diseases is needed. They resolved to collaborate and work towards an integrated approach to chronic vascular diseases with the establishment of a 5-year plan for the prevention and treatment of chronic vascular diseases, including public advocacy, advising international and national agencies, and improving education and the practice of established approaches.

Dialysis rationing in South Africa: A global message

Dialysis rationing resulting from limited facilities and health-care personnel in low- and middle-income countries such as South Africa must be addressed on several fronts. Prevention of kidney disease is an essential long-term approach, but in the short term, it is necessary to increase access to dialysis and transplantation, and to seek ways to limit the 'brain drain' to the developed world.

Toward global advancement of medicine: the International Society of Nephrology experience

BACKGROUND

Since its foundation in 1960, the International Society of Nephrology (ISN) has pursued the worldwide advancement of education, science and patient care in nephrology. This goal was achieved by means of the Society's journal and the organization of international congresses and symposia. In order to better reach its colleagues and patients in economically less developed countries, the ISN expanded its activities as of 1980 by a large number of specific programs aimed at these regions.

METHODS

The first phase of activities included teaching programs, fellowship and visiting scholar programs, and the provision of travel grants to enhance accessibility to the ISN congresses. A second phase consisted of the creation of a library enhancement program, a commission on acute renal failure and--to improve the organization and efficiency--a central commission on global advancement of nephrology (COMGAN). Currently, a third phase has been entered in which all activities have been intensified: (1) under the guidance of COMGAN, supported by a large number of teaching programs and fact finding missions; (2) by establishing a renal sister program; and (3) by initiating commissions on informatics and on clinical trials.

RESULTS

As a result, the ISN has reached most parts of the world, previously deprived of contact with renal science and renal patient care. The fellowship program now counts 160 fellows, who spend one or two years in training. The library enhancement program reaches 218 institutions worldwide. ISN membership has soared over the past two years with over 2,500 new members, mostly in the developing countries. They receive Kidney International and other relevant forms of information. Thus far, 135 pairs of renal units in developing and developed countries have been linked for support on a more continuous basis. ISN-sponsored congresses, symposia, and courses are being held in increasing numbers in the developing world. In many of its activities, the ISN closely collaborates with sister organizations, which also contribute financially. In total, the ISN spends annually over $1 million US from its own budget on the programs described above.

CONCLUSION

The various programs and initiatives are proving helpful in advancing renal medicine in areas in need. Expansion into supporting similar programs within other medical subspecialties is being explored.

Health research in Canada: a shifting paradigm

A personal review of medical research in Canadian medical schools over the past 25 years reveals extraordinary contributions. Over this time, medical research evolved from a by-product of faculty members to a commitment that determines the future success of a medical faculty. The increasing competition for health research funding and the high standards created internationally have changed the way research is organized in our medical faculties. Current trends include a move toward group and thematic research, an increased role of research institutes and the development of strategic partnerships with industry. Because of the need for more planning and more critical and timely review of research efforts, the benefits of collaboration enhance the quality and competitiveness of a medical faculty. A broadened vision of the Medical Research Council and provincial foundations and the need to increase resources for research foreshadow even greater change.

Effect of magnesium diet in gentamicin-induced acute renal failure in rats

Clearance experiments were conducted with metabolic cages to determine the effect of dietary magnesium on gentamicin nephrotoxicity. Three groups of male Wistar rats were given low, medium- or high-magnesium diets. Following baseline clearances, gentamicin was administered by intramuscular injection (20 mg/kg). Additional clearances were performed 6 and 11 days after gentamicin administration. 24-hour clearances were also taken 8 days after gentamicin withdrawal. The present experiments demonstrated that high magnesium intake protected the kidney against injury induced by gentamicin. This reduction in nephrotoxicity was probably due to competition of binding between magnesium and gentamicin to the renal membrane. Histological examinations were also done in these animals. The results showed that the most severe changes were seen in rats receiving a low-magnesium diet. Rats given a high-magnesium diet showed the least toxic changes while rats receiving a medium-magnesium diet showed changes of intermediate severity. These observations complemented the results obtained from 24-hour clearances and indicate the protective effect of dietary magnesium on the development of acute renal failure following administration of gentamicin.

Is lymphocyte magnesium concentration a reflection of intracellular magnesium concentration?

