Lung ventilation contributes to vertical lymph movement in anurans

Abstract

SUMMARY 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<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

“…Our description of lymphatic sac-related skeletal muscles and pressure and EMG data support our hypothesis that vertical transport of lymph must be principally accomplished by contraction of skeletal muscles rather than a compliance pump (Hillman et al, 2004;Hillman et al, 2005) or by negative pressures created by the lymph hearts alone (Toews and Wentzell, 1995). Lymphatic movement is also influenced by ventilatory events driven by skeletal muscles (Hedrick et al, 2007).…”

“…Hedrick et al have demonstrated that lung inflation forces lymph in the lateral, ventral and subvertebral lymph sacs anteriorly and posteriorly to the vicinity of the dorsal lymph hearts dorsally and to the axillae of the fore and hindlimbs ventrally (Hedrick et al, 2007). Here, we are concerned with the vertical movement of lymph pooling in sacs in the posterior ventral and inguinal regions of the frog up to the vicinity of the lymph hearts.…”

Regulation of theRana sylvaticabrevinin-1SY antimicrobial peptide during development and in dorsal and ventral skin in response to freezing, anoxia, and dehydration

“…Even in the face of high water loss, the skin remains important for cutaneous CO 2 excretion (Boutilier et al, 1979;Burggren and Vitalis, 2005). During anoxia, breathing rate has been shown to progressively decrease until it eventually ceases in Rana pipiens and as lymph movement is dependent on lung ventilation in frogs, the physiological responses to anoxia may inadvertently lower immunity (Winmill et al, 2005;Hedrick et al, 2007) as the lymphatics, as well as the blood, carry immune cells. With a decreased internal immune function, the protection of the skin by AMPs may become increasingly important.…”

Baroreflex regulation affects ventilation in the Cururu toad Rhinella schneideri

“…In addition, lymph heart destruction results in a marked increase in red cell volume, interstitial edema and also death (Zwemer and Foglia, 1943;Baustian, 1988). Generated extravascular fluid in anuran amphibians returns to the cardiovascular system through a combination of changes in pressure and volume of the lymph sacs that is driven by effectors such as lung ventilation (inflation and deflation) and skeletal muscle contraction Hedrick et al, 2007). Therefore, from an evolutionary perspective, ventilatory responses that may be modulated by baroreceptor loading and unloading in R. schneideri can contribute to our understanding of a number of pieces of evidence for the influence of the cardiovascular system on the respiratory system in mammals (Brunner et al, 1982;Walker and Jennings, 1998;Baekey et al, 2010;McMullan and Pilowsky, 2010;Stewart et al, 2011).…”

“…Based on the roles played by lung ventilation in blood volume homeostasis in anurans (Crossley and Hillman, 1999;Hedrick et al, 2007;Hillman et al, 2010Hillman et al, , 2013 and in O 2 delivery to match metabolic demands, we tested the hypothesis that pulmonary ventilation contributes to the baroreflex responses to blood pressure imbalances in R. schneideri, and that this effect is modulated by changes in temperature. To this end, baroreflex function was assessed through pharmacological interventions on arterial blood pressure, i.e.…”