Lymphatic regulation of hematocrit during hypoxia in the toad Bufo woodhousei

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

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.

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.

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.

Cardiorespiratory effects of forced activity and digestion in toads

Digestion and physical activity are associated with large and sometimes opposite changes in several physiological parameters. Gastric acid secretion during digestion causes increased levels of plasma bicarbonate ([HCO-3](pl)), whereas activity leads to a metabolic acidosis with increased lactate and decrease in plasma bicarbonate. Here we describe the combined effects of feeding and activity in the toad Bufo marinus to investigate whether the increased bicarbonate buffering capacity during digestion (the so-called alkaline tide) protects the acid-base disturbance during activity and enhances the subsequent recovery. In addition, we describe the changes in arterial oxygen levels and plasma ion composition, as well as rates of gas exchange, heart rates, and blood pressures. Toads were equipped with catheters in the femoral artery and divided into four experimental regimes: control, digestion, forced activity, and forced activity during the postprandial period (N=6 in each). Digestion induced a significant metabolic alkalosis with increased [HCO-3](pl) that was completely balanced by a respiratory acidosis; that is, increased arterial Pco(2) (P(a)co(2)), so that arterial pH (pH(a)) did not change. Forced activity led to a substantial reduction in pH(a) by 0.43 units, an increase in plasma lactate concentration by 12.5 mmol L(-1), and a reduction in [HCO-3](pl) of similar magnitude. While digesting animals had higher P(a)co(2) and [HCO-3](pl) at rest, the magnitude and duration of the changes in arterial acid-base parameters were similar to those of fasting animals, although the reduction in pH(a) was somewhat lower (0.32 units). In conclusion, while recovery from the acidosis following exercise did not seem to be affected by digestion, the alkaline tide did slightly dampen the reduction in pH(a) during activity.

Cardiovascular responses to hypoxia and anaemia in the toad Bufo marinus

Amphibians exhibit cardiorespiratory responses to hypoxia and, although several oxygen-sensitive chemoreceptor sites have been identified, the specific oxygen stimulus that triggers these responses remains controversial. This study investigates whether the cardiovascular response to oxygen shortage correlates with decreased oxygen partial pressure of arterial blood (Pa(O(2))) or reduced oxygen concentration ([O(2)]) in toads. Toads, equipped with blood flow probes and an arterial catheter, were exposed to graded hypoxia [fraction of oxygen in the inspired air (FI(O(2)))=0.21, 0.15, 0.10, 0.07 and 0.05] before and after reductions in arterial [O(2)] by isovolemic anaemia that reduced haematocrit by approximately 50%. Toads responded to hypoxia by increasing heart rate (fH) and pulmocutaneous blood flow (Q(pc)) and reducing the net cardiac right-to-left-shunt. When arterial [O(2)] was reduced by anaemia, the toads exhibited a similar cardiovascular response to that observed in hypoxia. While arterial CO(2) partial pressure (Pa(CO(2))) decreased significantly during hypoxia, indicative of increased alveolar ventilation, anaemia did not alter Pa(CO(2))). This suggests that reductions in [O(2)] mediate cardiovascular adjustments, while ventilatory responses are caused by reduced Pa(O(2)).

Whole-body systemic transcapillary filtration rates, coefficients, and isogravimetric capillary pressures in Bufo marinus and Rana catesbeiana

Whole-body and organ-level transcapillary filtration rates and coefficients are virtually unexamined in ectothermal vertebrates. These filtration rates appear to be greater than in mammals when plasma volume shifts and lymphatic function are analyzed. Gravimetric techniques monitoring whole-body mass changes were used to estimate net systemic filtration in Bufo marinus and Rana catesbeiana while perfusing with low-protein Ringer's and manipulating venous pressure. Capillary pressures were estimated from arterial and venous pressures after measuring the venous to arterial resistance ratio of 0.23. The capillary filtration coefficient (CFC) for the two species was 25.2+/-1.47 mL min-1 kg-1 kPa-1. Isogravimetric capillary pressure (Pci), the pressure at which net fluid is neither filtered nor reabsorbed, was 1.12+/-0.054 kPa and was confirmed by an independent method. None of these variables showed a significant interspecific difference. The anuran CFC and Pci are significantly higher than those found using the same method on rats (7.6+/-2.04 mL min-1 kg-1 kPa-1 and 0.3+/-0.37 kPa, respectively) and those commonly reported in mammals. Despite the high CFC, the high Pci predicts that little net filtration will occur at resting in vivo capillary pressures.

Effects of temperature and physical activity on blood flow shunts and intracardiac mixing in the toad Bufo marinus

Blood flow in systemic (.Qsys) and pulmocutaneous (.Qpul) arteries was measured as a function of body temperature (10 degrees, 20 degrees, and 30 degrees C) at rest and following enforced physical activity in conscious, adult cane toads (Bufo marinus). Arterial and mixed venous hemoglobin concentration (CHb) and total oxygen content (Co2, tot) were measured in a separate group under identical conditions. Heart rate (fH) and total flow (.Qtot) increased significantly (P<0.001) with elevated temperature and with activity, whereas stroke volume (VS) increased (P<0.001) only with activity. .Qtot ranged about 10-fold, from 10 degrees C (rest) to 30 degrees C (activity); increases in both fH and VS contributed to the increase in .Qtot. The overall distribution of blood to the pulmocutaneous circuit (net L-R shunt) increased with both temperature and activity and was significantly correlated with .Qtot. These data indicate that blood flow distribution in toads is a direct function of cardiac output, and this is linked to relative changes in resistance in the major outflow vessels. Arterial O2 saturation (Sa) was high (mean=93%) in all conditions except activity at 30 degrees C, when it decreased to 74% and contributed to a decrease in the arteriovenous O2 difference. Venous O2 saturation (Sv) was high at rest (76%) and dropped significantly during activity to about 30% at all temperatures. Intracardiac arterial-venous mixing (systemic mixing index) showed the strongest correlation with variation in fH with minimal mixing (17%) occurring at about 50 beats min-1. The most mixing occurred at the lowest fH (13 beats min-1) and at the highest fH (103 beats min-1). The results indicate that the heart of a 0.25-kg toad becomes more efficient from an oxygen transport perspective from low fH to 50 beats min-1 and then less efficient at higher fH, contributing to an uncoupling of blood flow and metabolic rates at these high rates.

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

The influence of increased pulmocutaneous arterial systolic pressure on amphibian lymph heart activity was determined in Bufo marinus. Arginine vasotocin-induced increases in systolic arterial pressure of greater than 0.5 kPa significantly (P<0.0002) decreased lymph heart rate. Denervation of the recurrent laryngeal nerve led to an increase in arterial pressure of 3.88 kPa in test animals compared to control animals. Denervation abolished lymph heart response to increased arterial pressure for an average of 38 min. Direct stimulation to the recurrent laryngeal nerve stopped lymph heart activity without an increase in arterial pressure. These data indicate that baroreceptors in the pulmocutaneous artery can decrease lymph heart rate during periods of either direct stimulation or elevated arterial pressure. The data also support lymph hearts as effectors for aortic and carotid baroreceptors.