Diuretics are known to cause magnesium depletion, and the aim of the present experiment is to establish the relationship between lymphocyte magnesium concentration and intracellular magnesium concentration during chronic diuretic therapy. Studies were conducted in male Wistar rats that were subjected to daily administration of furosemide (2 mg/kg/day IP) for 19 weeks. Clearance measurements were performed during the baseline week and subsequently during the third, seventh, eleventh, fifteenth, and nineteenth weeks in both furosemide-treated (n = 32) and control rats (n = 32). Lymphocyte magnesium concentration was also measured as a determinant of intracellular magnesium concentration. Magnesium concentrations in kidney, bone, skeletal, and heart muscle tissues were also quantitated at week 11 and at the end of the experiment. After 11 weeks of furosemide administration, furosemide-treated rats developed a lower plasma magnesium concentration (0.95 +/- 0.01 mmol/L) compared with that in the control group (0.99 +/- 0.01 mmol/L). This difference persisted from week 11 to week 19 of the experiment. Fractional excretion of magnesium was modestly elevated in the furosemide-treated group. After 7 weeks of furosemide treatment, lymphocyte magnesium concentration decreased significantly in furosemide-treated rats when compared with that in the control group (1.56 +/- 0.09 micrograms/mg protein vs 1.33 +/- 0.07 micrograms/mg protein). During week 19, the lymphocyte magnesium concentration had fallen to 0.75 +/- 0.04 micrograms/mg protein as compared with 1.45 +/- 0.08 micrograms/mg protein in the control rats. There is a significant correlation between lymphocyte magnesium concentration and plasma magnesium concentration. Our present results indicate that during long-term diuretic therapy, lymphocyte magnesium concentration mirrors the reduction in plasma magnesium concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclosporin-induced hypomagnesaemia and renal magnesium wasting in rats

1. Twenty-four hour clearance studies were performed on three groups of rats to determine the hypomagnesaemic effect of cyclosporin. Group I rats served as controls, whereas group II rats received 10 mg of cyclosporin/kg and group III rats received 20 mg of cyclosporin/kg daily. 2. After 7 days of cyclosporin treatment, plasma magnesium concentration was 1.04 +/- 0.01 mmol/l in control rats compared with 0.85 +/- 0.01 mmol/l in group II and 0.81 +/- 0.02 mmol/l in group III. After 14 days of cyclosporin administration, group III rats developed severe hypomagnesaemia (0.67 +/- 0.01 mmol/l). This reduction in plasma magnesium was associated with an increase in the urinary excretion of magnesium. 3. This reduction in plasma magnesium and increment in magnesium excretion returned to normal 7 days after cessation of treatment. 4. Additional three-phase acute clearance experiments were performed on eight normal and 12 cyclosporin-treated rats (20 mg/kg). These animals were subjected to acute magnesium chloride infusion. After graded magnesium chloride infusion, despite a similar rise in plasma magnesium, the fractional magnesium excretion was significantly higher in the cyclosporin-treated animals. 5. Analysis of the fractional intestinal absorption of magnesium suggested that the development of hypomagnesaemia after cyclosporin treatment is due to magnesium loss by the kidney. Furthermore, this effect of cyclosporin on magnesium transport is only present during cyclosporin treatment and is reversible when treatment with cyclosporin is withdrawn.

Renal magnesium wasting and hypocalciuria in chronic cis-platinum nephropathy in man

1. The renal handling of calcium and magnesium was studied in six patients with persistent hypomagnesaemia after cis-platinum treatment for testicular tumours. 2. In comparison with normal subjects, the patients showed hypomagnesaemia (mean 0.54 mmol/l), which was associated with a normal urinary magnesium excretion (mean 4.83 mmol/24 h). Urinary calcium excretion was significantly lower in the patients than in the normal subjects (mean 2.05 vs 5.15 mmol/24 h, respectively; P less than 0.01), despite slightly higher total serum calcium levels (2.53 vs 2.38 mmol/l, respectively; P less than 0.05). During magnesium chloride infusion, when serum magnesium levels were comparable in patients and controls, urinary calcium excretion remained lower in the patients, indicating that hypomagnesaemia was not the cause of the hypocalciuria. 3. Dietary magnesium supplementation resulted in a significant increase in the serum magnesium levels in the patients, while dietary magnesium deprivation resulted in a comparable decrease in urinary magnesium excretion in patients and controls (to 1.46 and 2.00 mmol/day, respectively), although the serum magnesium level fell further (to 0.46 mmol/l) in the patients. 4. The dissociation of renal calcium and magnesium excretion appears to be part of the intrinsic tubular defect caused by cis-platinum. This dissociation of urinary calcium and magnesium excretion, which resembles that seen in Bartter's syndrome, may result from a lesion in the distal convoluted tubule.

