The Role of the Lymphatic System for Water Balance and Acid-Base Regulation in the Amphibia. In: Heisler N, editor. Mechanisms of Systemic Regulation. Berlin: Springer Berlin Heidelberg; 1995. pp. 201-14. [DOI: 10.1007/978-3-642-79666-1_9]

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.

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.

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.

Cardiovascular and behavioural changes during water absorption in toads, Bufo alvarius and Bufo marinus

Blood cell flux (BCF) in the pelvic skin of Bufo marinus was lower than Bufo alvarius when toads rehydrated from deionised water (DI) or 50 mmol l–1 NaCl (NaCl). Despite the lower BCF in B. marinus, water absorption was not different between the species when toads rehydrated from DI or NaCl. When fluid contact was limited to the pelvic skin, water uptake from NaCl was lower than from DI, but became greater than uptake from DI as the immersion level increased. Hydrophobic beeswax coating the lateral sides reduced absorption from NaCl but not from DI. Toads settled into water absorption response posture well after maximal BCF was attained in both DI and NaCl, indicating that the behavioural response requires neural integration beyond the increase in BCF. Water exposure increased BCF in hydrated B. alvarius with empty bladders but not in those with stored bladder water. Hydrated B. marinus with an empty bladder did not increase BCF when given water. Handling stress depressed BCF but increased central arterial flow (CAF), measured using a flow probe around the dorsal aorta. In undisturbed toads, CAF increased with the same time course as BCF while heart rate remained relatively constant, suggesting redistribution of blood flow.

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.

General Function and Endocrine Control of the Posterior Lymph Hearts inBufo marinusandRana catesbeiana

The effects of hypervolemia and graded increases in arginine vasotocin (AVT), angiotensin II (ANGII), and atrial natriuretic peptide (ANP) on lymph heart pressure (P(lh)) and rate (f(lh)) were examined in Bufo marinus and Rana catesbeiana. The P(lh) and f(lh) for normally hydrated B. marinus at rest were 1.45+/-0.01 kPa and 52.8+/-0.38 beats min(-1). The P(lh) and f(lh) were significantly lower in R. catesbeiana, 1.05+/-0.01 kPa and 48.4+/-0.35 beats min(-1). Hypervolemia, induced by intravenous infusion of isotonic saline, stopped the lymph hearts at volumes of 0.48%+/-0.06% and 0.32%+/-0.04% body mass in B. marinus and R. catesbeiana, respectively, equivalent to an 8% increase of their respective plasma volumes. ANP had no effect on P(lh) or f(lh) at any of the dosages tested. ANGII decreased f(lh) in both species, approximating the physiological range of concentrations. AVT, at physiological concentrations, increased P(lh) 48% in B. marinus and 38% in R. catesbeiana without changing f(lh) in either species. At higher than physiological dosages, P(lh) and f(lh) in both species declined. The results suggest that AVT, normally released during hemorrhage and dehydration, would increase lymph heart output and help compensate for the hypovolemia. This is a contrary result to previous work using supraphysiologic doses of AVT.

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

Amphibians are a vertebrate group transitional between aquatic and terrestrial environments. Consequently, both increases and decreases in blood volume are a natural biological stress associated with aquatic and terrestrial environments. In comparison with other vertebrate classes, anuran amphibians have the most rapid compensation and greatest capacity to compensate for changes in blood volume and survive dehydration. Unlike in mammals, a Starling transcapillary uptake mechanism does not account for this fluid mobilization because lymph flow is a substantial and important additional factor. The role of the lymphatic system in flux of fluids back into the circulation varies interspecifically in anurans and is an order of magnitude greater in anurans than in mammals. Current models of lymph movement in anurans are centered on the role of lymph hearts, but we suggest that these models are untenable. We present a new hypothesis for lymph movement involving (1) pressure differences created by compartmentalization of the hind limb lymph spaces into sacs of serially graded compliance to move lymph horizontally and (2) both negative and positive pressure differences created by contraction of skeletal muscles to move lymph vertically. The primary function of some of these skeletal muscles may be solely for lymph movement, but some may also be involved with other functions such as pulmonary ventilation.

Cardiac role of frog ANF: negative inotropism and binding sites in Rana esculenta

To elucidate the role of atrial natriuretic peptides (NPs) in the amphibian heart, the myotropic effects and the cardiac distribution of frog atrial natriuretic factor (fANF) have been studied in Rana esculenta. Spontaneously, beating in vitro isolated working heart preparations were treated with increased concentrations (10(-11)-10(-8) M) of fANF-(1-24). The peptide at 10(-9) and 10(-8) M significantly reduced heart rate (HR) and, on the electrically paced preparations, decreased cardiac output (CO), stroke volume (SV) and work. Such negative inotropism was abolished by pretreatment with the pertussis toxin or by blocking the particulate guanylate cyclase (GC) with anantin while it was independent both from the functional impairment of the endocardium-endothelium by Triton X-100 and the inhibition of the soluble guanylate cyclase by 1 H-(1,2,4,) oxadiazolo-(4,3-a) quinoxalin-1-one (ODQ). By autoradiography, two classes of high and low affinity NPs binding sites were detected in the ventricular endocardium and myocardium and in the bulbus arteriosus. The analysis of displacement binding data using the radioligand [125I]-rat atrial natriuretic peptide [125I-rANP-(1-28)], its cold counterpart and the fANF-(1-24) showed that in the ventricular myocardium, the low affinity NPs sites bound both the heterologous and the homologous ligands at a concentration close to that responsible for the negative inotropism and chronotropism.

Pericardium of the frog, Rana esculenta, is morphologically designed as a lymphatic space

The importance of the pericardium and the pericardial fluid (PF) in the control of cardiac function has emerged over the past few years. Despite the acknowledgment that amphibians are exposed to both dehydration and excessive water accumulation, nothing is known about their pericardial structure and the morphological basis of the PF formation. We have studied the parietal pericardium (PP) morphology in Rana esculenta by electron microscopy. SEM images of the inner surface, which lines the pericardial cavity, revealed the presence of large vesicles and many small circular openings. TEM observations showed that the PP is made up of an inner mesothelial lining, often constituted by two layers of very flat cells lying on a basal membrane and of regularly oriented collagen bundles. The PP outer surface is lined by a layer of flat cells, without a basal membrane. The mesothelial cells had overlapping boundaries with complex intercellular connections and a rich pool of caveolae opened in the direction of both the pericardial cavity and intercellular spaces. These cells indicate an intense intracellular and/or intercellular transfer of fluids and substances. The intraperitoneal injection of the idromineral hormone, Val(5)-ANG II, induced PP modifications, particularly evident at the level of the structures involved in the transmesothelial traffic. These lymphatic-like traits suggest that the frog PP represents a large lymphatic sac, subject to paracrine-endocrine remodeling, which can actively adjust the PF, influencing the composition and volume of the myocardial interstitial fluid.