The role of pulmocutaneous baroreceptors in the control of lymphatic heart rate in the toad Bufo marinus

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 regulation affects ventilation in the Cururu toad Rhinella schneideri

Anurans regulate short-term oscillations in blood pressure through changes in heart rate (fH), vascular resistance and lymph hearts frequency. Lung ventilation in anurans is linked to blood volume homeostasis by facilitating lymph return to the cardiovascular system. We hypothesized that the arterial baroreflex modulates pulmonary ventilation in the Cururu toad Rhinella schneideri, and that this relationship is temperature-dependent. Pharmacologically induced hypotension (sodium nitroprusside) and hypertension (phenylephrine) increased ventilation (25°C: 248.7±25.7; 35°C: 351.5±50.2 ml kg−1 min−1) and decreased ventilation (25°C: 9.0±6.6; 35°C: 50.7±15.6 ml kg−1 min−1), respectively, relative to control values from Ringer's injection (25°C: 78.1±17.0; 35°C: 137.7±15.5 ml kg−1 min−1). The sensitivity of the ventilatory response to blood pressure changes was higher during hypotension than hypertension (25°C: -97.6±17.1 vs. -23.6±6.0 breaths min−1 kPa−1; 35°C: -141.0±29.5 vs. -28.7±6.4 breaths min−1 kPa−1, respectively), while temperature had no effect on those sensitivities. Hyperoxia (30%; 25°C) diminished ventilation, but did not abolish the ventilatory response to hypotension, indicating a response independent of peripheral chemoreceptors. Although there are previous data showing increased fH baroreflex sensitivity from 15 to 30°C in this species, further increases in temperature (35°C) diminished fH baroreflex gain (40.5±5.62 vs. 21.6±4.64 % kPa−1). Therefore, besides a pulmonary ventilation role in matching O2 delivery to demand at higher temperatures in anurans, it also plays a role in blood pressure regulation, independent of temperature, possibly owing to an interaction between baroreflex and respiratory areas in the brain, as previously suggested for mammals.

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.

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.

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.

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.

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.

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.

Aspects of lymph-heart function inRana catesbeiana

The effect of voluntary dives on the posterior lymph heart rate of the bullfrog, Rana catesbeiana, was tested and compared with the blood-heart rate (n = 6). This was performed by cannulating the posterior lymph heart and femoral artery simultaneously. Blood-heart rates during submergence were significantly lower (α = 0.05) then pre-submergence rates at all sampling times. In contrast, the lymph hearts showed significantly lower rates only during the first and last submergence intervals. It is believed that the lymph-heart bradycardia found during these intervals is due in part to the physiological "preparations" for diving by the frog. Further information regarding posterior lymph heart contractions was gained by cannulating two posterior lymph hearts on one side of the frog (n = 5). It was found that these hearts beat within 100 ms of each other between 66 and 97% of the time (α = 0.05). The combined contraction of the three posterior lymph hearts could facilitate the movement of lymph through the outflow valve and into the venous circulation. This study represents the first time the axial coordination of homolateral lymph hearts has been shown to extend to the multiple posterior lymph hearts.