Lymph Pools in the Basement, Sump Pumps in the Attic: The Anuran Dilemma for Lymph Movement

Fluid Therapy in Exotic Animal Emergency and Critical Care

Many new concepts are emerging in the understanding of fluid therapy in human and mammalian medicine, including the role of the glycocalyx, increased understanding of fluid, sodium, and chloride overload, and the advantages of colloid administration in the form of albumin. None of these concepts, however, appear to be directly applicable to non-mammalian exotic patients, and careful consideration of their alternate physiology is required when formulating fluid plans for these patients.

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Anuran amphibians as comparative models for understanding extreme dehydration tolerance: a unique negative feedback lymphatic mechanism for blood volume regulation

Anurans are the most terrestrial order of amphibians. Couple the high driving forces for evaporative loss in terrestrial environments and their low resistance to evaporation, dehydration is an inevitable stress on their water balance. Anurans have the greatest tolerances for dehydration of any vertebrate group. Some species can tolerate evaporative losses up to 45% of their standard body mass. Anurans have remarkable capacities to regulate blood volume with hemorrhage and dehydration compared with mammals. Stabilization of blood volume is central to extending dehydration tolerance, since it avoids both the hypovolemic and hyperviscosity stresses on cardiac output and its consequential effects on aerobic capacity. Anurans, in contrast to mammals, seem incapable of generating a sufficient pressure difference, either oncotically or via interstitial compliance, to move fluid from the interstitium into the capillaries. Couple this inability to generate a sufficient pressure difference for transvascular uptake to a circulatory system with high filtration coefficients and a high rate of plasma turnover is the consequence. The novel lymphatic system of anurans is critical to a remarkable capacity for blood volume regulation. This review summarizes what is known about the anatomical and physiological specializations that are involved in explaining differential blood volume regulation and dehydration tolerance involving a true centrally mediated negative feedback of lymphatic function involving baroreceptors as sensors and lymph hearts, arginine vasotocin, pulmonary ventilation and specialized skeletal muscles as effectors.

Baroreflex function in anurans from different environments

Anurans from terrestrial environments have an enhanced ability to maintain mean arterial blood pressure (P(m)) through lymph mobilization in response to desiccation or hemorrhage compared with semiaquatic or aquatic species. Because short term blood pressure homeostasis is regulated by arterial baroreceptors, we compared baroreflex function in three species of anurans that span a range of environments, dehydration tolerance and an ability to maintain P(m) with dehydration and hemorrhage. The cardiac limb of the baroreflex loop was studied using pharmacological manipulation of P(m) with phenylephrine and sodium nitroprusside (20–200 μg kg(− 1)), and the resulting changes in heart rate (f(H)) were quantitatively analyzed using a four-parameter sigmoidal logistic function. Resting P(m) in the aquatic species, Xenopus laevis, was 3.6 ± 0.3 kPa and was significantly less (P < 0.005) than for the semiaquatic species, Lithobates catesbeianus (4.1 ± 0.2 kPa), or the terrestrial species, Rhinella marina (4.7 ± 0.2 kPa). The maximal baroreflex gain was not different among the three species and ranged from 12.1 to 14.3 beats min( −1) kPa( −1) and occurred at P(m )ranging from 3.0 to 3.8 kPa, which were slightly below the resting P(m) for each species. Mean arterial blood pressures at rest in the three species were near the saturation point of the baroreflex curve which provides the animals with a greater fH response range to hypotensive, rather than hypertensive, changes in P(m). This is consistent with the hypothesis that arterial baroreceptors are key sensory components that allow anurans to maintain P(m) possibly by mobilization of lymphatic return in response to hypotension.

Reprint of "Baroreflex function in anurans from different environments"