Effect of cisplatin on proximal straight tubule transport of divalent cations in the rabbit

Clearance and in vitro microperfusion studies were performed in rabbits to determine the effect of cisplatin on proximal straight tubule transport of calcium and magnesium. Rabbits were injected with cisplatin (2.5 mg/kg i.p. once weekly) for 3 weeks, whereas control rabbits received normal saline solution which served as a diluent for cisplatin. In 5 rabbits, 24-hour clearance studies were performed with the aid of a metabolic cage. Following cisplatin treatment, fractional excretion of magnesium rose significantly (73.3 +/- 11.5 vs. 111.4 +/- 17.5%). Glomerular filtration rate fell with cisplatin treatment (4.05 +/- 0.76 vs. 2.81 +/- 28 ml/min). There was no difference in fractional excretion of calcium (26.3 +/- 9.5 vs. 22.7 +/- 3.2%). The cortical and juxtamedullary proximal straight tubules were perfused in vitro. Net volume absorption was the same in the control and cisplatin-treated rabbits. However, there was a significant reduction in JCa (cortical 0.57 +/- 0.10 vs. -0.10 +/- 0.12 pmol/min/mm; juxtamedullary 0.96 +/- 0.17 vs. 0.31 +/- 0.37 pmol/min/mm) and JMg (cortical 0.43 +/- 0.08 vs. -0.15 +/- 0.07 pmol/min/mm; juxtamedullary 0.40 +/- 0.27 vs. -0.30 +/- 0.28 pmol/min/mm). In contrast to chronic administration, acute addition of cisplatin into the bath had no effect on JCa and JMg in the cortical and juxtamedullary proximal straight tubules. These data indicate that chronic but not acute cisplatin treatment depresses the transport of calcium and magnesium in the cortical and juxtamedullary nephrons of the proximal straight tubule of the rabbit.

Sodium thiosulfate prevents cisplatin-induced hypomagnesemia

Clearance studies were performed in four groups of male Wistar rats to assess the protective effect of sodium thiosulfate on cisplatin-induced hypomagnesemia. In group I, sodium thiosulfate (400 mg/kg) was injected intraperitoneally once weekly for 3 consecutive weeks. In group II, only cisplatin (2.5 mg/kg) was administered. In group III, both cisplatin (2.5 mg/kg) and sodium thiosulfate (400 mg/kg) were injected via the intraperitoneal route. When both drugs were administered together, they were injected into different parts of the peritoneal cavity. In group IV cisplatin was administered intraperitoneally and sodium thiosulfate intravenously. Sodium thiosulfate prevented a rise in plasma creatinine. The overall glomerular filtration rates of groups III and IV were the same as in group I. Hypomagnesemia was noted in group II, whereas in groups I, III, and IV the plasma magnesium level remained unchanged. The fractional excretion of magnesium was also higher in group II than in groups I, III, and IV. These differences persisted for the duration of the study. These results suggest that concurrent injections of sodium thiosulfate intraperitoneally or intravenously prevented the hypomagnesemic and the nephrotoxic effects of cisplatin and can be of clinical significance.

Effect of acidosis and alkalosis on divalent ion transport across the proximal straight tubule of the rabbit

In vitro microperfusion experiments were performed on the cortical proximal straight tubule of the rabbit to examine the effect of acid base disturbances on calcium and magnesium transport by this segment of the nephron. During acidosis (pH 7.22 +/- 0.01) net calcium reabsorption was reduced, whereas during alkalosis (pH 7.82 +/- 0.02) net calcium reabsorption was increased. Our flux experiments with 45Ca indicate that the efflux of Ca from the lumen to the bath remained the same during acidosis; however, the back-flux (bath to lumen) of calcium increased. On the other hand, during alkalosis the efflux of calcium rose and the influx of calcium remained unchanged. These changes in fluxes during acid-base disturbances can be accounted for mainly by changes in ionized calcium as a result of pH changes. We failed to observe any effect of acid-base changes on net magnesium transport by the superficial proximal straight tubule.

