Accessory lymph sacs and body fluid partitioning in the lizard, Sauromalus hispidus

Chuckwalla lizards (genus Sauromalus) may accumulate substantial quantities of body fluid in extracoelomic, lateral abdominal spaces called accessory lymph sacs. The lymph sac fluid (LSF) of S. hispidus is similar to that of serum in Na+, K+ and Cl- concentrations, but the total protein content (3.58 +/- 0.20 g dl-1) is only half that measured in serum (7.05 +/- 0.26 g dl-1). These analyses confirm that LSF is an extravascular form of extracellular fluid, similar in composition to true lymph. Measurements of body fluid partitioning by dilution analysis indicate that Sauromalus hispidus Stejnejer possesses a comparatively large (38.9% body mass) and labile extracellular fluid volume (ECFV), and that the volume of LSF is dependent on the ECFV. Expansion of the ECFV (and subsequent accumulation of LSF) is observed following large, intercompartmental fluid shifts from intracellular to extracellular locations when lizards are kept inactive in simulated hibernation, are injected with KCl in amounts similar to those found in their field diet, and are hydrated with NaCl that is isotonic to their body fluids. These data collectively suggest that the lymph sacs of chuckwallas facilitate expansion of the ECFV, and may be adaptive not only as a means to store body water, but to accommodate transient shifts in body fluid from intracellular to extracellular locations.