Anurans from terrestrial environments have an enhanced ability to maintain mean arterial blood pressure (Pm) through lymph mobilization in response to desiccation or hemorrhage compared with semiaquatic or aquatic species. Because short term blood pressure homeostasis is regulated by arterial baroreceptors, we compared baroreflex function in three species of anurans that span a range of environments, dehydration tolerance and an ability to maintain Pm with dehydration and hemorrhage. The cardiac limb of the baroreflex loop was studied using pharmacological manipulation of Pm with phenylephrine and sodium nitroprusside (20-200μgkg(-1)), and the resulting changes in heart rate (fH) were quantitatively analyzed using a four-parameter sigmoidal logistic function. Resting Pm in the aquatic species, Xenopus laevis, was 3.6±0.3kPa and was significantly less (P<0.005) than for the semiaquatic species, Lithobates catesbeianus (4.1±0.2kPa), or the terrestrial species, Rhinella marina (4.7±0.2kPa). The maximal baroreflex gain was not different among the three species and ranged from 12.1 to 14.3beatsmin(-1)kPa(-1) and occurred at Pm ranging from 3.0 to 3.8kPa, which were slightly below the resting Pm for each species. Mean arterial blood pressures at rest in the three species were near the saturation point of the baroreflex curve which provides the animals with a greater fH response range to hypotensive, rather than hypertensive, changes in Pm. This is consistent with the hypothesis that arterial baroreceptors are key sensory components that allow anurans to maintain Pm possibly by mobilization of lymphatic return in response to hypotension.

Visualising lymph movement in anuran amphibians with computed tomography

Lymph flux rates in anuran amphibians are high relative to those of other vertebrates owing to ‘leaky’ capillaries and a high interstitial compliance. Lymph movement is accomplished primarily by specialised lymph muscles and lung ventilation that move lymph through highly compartmentalised lymph sacs to the dorsally located lymph hearts, which are responsible for pumping lymph into the circulatory system; however, it is unclear how lymph reaches the lymph hearts. We used computed tomography (CT) to visualise an iodinated contrast agent, injected into various lymph sacs, through the lymph system in cane toads (Rhinella marina). We observed vertical movement of contrast agent from lymph sacs as predicted, but the precise pathways were sometimes unexpected. These visual results confirm predictions regarding lymph movement, but also provide some novel findings regarding the pathways for lymph movement and establish CT as a useful technique for visualising lymph movement in amphibians.

Comparative cardiovascular physiology: future trends, opportunities and challenges

The inaugural Kjell Johansen Lecture in the Zoophysiology Department of Aarhus University (Aarhus, Denmark) afforded the opportunity for a focused workshop comprising comparative cardiovascular physiologists to ponder some of the key unanswered questions in the field. Discussions were centred around three themes. The first considered function of the vertebrate heart in its various forms in extant vertebrates, with particular focus on the role of intracardiac shunts, the trabecular ('spongy') nature of the ventricle in many vertebrates, coronary blood supply and the building plan of the heart as revealed by molecular approaches. The second theme involved the key unanswered questions in the control of the cardiovascular system, emphasizing autonomic control, hypoxic vasoconstriction and developmental plasticity in cardiovascular control. The final theme involved poorly understood aspects of the interaction of the cardiovascular system with the lymphatic, renal and digestive systems. Having posed key questions around these three themes, it is increasingly clear that an abundance of new analytical tools and approaches will allow us to learn much about vertebrate cardiovascular systems in the coming years.

Lymphatic regulation in nonmammalian vertebrates

All vertebrate animals share in common the production of lymph through net capillary filtration from their closed circulatory system into their tissues. The balance of forces responsible for net capillary filtration and lymph formation is described by the Starling equation, but additional factors such as vascular and interstitial compliance, which vary markedly among vertebrates, also have a significant impact on rates of lymph formation. Why vertebrates show extreme variability in rates of lymph formation and how nonmammalian vertebrates maintain plasma volume homeostasis is unclear. This gap hampers our understanding of the evolution of the lymphatic system and its interaction with the cardiovascular system. The evolutionary origin of the vertebrate lymphatic system is not clear, but recent advances suggest common developmental factors for lymphangiogenesis in teleost fishes, amphibians, and mammals with some significant changes in the water-land transition. The lymphatic system of anuran amphibians is characterized by large lymphatic sacs and two pairs of lymph hearts that return lymph into the venous circulation but no lymph vessels per se. The lymphatic systems of reptiles and some birds have lymph hearts, and both groups have extensive lymph vessels, but their functional role in both lymph movement and plasma volume homeostasis is almost completely unknown. The purpose of this review is to present an evolutionary perspective in how different vertebrates have solved the common problem of the inevitable formation of lymph from their closed circulatory systems and to point out the many gaps in our knowledge of this evolutionary progression.