Effects of acid-base disturbances on renal handling of magnesium in the dog

Clearance and micropuncture studies were performed in four groups of acutely thyroparathyroidectomized animals to study the effects of alkalosis and acidosis on the renal handling of magnesium. Our results indicate that chronic metabolic acidosis reduces, whereas acute metabolic alkalosis enhances, magnesium reabsorption. The site within the nephron where absorption of magnesium increases or decreases during acid-base disturbances was beyond the late proximal tubule. Tubular fluid bicarbonate was also measured in these experiments, and the results indicated that magnesium reabsorption in the distal tubule correlated to bicarbonate delivery. However, whether this was a direct or an indirect effect of bicarbonate on magnesium transport could not be delineated.

Long-term morphological and biochemical observations in cisplatin-induced hypomagnesemia in rats

A model of cisplatin (cis-diamminedichloroplatinum II) induced hypomagnesemia with renal magnesium wasting was developed in rats. Following three weekly intraperitoneal injections of cisplatin (2.5 mg/kg body weight) hypomagnesemia was evident after the second injection of cisplatin and persisted for at least 24 weeks after the last injection. The plasma magnesium concentration was 0.69 +/- 0.01 mM in cisplatin-treated rats and 0.79 +/- 0.01 mM in control rats 24 h after the second injection (less than 0.01). Despite a lower plasma magnesium concentration in cisplatin-treated rats, the fractional urinary excretion of magnesium was similar in both groups, indicating inappropriate renal magnesium excretion in cisplatin-treated rats. Rat kidneys fixed in situ by arterial infusion were examined by light and electron microscopy at the following times: 24 h after the first and second injections and 48 h and 1, 4, 8, 16, and 24 weeks after the third injection. Focal necrosis of the proximal tubule cells in the outer stripe of the medulla (S3 segment) was noted 24 h after the second injection. Following this, there was dilatation of the S3 segment proximal tubules which were lined by variably flattened epithelial cells devoid of brush borders. Many of these cells had enlarged, hyperchromatic nuclei, and few mitoses were observed. These changes were most extensive at 4 weeks, diminished after this, but were still present focally at 24 weeks. In this model the cisplatin-induced hypomagnesemia persists long after discontinuation of therapy as in humans, and the S3 segment is the prime target of the cisplatin.(ABSTRACT TRUNCATED AT 250 WORDS)

The physiology of renal magnesium handling

Present evidence suggests that the renal handling of magnesium is normally a filtration-reabsorption process as evidence for secretion is unsubstantiated. Magnesium reabsorption has distinctive features when compared with that of sodium and calcium. The proximal tubule concentration of magnesium rises to levels about 1.5 times greater than the glomerular filtrate and only 20-30% of the filtered magnesium is reabsorbed in this segment. Although the fractional reabsorption of magnesium is only half that of sodium, it changes in parallel with that of sodium in response to changes in extracellular fluid volume. The major portion of filtered magnesium (some 65%) is reabsorbed in the loop of Henle and evidence indicates that the thick ascending limb is the principal segment involved in magnesium absorption. Recent observations suggests that magnesium reabsorption in the ascending limb may be voltage dependent and secondary to active sodium chloride reabsorption. The loop of Henle appears to be the major nephron site where magnesium reabsorption is regulated possibly by cAMP-mediated hormones including parathyroid hormones, calcitonin, glucagon and antidiuretic hormone. About 10% of the filtered magnesium is delivered into the distal nephron. The distal tubule reabsorbs only a small fraction of the filtered magnesium which may be regulated by the same cAMP-mediated hormones involved in control of magnesium in the loop.