Evolutionary implications of the distribution and variation of the skeletal muscles of the anuran lymphatic system

Lymphatic return to the circulation in anurans is dependent upon the interaction of a number of skeletal muscles and lung deflation. We define character states and describe variation of these putative lymphatic skeletal muscles: the M. cutaneus pectoris (CP), M. cutaneus dorsi (CD), M. piriformis (P), M. sphincter ani cloacalis (SAC), and the complex of the M. gracilis minor/M. abdominal crenator (GM/AC). We include examination of over 400 specimens of 377 species belonging to 40 of the 42 currently recognized anuran families. Some muscles show limited variation (P) or are clearly linked to phylogeny (CP; CD) and thus have limited value in the determination of form and function. However, the GM/AC and SAC show a high degree of structural variation that appears in taxa across the phylogenetic spectrum. This allows us to make phylogenetically independent determinations of form and function. We define an ancestral state of the GM and conclude that evolution of the GM/AC and SAC has progressed in two directions from this ancestral state: toward either elaboration or reduction. Where present, the character states of both of these muscle groups were observed in all species examined and the number of states correlated within each family as well. The degree of development of the GM/AC and SAC compliance pump system is strongly correlated with previously determined lymph flux rates in a three species test. Our data suggest there may be a relationship between greater elaboration of the GM/AC and SAC system and terrestriality among the Anura.

Posterior lymph heart function in two species of anurans: analysis based on both in vivo pressure-volume relationships by conductance manometry and ultrasound

Rhinella marina and Lithobates catesbeianus have known differences in the capacity to mobilize lymph to stabilize blood volume following dehydration and hemorrhage. The purpose of these experiments was to assess whether there are interspecific differences in basic lymph heart functions. The end diastolic volumes of posterior lymph hearts averaged 10.8 μl kg⁻¹ in R. marina and 7.9-10.8 μl kg⁻¹ in L. catesbeianus by conductance manometry, and 9-32 μl kg⁻¹ in R. marina by ultrasound techniques, which correlated with body mass. Stroke volumes were approximately 20% of end diastolic volumes in both species. Peak systolic pressures and stroke work were correlated with the index of contractility (dP/dt(max)) in both species. Stroke volume was correlated to stroke work but not peak systolic pressure, end diastolic volume or end diastolic pressure indicating the preload variables do not seem to determine stroke volume as would be predicted from Starling considerations of the blood heart. Renal portal elastance (end systolic pressure/stroke volume) an afterload index did not differ interspecifically, and was equivalent to values for systemic flow indices from mice of equivalent ventricular volume. These data, taken together with predictions derived from mammalian models on the effect of high resistance indicate afterload (renal portal pressure), may be important determinants of posterior lymph heart stroke volume. The shape of the pressure-volume loop is different from an idealized version previously reported, and is influenced by end diastolic volume. Our data indicate that increasing end diastolic pressure and volume can influence the loop shape but not the stroke volume. This indicates that lymph hearts do not behave in a Starling Law manner with increased preload volume.

Lymph flux rates from various lymph sacs in the cane toad Rhinella marina: an experimental evaluation of the roles of compliance, skeletal muscles and the lungs in the movement of lymph

A new method for quantitatively determining lymph flux from various lymphatic sacs of an anuran, the cane toad, was developed. This method used the dye dilution principle of CiVi=CfVf following injection of Evans Blue into specific lymph sacs and measuring its appearance in the venous circulation. The apparent lymph volume was 57 ml kg–1. The greatest rate of lymph return (0.5–0.8 ml kg–1 min–1) and best linear fit of Evans Blue appearance in the circulation with time followed injections into the subvertebral lymph sac, which has direct connections to both the anterior and posterior pairs of lymphatic hearts. Rate of lymph flux from the pair of posterior lymph hearts was three times greater than the anterior pair. Rates of lymph flux were only influenced by injection volume in the crural lymph sacs, implicating lymph sac compliance as the source of the pressure for lymph movement from these sacs. Femoral lymph sac fluxes were decreased by 60% following ablation of the tendons of the sphincter ani cloacalis, abdominal crenators and piriformis. This supports a role for these muscles in generating the pressure for vertical lymph movement. Femoral lymph sac fluxes were also decreased by 70% by the insertion of a coil in the subvertebral lymph sac, preventing normal compression and expansion of this sac by the lungs. This supports a role for lung ventilation in generating the pressure for vertical movement of lymph. Contrary to previous hypotheses, fluxes from the brachial sac were not influenced by insertion of the coil into the subvertebral sac. A haemorrhage equivalent to 50% of the blood volume did not change lymph flux rates from the femoral lymph sacs. These data provide the first experimental evidence that actual lymph fluxes in the cane toad Rhinella marina depend on lymph sac compliance, contraction of specific skeletal muscles and lung ventilation to move lymph laterally and vertically to the dorsally located lymphatic hearts.

Physiological analysis of the lymphatic system in the eastern painted turtle (Chrysemys picta picta)

Examination into the anuran lymphatic system has led to a comprehensive understanding of lymphatics, including the importance of synchrony in fluid-balance maintenance. However, little research has been conducted on the lymphatics of turtles and other reptilian vertebrates. Using pressure-peak recordings created through cannulation of both lymph hearts of the eastern painted turtle, Chrysemys picta picta (Schneider, 1783), the lymph heart contraction rate was verified and the interbeat interval patterns were examined using Poincaré plots. The lymph heart beating rate was determined to be 38.2 beats·min–1with a mean pulse pressure of 2.40 ± 1.44 mm Hg (1 mm Hg at 0 °C = 133.3224 Pa). Poincaré plots are useful in displaying nonlinear sequential data and are often given descriptive names related to the overall pattern. The Poincaré plot resembled a garden hose nozzle spray, indicating a large variability in interbeat time intervals with periods of multiple-beat patterns. The degree of bilateral lymph heart synchrony was determined in the turtle using the mean time difference between right and left lymph heart systoles. Results show that chelonian lymph hearts do in fact beat in synchrony, with over 50% of contractions occurring within 100 ms of each other. This indicates shared neuronal control and may suggest an energetic advantage to fluid homeostasis.

Lung ventilation contributes to vertical lymph movement in anurans

Anurans (frogs and toads) generate lymphatic fluid at 10 times the rate in mammals, largely as a consequence of their very `leaky' vasculature and high interstitial compliance. Lymph is ultimately pumped into the venous system by paired, dorsally located lymph hearts. At present, it is unclear how lymphatic fluid that accumulates in central body subcutaneous lymph sacs is moved to the anterior and posterior lymph hearts in the axillary regions and how lymph is moved, against gravity, to the dorsally located lymph hearts. In this study,we tested the hypothesis that lung ventilation, through its consequent effects on lymph sac pressure, contributes to the vertical movement of lymphatic fluid in the cane toad (Chaunus marinus) and the North American bullfrog(Lithobates catesbeiana). We measured pressure in the dorsal, lateral and subvertebral lymph sacs of anesthetized cane toads and bullfrogs during artificial lung inflation and deflation. We also measured pressure in the subvertebral lymph sac, which adheres to the dorsal surface of the lungs,simultaneously with brachial (forelimb) and pubic (posterior) sac pressure during ventilation in freely behaving animals. There were highly significant(P&lt;0.001) relationships between lung pressure and lymph sac pressures (r2=0.19–0.72), indicating that pulmonary pressure is transmitted to the highly compliant lymph sacs that surround the lungs. Subvertebral sac pressure of resting animals was not significantly different between L. catesbeiana (518±282 Pa) and C. marinus (459±111 Pa). Brachial sac compliance (ml kPa–1 kg–1) also did not differ between the two species (33.6±5.0 in L. catesbeiana and 37.0±9.4 in C. marinus). During expiration (lung deflation), reductions in expanding subvertebral sac pressure are communicated to the brachial lymph sac. Changes in brachial and pubic lymph sac pressures were correlated almost entirely during expiration rather than inspiration. The change in brachial sac pressure during expiration was 235±43 Pa for C. marinus and 215±50 Pa for L. catesbeiana, which is of sufficient magnitude to move lymph the estimated 0.5–1.0 cm vertical distance from the forelimb to the vicinity of the anterior lymph hearts. We suggest that lymph is moved during expiration to the subvertebral sac from anterior and posterior lymph sacs. During lung inflation, increased lymph sac pressure moves lymph to axillary regions, where lymph hearts can return lymph to the vascular space. Consequently, pulmonary ventilation has an important role for lymph movement and, hence, blood volume regulation in anurans.