Studies on the pathogenesis of cisplatin-induced hypomagnesemia in rats

After three weekly intraperitoneal injections of cisplatin (2.5 mg/kg body wt), male Wistar rats developed chronic hypomagnesemia, which was evident from the second week and persisted throughout the 8-week experiments. Plasma magnesium concentration was 0.69 +/- 0.01 mM in cisplatin-treated rats compared to 0.80 +/- 0.02 mM in pair-fed control rats (P less than 0.01) in the eighth week of experimentation. Despite a similar dietary magnesium intake, urinary excretion of magnesium in cisplatin-treated rats was inappropriately high, relative to the lower plasma magnesium concentration, indicating the presence of renal magnesium wasting induced by cisplatin. During the 3 weeks of cisplatin injections, metabolic balance studies indicated abnormal renal excretion and a reduction in the fractional intestinal absorption of magnesium. A compensatory period of significantly greater retention of magnesium then occurred in cisplatin-treated rats beginning in the fourth week. Clearance and recollection micropuncture studies in a separate group of rats revealed normal magnesium and calcium transport in the superficial proximal and distal nephron. Following acute MgCl2 infusion, the urinary excretion of magnesium and calcium were significantly higher in cisplatin-treated rats than in control rats; however, micropuncture studies of superficial nephrons failed to demonstrate abnormal transport of these divalent cations. It is possible, therefore, that 7 weeks of cisplatin treatment led to tubular adaptation that might have obscured the defect in magnesium reabsorption. Morphological examination indicated that pathological changes were confined to the S3 segment of proximal corticomedullary nephrons.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced distal absorption of potassium by magnesium-deficient rats

The effect of acute potassium infusion on renal tubular reabsorption of potassium by magnesium-deficient and pair-fed control rats has been studied by the recollection micropuncture method. During potassium chloride infusion, the amount of potassium remaining in the distal tubule is lower in magnesium-deficient than in pair-fed rats. This could be due to a reduction in potassium secretion or to an enhancement of potassium reabsorption. The present study demonstrates enhanced renal potassium retention in the magnesium-deficient rat, and is in contrast to previous reports of renal potassium wasting in this circumstance.

Actions of parathyroid hormone are not impaired during chronic metabolic acidosis

The effect of parathyroid hormone (PTH) on renal excretion of calcium, phosphate, and bicarbonate was studied in acutely thyroparathyroidectomized dogs with metabolic acidosis. Ammonium chloride 10 gm/day for 3 days was given to 15 dogs to induce chronic metabolic acidosis. Clearance results obtained from these experiments showed that infusion of Beckman 1-34 PTH into normal and acidotic dogs resulted in a marked increase in phosphate and bicarbonate excretion, accompanied by a small reduction in calcium excretion and a slight but significant increase in plasma ultrafilterable calcium. Complementing our clearance data, micropuncture results obtained from the proximal and distal tubules of these animals indicated an enhancement of calcium reabsorption in the distal tubule after administration of PTH. Phosphate reabsorption by the proximal and distal tubules was reduced after infusion of PTH in both normal and acidotic dogs. The fraction of bicarbonate reabsorbed in the proximal tubule was reduced after PTH infusion, but the fraction of bicarbonate reabsorbed in the distal tubule remained unchanged. These data suggest that the phosphaturic, the hypocalciuric, and the bicarbonaturic effects of PTH were not impaired during chronic metabolic acidosis.

Effects of phosphate and calcium infusion on renal phosphate transport in the dog

The effect of phosphate infusion on renal tubular handling of calcium and phosphate was examined in dogs which had been thyroparathyroidectomized (TPTX) immediately prior to the studies. Phosphate infusions in TPTX animals caused a small decrease in total and ultrafilterable plasma calcium, and decreased phosphate reabsorptive capacity in the proximal tubule and loop segment. Infusion of CaCl2 during phosphate loading to offset the fall in plasma calcium prevented the reduction in proximal phosphate reabsorptive capacity. However, between the proximal and distal sampling site, the reduction in phosphate reabsorptive capacity could not be prevented by CaCl2 administration. These data are consistent with the presence of two phosphate transport systems; one in the early proximal tubule, modulated by changes in plasma calcium level, and a second in the loop segment, which is independent of calcium. While the data suggest that the depression of proximal phosphate reabsorption during phosphate infusion may be secondary to the fall in plasma calcium concentration, they do not exclude a direct effect of infused phosphate on proximal phosphate reabsorption that may be antagonized by an opposing direct effect of the calcium infusion.