Unique role of skeletal muscle contraction in vertical lymph movement in anurans

Electromyographic (EMG) activity of skeletal muscles that either insert on the skin or are associated with the margins of subcutaneous lymph sacs was monitored for two species of anurans, Chaunus marinus and Lithobates catesbeiana (formerly Bufo marinus and Rana catesbeiana). Our hypothesis was that contraction of these muscles varies the volume, and hence pressure, within these lymph sacs, and that this pressure is responsible for moving lymph from ventral, gravitationally dependent reaches of the body to dorsally located lymph hearts. EMG activity of M. piriformis, M. gracilis minor, M. abdominal crenator, M. tensor fasciae latae, M. sphincter ani cloacalis, M. cutaneous pectoris and M. cutaneous dorsi was synchronous with pressure changes in their associated lymph sacs. These muscles contracted synchronously, and the pressures generated within the lymph sacs were sufficient to move lymph vertically against gravity to the lymph hearts. The pressure relationships were complex; both negative and positive pressures were recorded during a contractile event, a pattern consistent with the addition and loss of lymphatic fluid to the lymph sacs. Severing the tendons of some of the muscles led to lymph pooling in gravitationally dependent lymph sacs. These data are the first to: (1)describe a function for many of these skeletal muscles; (2) document the role of skeletal muscles in vertical lymph movement in anurans; and (3) reinterpret the role of the urostyle, a bony element of the anuran pelvic girdle.

Measurements of common iliac arterial blood flow in anurans using Doppler ultrasound

Color Doppler ultrasonography was used to determine time-average mean velocity and cross-sectional area of the common iliac artery in bullfrogs (Rana catesbeiana) and marine toads (Bufo marinus). Volumetric blood flow and weight-adjusted blood flow measurements were calculated from this data. Volumetric flow rates of frogs (31.8 ml/min) and toads (23.6 ml/min) did not differ statistically. However, when flow rates were adjusted for body mass, toads displayed a significantly greater flow rate of 238.1 ml/min/kg compared to 114.4 ml/min/kg for frogs.

Synchrony in the amphibian lymphatic system: evidence for bilateral posterior lymph heart synchrony and cardiac–lymphatic synchrony inRana catesbeianaandBufo marinus

Early investigations into amphibian lymph heart function established that lymph heart contractions were synchronous with neither the systemic heart, nor the lungs, nor each other. However, the present study concludes that there is synchronization between the cardiac heart and the lymph hearts and that the posterior lymph hearts in both Rana catesbeiana Shaw, 1802 and Bufo marinus (L., 1758) beat synchronously as well. Pressure peaks were recorded through cannulation of the ischiatic artery and each posterior lymph heart and subsequently analyzed to determine the time differences between arterial diastole and lymph heart systole or between two bilateral lymph heart systoles. Results show that there is clear synchronization between the lymph heart systoles of two bilateral posterior lymph hearts. This lymph heart synchrony is further supported by using Poincaré plot analysis to visually compare the lymph heart inter-beats. Cardiac heart and lymph heart contractions also show a degree of synchronization, even though the lymph hearts beat up to three times as fast as the cardiac heart. These results support the conclusion that synchrony is characteristic of the anuran lymphatic system and that synchronization of the cardiac heart and the lymph hearts could impart an energetic advantage that benefits fluid homeostatic mechanisms.

Functional Roles for the Compartmentalization of the Subcutaneous Lymphatic Sacs in Anuran Amphibians

Compliance of the subcutaneous lymph sacs of the hindlimbs increases from distal to proximal, as does limb segment mass (and presumably rate of lymph formation), for the semiaquatic bullfrog Rana catesbeiana and the cane toad Bufo marinus but not the aquatic clawed toad Xenopus laevis. Subcutaneous lymph-sac compliances vary interspecifically. The distal-to-proximal increase in lymph-sac compliance and estimates of lymph formation rate in the various hindlimb segments indicate that partitioning of hindlimb subcutaneous lymphatic sacs establishes a differential decrease in the intra-lymph-sac pressure for R. catesbeiana and B. marinus. These pressure differentials constitute a "compliance pump" that drives distal-to-proximal intersac lymph flow. The compliance pump alone explains lymphatic return for the aquatic frog X. laevis but does not explain how lymph would reach the dorsally located lymph hearts for terrestrial anurans, so we hypothesize that skeletal muscle pumps return lymph from the femoral and pubic lymph sacs to the lymph heart. This is a fundamentally different role of the subcutaneous lymph-sac system than has been previously proposed. We suggest that the more proximal subcutaneous lymph sacs are important for fluid storage because they have a relatively high compliance.