Tubular handling of bicarbonate in dogs with experimental renal failure

In this study, micropuncture experiments were performed to examine the segmental reabsorption of bicarbonate and chloride in the normal dog kidney (stage 1) and in the remnant kidney (stage 3) of dogs with experimental renal failure. The protocol consisted of three phases: (1) 3% extracellular fluid volume expansion, (2) infusion of Ringer's solution containing 90 mM of sodium bicarbonate, and (3) infusion of 150 mM of sodium bicarbonate in Ringer's solution. In the animals with remnant stage 3 kidneys, the ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was increased compared to dogs with stage 1 kidneys at both the proximal and distal sampling sites. These data suggest that bicarbonate reabsorption was elevated in both the distal and the proximal tubules in experimental chronic renal failure. In contrast to the reabsorption of bicarbonate, chloride reabsorption was depressed in stage 3 kidneys at the late proximal puncture site. This resulted in greater delivery to the distal nephron. The distal segments reabsorbed a constant fraction of delivered chloride, resulting in an increase in fractional chloride excretion in chronic renal failure compared to that in normal animals.

Renal tubular calcium transport: effects of changes in filtered calcium load

Micropuncture experiments were performed in thyroparathyroidectomized dogs to examine the influence of changes in filtered calcium load on segmental tubular calcium reabsorption. Filtered calcium load was changed either by reducing glomerular filtration rate (GFR) by aortic clamping (group I) or by progressive calcium infusion (group II) to increase plasma ultrafilterable calcium concentration (UFCa). The results suggest that fractional proximal calcium reabsorption responds similarly to altered filtered load, whether produced by changes in GFR or UFCa. In contrast, fractional reabsorption by the loop segment is progressively reduced as UFCa is increased but is relatively unchanged by alterations in filtered load secondary to altered GFR. These data indicate a specific parathyroid hormone-independent reciprocal effect of UFCa on calcium reabsorption in the loop segment, which may be an important determinant of urinary calcium excretion.

Effects of metabolic alkalosis on calcium excretion in the conscious dog

The tubular reabsorption of calcium has been studied in chronic metabolic alkalosis and acidosis. Clearance studies were performed in five conscious mongrel dogs during control periods and during alkalosis and acidosis both before and after thyroparathyroidectomy. After thyroparathyroidectomy, normocalcemia was maintained with oral dihydrotachysterol, and thyroxine was replaced. An initial control study was followed by the induction of chronic metabolic alkalosis by repeated gastric drainage for 4 days through a previously implanted gastric fistula. A second control study 1 week after alkalosis was followed by the induction of acidosis by feeding ammonium chloride, 10 gm daily for 3 days. A final control study was performed 1 week later. Serum proteins, GFR, and filtered calcium load were not significantly different in control, alkalosis, or acidosis. Alkalosis, in both intact and TPTX dogs, was associated with no change in TRNa in comparison with controls (94.6% vs. 95.4 in intact, p greater than 0.05; 95.0% vs. 96.3% in TPTX, p greater than 0.05). By contrast, TRCa increased significantly (97.2% vs. 95.5, p less than 0.05 in intact; 98.0% vs. 95.0, p less than 0.05 in TPTX). In acidosis, TRNa in both intact and TPTX dogs was unchanged in comparison with control (96.4%, p greater than 0.05 and 96.6%, p greater than 0.05, respectively), and TRCa was significantly decreased (to 91.9%, p less than 0.01 and 83.3%, p less than 0.001, respectively). These data indicate that chronic metabolic alkalosis increases the TRCa independently of changes in TRNa, filtered calcium load, or parathyroid activity.

Tubular reabsorptive capacity for magnesium in the dog kidney

Micropuncture studies were performed on 27 thyroparathyroidectomized dogs to determine the segmental reabsorption of magnesium before and after graded magnesium infusion. Overall kidney reabsorption, as determined by the difference between filtered magnesium and urinary excretion, initially increased with elevation of plasma magnesium up to 3.5 meq/liter. Further elevation of plasma magnesium resulted in the appearance in the urine of all additional increments in filtered magnesium. Thus, renal magnesium reabsorption followed a pattern characteristic of a maximal transport rate (Tm). Evaluation of the proximal tubule by micropuncture demonstrated that net reabsorption of magnesium rose proportional to the increase in filtered magnesium and accounted for a constant fractional reabsorption of 14%. In contrast, magnesium reabsorption in Henle's loop initially increased with low delivery rates but peaked and fell with high plasma magnesium concentrations. Little magnesium reabsorption was observed between the distal collection site and final urine. Accordingly, the overall urinary excretion pattern was a summation of the different effects occurring in the proximal tubule and Henle's loop. Thus, renal magnesium reabsorption is not characterized by a Tm process but is a composite of distinct transport properties of the proximal tubule and the loop of Henle.